a literature review of soil carbon under pasture, horticulture and

A literature review of soil carbon under pasture, horticulture and

arable land uses

Report prepared for AGMARDT

October 2009

A literature review of soil carbon under pasture,

horticulture and arable land uses

Report prepared for AGMARDT

October 2009

Anwar Ghani, Alec Mackay, Brent Clothier, Denis Curtin and Graham Sparling

DISCLAIMER: While all reasonable endeavour has been made to ensure the accuracy of the investigations and the information contained in this report, AgResearch expressly disclaims any and all liabilities contingent or otherwise that may arise from the use of the information.

COPYRIGHT: All rights are reserved worldwide. No part of this publication may be

copied, photocopied, reproduced, translated, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of AgResearch Ltd.

3

Soil Carbon Report prepared for AGMARDT

Table of Contents

1. Executive Summary ................................................................................................................... 6

2. Carbon: a global perspective .................................................................................................... 9

2.1 Carbon and our world ...................................................................................................... 9

2.2 Carbon cycling in terrestrial environments .................................................................... 10

2.3 Carbon content of New Zealand Soils ........................................................................... 11

2.4 Why is soil carbon important? ....................................................................................... 16

2.4.1 Physical characteristics ................................................................................................. 16

2.4.2 Chemical characteristics ............................................................................................... 17

2.4.3 Biological characteristics ............................................................................................... 18

2.5 How is soil organic carbon measured? ......................................................................... 18

2.5.1 Soil sampling for carbon measurements....................................................................... 19

2.5.2 Preferred units ............................................................................................................... 22

2.6 Characterising soil organic matter ................................................................................. 23

2.7 Climate change and soil carbon .................................................................................... 25

2.8 Soil carbon and soil quality ............................................................................................ 25

2.9 Sustaining the soils carbon ............................................................................................ 26

2.10 Why is it so difficult to increase the soil carbon content over the long term? ............... 29

3. Carbon cycling in grassland/pasture systems .....................................................................30

Suggestions for increasing soil carbon under pastoral agriculture ......................................... 30

3.1 Soil carbon in grassland and pasture soil – A global view ............................................ 31

3.2 Carbon cycle in grazed pastures ................................................................................... 32

3.3 What are the levels of soil carbon in temperate pastures? ........................................... 34

3.4 Soil carbon changes, turnover and half-life in NZ pastures .......................................... 35

3.5 Effects of livestock on soil carbon ................................................................................. 37

3.6 Effects of irrigation ......................................................................................................... 38

3.7 Effects of effluent application ......................................................................................... 39

3.8 Effects of fertilisers ........................................................................................................ 40

3.9 Effects of drainage of wetlands ..................................................................................... 41

3.10 Effects of farm systems (organic, biodynamic, conventional) ....................................... 42

4


3.11 Total Vs. labile carbon as indicator of early changes in soil carbon ............................. 44

4. Carbon cycling in horticultural systems................................................................................45

Suggestion for improving C in soils under horticulture land use ............................................. 45

4.1 Soil carbon in horticultural soils – A world view ............................................................ 46

4.2 Soil carbon in horticultural soils – A New Zealand view ................................................ 46

4.3 Land use and land-use change and impacts on soil carbon ......................................... 47

4.4 Carbon stocks and flows: The Big Three - Kiwifruit, Grapes, and Apples .................... 51

4.4.1 Kiwifruit .......................................................................................................................... 51

4.4.2 Grapes ........................................................................................................................... 53

4.4.3 Apples ............................................................................................................................ 54

4.5 Understorey management ............................................................................................. 57

4.6 Irrigation ......................................................................................................................... 57

4.7 Cultivation ...................................................................................................................... 58

4.8 Production systems: Integrated and Organic ................................................................ 59

4.9 Soil carbon and its impact on soil functioning and soil quality ...................................... 60

5. Arable Soils ...............................................................................................................................64

Suggestions for increasing soil Carbon under cropping ......................................................... 64

5.1 Carbon cycling in arable soils ........................................................................................ 65

5.2 Soil carbon in arable soils – world view ......................................................................... 66

5.3 Soil carbon in NZ arable soils ........................................................................................ 67

5.4 Effect of cultivation practices ......................................................................................... 68

5.5 Effects of irrigation ......................................................................................................... 69

5.6 Effects of fertiliser .......................................................................................................... 69

5.7 Effect of plant type and crop rotation ............................................................................. 69

5.8 Effects of farm systems (organic and conventional) on soil carbon .............................. 69

5.9 Vegetable production and soil carbon ........................................................................... 70

5.10 Predicting effects of management practices on soil carbon .......................................... 74

6. Opportunities for increasing carbon sequestration .............................................................76

6.1 Bio-solids, green wastes, manures ............................................................................... 76

6.1.1 Availability of organic solid and semi-solid waste materials in NZ ............................... 77

5


6.1.2 Carbon accumulation in soils through application of waste .......................................... 78

6.1.3 Co-benefits of carbon sequestration in soils ................................................................. 78

6.1.4 Risks association with addition of organic carbon in soils ............................................ 79

6.2 Pastoral fallow................................................................................................................ 79

6.3 Tree pasture systems .................................................................................................... 80

6.4 Managing soil carbon sequestering through biochar .................................................... 81

6.4.1 Stability of biochar in soils ............................................................................................. 82

6.4.2 Net reduction of equivalent CO2 emissions due to the use of biochar ......................... 83

6.4.3 Practicality and cost-effectiveness of biochar use ........................................................ 83

6.4.4 Possible short- or long-term consequences of biochar applications ............................ 84

6.4.5 Impact of biochar on physical and chemical soil properties ......................................... 85

7. Soil carbon as a soil quality indicator and soil natural capital and ecosystems

services .....................................................................................................................................86

7.1 Soil carbon and soil quality indicators ........................................................................... 87

7.1.1 Soil quality indicators..................................................................................................... 87

7.1.2 Soil quality indicators for NZ soils ................................................................................. 88

7.1.3 Soil carbon as a soil quality indicator ............................................................................ 88

7.2 Natural capital and ecosystems services ...................................................................... 90

7.2.1 Soil Natural Capital ........................................................................................................ 90

7.3 Classifying and measuring soil natural capital and ecosystem services ...................... 93

7.4 Application of soil Natural capital in land management................................................. 92

8. References ................................................................................................................................97

9. Glossary ..................................................................................................................................113

10. Appendix 1 PAS 2050 protocol ............................................................................................118

6


1. Executive Summary

Carbon (C) is an essential constituent of biological life on earth. Most C on earth is not in living

organisms but is present in vast amounts in rocks such as limestone, and in dead organic matter.

For terrestrial ecosystems, there is more C in soil organic matter (SOM), than in the living plants

and animals, and the atmosphere. It has been estimated that soil contains 1,200–1,550 Pg C to a

depth of 1 m, and 2,370–2,450 Pg C to a depth of 2 m. Comparative estimates of organic C

contained in living biomass (560 Pg) and atmospheric CO2-C (760 Pg) indicates that a small shift

in the soil organic C pool has the potential to have a significant impact on atmospheric C

concentrations.

Organic matter helps soils to retain and store water and plant nutrients, to resist erosion, form

stable aggregates, improve water infiltration and drainage, and provide a food source and habitat

for soil dwelling organisms. The C in organic matter is from a complex mix of different compounds

that is undergoing constant change as fresh organic matter is added, and organic matter

decomposes. Once lost from the soil organic C pool, replenishment can take many years, even

centuries. For New Zealand soils the total C content, measured by high temperature combustion,

provides a reliable measure of the organic C content. Important considerations for estimating C

stocks in a soil are a representative sample, the depth of sampling and bulk densities. This

literature review summarizes the impacts of different land uses namely pasture, arable and

horticulture on soil C levels. It also covers options for increasing C sequestration in soils through

adaptation of soil and crop management and organic inputs, soil C as a soil quality indicator and

emerging approaches for valuing soil C.

Nearly 1/3 of the terrestrial C stock is stored in pasture and rangeland soils. At present, for most

pastoral soils in New Zealand production gains are unlikely from increasing soil C levels above

current levels. They are in general described as being ―rich‖ in organic C ranging between 3.5-

15% C (w/w). Recent study in New Zealand indicates that in intensive lowland livestock systems,

soils have lost C over the last approximately 2 decades, while in hill land soil C levels have

increased. Both were unexpected findings. Further work is required to identify the reasons for the

measured losses of soil C under intensive livestock farming and establish if these losses are

ongoing. While numerous international studies show inputs such as fertilisers, irrigation and

lenient grazing can increase C sequestration, many of these come from native grasslands and

degraded rangelands with low initial soil C contents. This is not the general case in New Zealand

with many of our pasture soils already ―rich‖ in organic matter. The opportunity for increasing C

sequestration in soil is therefore likely to be limited primarily to hill land. Retaining existing soil C

contents may pose more of a challenge on flat and lowland pastures.

7


Horticultural production systems are often managed to increase the production of the fruiting part

of the plant, rather than the vegetative component. Thus soils with low soil C and with limited

fertility are favoured for horticulture to ensure quality production of fruit. The combination of

naturally low vegetative vigour, pruning practices, and high harvest index in horticultural systems,

return low amounts of C to the soil, resulting in observed losses in soil C. Because soil C is

critical for soil functioning and health (e.g. aeration and N mineralization), it is imperative that C

levels be maintained even if this is at levels lower than pastoral soils. In orchards and vineyards

this can be achieved through mulches and composts, and in the future there is the possibility of

gains from using biochar (charcoal produced by pyrolysis of woody plant biomass).

Levels of C in arable and, especially, vegetable-producing soils are much lower than in pastoral

soils. To raise soil C levels, it is necessary to either increase C inputs from crop residues or

decrease the rate of decomposition. Switching from intensive cultivation to low-disturbance tillage

may be beneficial in increasing C in soils with low C contents, though further work is needed to

quantify the C sequestration potential of no tillage under New Zealand conditions. Although a

reduction in tillage intensity can sometimes increase C inputs (by improving yield), the effect of

tillage on soil C is mainly related to its influence on the rate of decomposition. Managing crops to

maximise yields (e.g., by providing adequate nutrition for the crop) should also maximise C

returns in post-harvest residues. Crop type has a strong influence on C returns in plant residues,

with perennial grasses (pasture) returning largest amounts, vegetable crops the least, and small-

grain cereals being intermediate. In the long-term, burning of crop residues can cause depletion

of soil C.

Environmental benefits of sequestering greater quantities of C in soils are well documented but

are harder to quantify. Other than some subtle land management options that slow down

decomposition of SOM, regular inputs of low cost materials that are rich in organic C can improve

soil C stocks. These include addition of organic wastes (e.g. biosolids, pulp and paper waste,

green waste, manures), fallowing, and the inclusion of spaced-tree in pasture systems as part of

a soil conservation practice and for providing shade, shelter and fodder. More recently addition of

agrichar or biochar in agricultural soils has also been suggested. It is too early to comment on the

potential value and the practical challenges associated with use of these compounds.

Because of the pivotal role soil C plays in soil function, it is a very useful soil quality indicator for a

wider range of soil services. For each of the major soil orders and land uses, there is an optimum

range of soil C to achieve the desired production and environmental goals. There is also a

significant range in that optimum target range for which there appears to be little measurable

change in production or environmental outcomes. For example for the allophanic soil the optimum

range under a pastoral use varies from 4-9% soil C. Inclusion of soil C as a potential C-offset for

greenhouse gas emissions, will require a rethink of the current target range or fit-for-purpose

definition for this soil quality indicator and a revision of the provisional targets for state-of-the-

8


environment indicators published in Provisional targets for soil quality indicators in New Zealand.

A limitation of soil quality indicators is that they only inform us about the characteristics or

condition of the soil. Linked to soil processes and built into an ecosystems service framework an

insight into the influence of a change in soil C on a wider range of soil services could be

quantified and ultimately valued, help to highlight just how closely our well-being is linked to the C

economy.

9


2. Carbon: a global perspective

Summary Carbon (C) is an essential constituent of biological life on earth; our bodies are built up of a huge

range of different complex organic molecules. But on a global scale, most C on earth is not in

living organisms but is present in vast amounts in rocks such as limestone, and in dead organic

matter. For terrestrial ecosystems, there is more C in soil organic matter, than in the living plants

and animals, plus the atmosphere. Soils form an important role in the global C balance.

Organic matter modifies the characteristics of soil, usually in ways that benefit human use.

Organic matter helps soils retain and store plant nutrients, resist erosion, form stable aggregates,

store and retain moisture, improve drainage and water infiltration, and provide a food source and

habitat for soil dwelling organisms.

Most organic matter C is concentrated in the surface layers of soil (0-300 mm) and has taken

many hundreds of years to form. Soil C is not a single compound but a complex mix of different

compounds that is constantly undergoing change as fresh organic matter is added, and older

organic matter decomposes. Once lost from soil, organic C can take many years, even centuries,

to be restored. For New Zealand soils the total C content, measured by high temperature

combustion, provides a reliable measure of the organic C content.

2.1 Carbon and our world

Carbon (C) is the major chemical constituent of all organic matter and all life on our planet. This C

comes in a vast range of forms from the living tissues of your body to non-living forms such as

coal and oil and diamonds. The amount present as living plants and animals or as dead plants

and animals (litter, organic matter and fossil fuel deposits) is very small compared to inorganic

forms in rocks such as limestone and sediments (Table 1.1). The C stored in soil organic matter

is an important component in the global C balance. The organic forms of C in soil are often

referred to collectively as soil organic matter, humus, or soil C. In fact soil organic matter is about

60% C, and comprises the largest terrestrial store for organic C, more than the total amount in

living land plants and animals and the atmosphere.

Soil organic matter is a complex mixture of compounds and differs in composition from soil to soil.

There is no single ―description‖ for soil organic matter, and it is actually very difficult to

characterize except in a general way, using chemical and physical methods to separate out

various fractions. As well as C, soil organic matter also contains large amounts of oxygen (O),

hydrogen (H), nitrogen (N), smaller amounts of sulphur (S) and phosphorus (P) and a range of

trace elements. Derived initially from the decomposing remains of plants, animals and soil

microbes, the composition of soil organic matter is not static but is constantly undergoing

10


continuing decay and transformation. It is organic matter that gives topsoil and composts a dark

colour.

Table 1.1 Estimated major stores of C on the Earth

From ―The Encyclopedia of Earth. http://www.eoearth.org/article/Carbon_cycle

2.2 Carbon cycling in terrestrial environments

Virtually all the C in soil organic matter has originally been ―fixed‖ by green plants using energy

from the sun. In the leaves of green plants, the C gas in the atmosphere (carbon dioxide, CO2) is

converted into complex organic C molecules and becomes part of the plant structure. If the plant

is eaten by an animal then that C is respired for energy or becomes part of the animal‘s body.

When plants or animals die, or when animals excrete wastes, the C from their bodies and wastes

enters the soil and begins to decay, forming soil organic matter. The amount of ―dead‖ organic

matter in soil is substantial; 3-4 times more than the mass of the living terrestrial organisms. This

dead soil organic matter was initially all derived from the ―fixation‖ of atmospheric CO2 by living

photosynthetic organisms. The decay of dead organic matter by soil organisms completes the C

cycle converting the organic matter back to gaseous CO2 in the atmosphere (Fig. 1.1) that living

plants can use. Under some conditions during decomposition small amounts of methane may be

formed. This is a concern as methane is a potent greenhouse gas (Nayak et al. 2007). Much

methane is formed when grazing ruminant animals digest pasture.

Store Amount in Billions of Metric Tons

Marine Sediments and Sedimentary Rocks 66,000,000 to 100,000,000

Ocean 38,000 to 40,000

Fossil Fuel Deposits 4000

Soil Organic Matter 1500 to 1600

Atmosphere 578 (as of 1700) - 766 (as of 1999)

Terrestrial Plants 540 to 610

11


Figure 1.1 Average amounts of C in one hectare of terrestrial ecosystems and yearly inputs and losses of C. Adapted from Janzen (2004).

2.3 Carbon content of New Zealand Soils

The large scale distribution of organic C in New Zealand is shown in Figure 1.2. The low C areas

are clearly shown in red and the very high C soils (peats) in dark green. In general, North Island

soils have a greater C content than those in South Island.

The C content of soils depends on the type of soil (its soil class or order) which itself depends on

the parent rock materials that made up the soil, climatic factors and the length of time for soil

formation. Note the high C content of Allophanic soils and the much lower amounts in raw and

Semi-arid soils (Fig. 1.3).

12


Figure 1.2 The pattern of total C in New Zealand soils taken from Landcare Research’s

Fundamental Soil Layers (NZFSL).

13


Figure 1.3 Organic C as percentage of soil mass in different New Zealand soil orders. Note

high C in Allophanic Soils (volcanic ash soils) and lower C in Pallic Raw (newly forming

soils) and Semi-arid Soils (typically from dry Central Otago). From New Zealand National Soils

Database

In New Zealand by far the largest amount of soil C is stored under pastures (grazing land) than

other land uses because this is the dominant land use, and also because the C content of soils

under pastures is greater than under most other land uses (Table 1.2)

In undisturbed soils most organic matter is in the surface layer (0–10 cm), and declines rapidly

with increasing depth. An example is shown in Fig. 1.4 for Horotiu soil from the Allophanic Soil

Order. An exception to this general rule is the Organic peat soils where the organic matter is

distributed evenly throughout the soil profile Most New Zealand soils contain around 50–150

tonnes C in 1 ha (to 10 cm depth).

14


Table 1.2 Estimates of the amounts of soil organic C under different land uses summed for the whole of New Zealand and average amounts per hectare (as in 2000, 0–30 cm depth).

Adapted from: Tate et al (2005)

Figure 1.4 Total soil C stored in a Horotiu soil at different depths (Schipper, unpublished).

Land use

Area

(Mha)

Soil C (Millions of

tonnes)

Mean and

standard error

Soil C content

( Tonnes ha-1

)

Grazing land 14.0 1480±58 105.7

Natural shrub vegetation 2.7 244±18 90.4

Cropland 0.3 26±3 86.7

Exotic forest 1.3 77±23 59.2

15


Figure 1.5 The decreasing C:N ratio in soil organic matter due to the accumulation of N

under various land uses on Allophanic soils. (Sparling and Schipper, unpublished data from

the 500 Soils Project)

Even within a single soil order the C content and its characteristics vary greatly with land use,

reflecting differences in inputs of C and N. This is especially noticeable when the N content of the

organic matter is examined, with more intensive lands uses having markedly higher stored N in

the organic matter, reducing the soil C:N ratio (Figure 1.5)

The lowest C:N ratios known globally are about 10 (see database of world soils, Batjes 1996).

The values in New Zealand dairy soils approach this lower limit. Under decades of dairy farming

New Zealand soils have accumulated large amounts of N in soil organic matter and look to have

little further capacity to store additional N. Schipper et al. (2004) estimated that within 40 years,

most soils under intensive livestock farming would be near N saturated. An N saturated soil can

no longer store more organic N. Any additional N will be lost from the soil and ultimately

accumulate in drainage waters and aquifers as nitrates.

Allophanic soils

Dairy

Drysto

ck

Arable cr

op

Horticu

lture

Forestr

y

Indigenous

Land use

5

10

15

20

25

C:N

Allophanic soils

Dairy

Drysto

ck

Arable cr

op

Horticu

lture

Forestr

y

Indigenous

Land use

5

10

15

20

25

C:N

Allophanic soils

Dairy

Drysto

ck

Arable cr

op

Horticu

lture

Forestr

y

Indigenous

Land use

5

10

15

20

25

C:N

16


2.4 Why is soil carbon important?

Organic matter helps to bind the primary mineral particles, forming crumbs and aggregates.

Aggregates help to reduce soil erosion and also give soil its crumbly texture, or ‗structure‘. A well

structured soil makes it easier for plant roots to penetrate, for air to enter the soil and for the soils

to drain freely and still retain water.

The presence of organic matter helps the soil to store plant nutrients. Among the most important

of these nutrients is nitrogen (N), an essential macronutrient for plants and animals. Mineral rocks

contain very little N. Most N in soils has been accumulated through biological processes such as

N-fixation by legumes and their symbiotic bacteria, and by chemical inputs from fertilisers and

from nitrogen oxides formed in the atmosphere by lightning. The ability of a soil to hold onto these

sparse N sources is highly dependent on the amount of soil organic C (see Fig 1.6b). When

micro-organisms decompose soil organic matter, this organic N becomes available to plants.

2.4.1 Physical characteristics

Organic matter helps bind soils together and assist with the formation of porous aggregates,

which reduces bulk density (Figure 1.6a). These soil aggregates increase the porosity of the soil,

and improve aeration (Soane 1990). Aeration and porosity are essential for good root growth.

Organic matter also makes the aggregates more resistant to being compacted either by animal

hooves or vehicles (Soane 1990, Shepherd et al. 2001). The larger soil aggregates are less

susceptible to movement by wind or water flows which makes the soil less erodible. Infiltration,

the rate at which water soaks into soil, is also improved by good aggregate structure, meaning

the soils are better draining, and less susceptible to water-logging and treading damage (Drewry

2006).

Organic matter helps soils to retain water once they have wetted. Soils with higher organic matter

release water gradually, providing a valuable ecological service. Without these characteristics,

during heavy rainfall, the water may run off the soil surface and cause surface flooding. Also the

ability to retain water within the soil structure means that rivers and streams are buffered from

rapid changes in flow rates and the soil water gets released gradually, helping to avoid ―flash

flood‖ events.

An unusual effect of soil organic matter is the potential to slightly increase soil temperature and

cause the soil to warm up more quickly. This can be useful if low soil temperatures are limiting

root growth and biological activity. The dark colour of soil organic matter means that the surface

of the soil is darker than it would be if no organic matter was present. This darker colour

decreases the albedo meaning that less radiant heat from the sun is reflected and the soil warms

more quickly. A bare topsoil with some organic matter has an albedo (reflectance) of about 0.17,

17


compared to about 0.40 for a desert sand with no organic matter (scale 0–1, low to high

reflectance) (http://en.wikipedia.org/wiki/Albedo).

2.4.2 Chemical characteristics

Organic matter in soils provides a store of nutrient for plants and soil fauna. Soil C is covalently

bonded to nitrogen (N) and through other bonds to phosphorus (P) and sulphur (S). Fig 1.6b

shows the close relationship between the amount of total C in soil and the amount of N stored in

soil.

Figure 1.6. Examples of the influence of soil organic C (measured as total C%), on (a) bulk

density, (b) N content and (c) cation exchange capacity (CEC) (Sparling, unpublished

data).

Soil C is not a direct food source to plants, but when organic matter mineralises the organic N is

released in plant available inorganic forms. A similar process releases P and S. Soil organic

matter has a high surface area and the charged surfaces give the organic matter the ability to

retain charged ions. Soils with high organic matter are much better buffered against pH change

than low organic matter soil, and the charged surfaces enable the soil to retain cations such as

Ca, Mg, and K (cation exchange capacity) (Fig 1.6c). Soil organic matter also has the capacity to

absorb trace elements such as Ni, Co, Cu, Pb, Mn, Zn, Ca, and Mg which lowers the effective

concentration in soil solution. However, the soluble fraction of organic matter can form complexes

with ions,keeping them in solution. The mechanisms of complexing, whether by chelation or other

mechanisms, are poorly understood, but the overall effect is to keep the trace elements in

solution where they are available for plant uptake and metabolism (Huang and Schnitzer 1986).

In some cases this may benefit the plant or soil organisms where the trace metal is essential for

growth (e.g., Mg, Fe) in other cases there may be a potentially toxic result if contaminant remains

in bio-available form.

Organic matter and nitrogen

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60

Total C %

To

ta

l N

%

Organic matter and CEC

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50 60

Total C %C

atio

n e

xch

an

ge

Organic matter and bulk density

0

0.5

1

1.5

2

0 10 20 30 40 50 60

Total C %

Bu

lk d

en

sit

y

a b c

18


2.4.3 Biological characteristics

Soil organic matter forms both a habitat and food source for soil organisms. Many soil fauna and

flora are detritus feeders ingesting organic matter particles which they digest as a source of

energy. Bacteria and fungi are often intimately embedded within an organic matter matrix. They

produce extracellular enzymes that are able to break down the organic matter to simpler

molecules which they are subsequently able to absorb into their bodies (Huang and Schnitzer

1986). Plant roots have been shown to be able to absorb even high weight organic molecules

and incorporate them into their chemical structure. Chemical fractions such as humic and fulvic

acids (See Table 3) have been claimed to stimulate plant growth, although the effects have not

always been consistent (Vaughan and Ord 1985; Huang and Schnitzer 1986).

An effect of SOM content on microbial diversity has been noted. In a survey of different soils and

land uses. Degens et al. (2000) noted that the biodiversity measured by the catabolic response

profile (CRP) test was reduced on matched pairs of soils where one of the pair had lost organic C

through land use change. Further work (Degens et al. 2001) showed that the microbial

communities of C depleted soils were less resistant to imposed stresses such as fluctuating

temperature, drying of the soil, salt stress, and metal toxicity.

2.5 How is soil organic carbon measured?

Once a soil sample has been obtained, there are well-established chemical methods to measure

the amount of C in the sample. For New Zealand a measure of total C gives a good estimate of

the soil organic C content (Blakemore et al. 1987; Metson et al.1979). This is because most New

Zealand soils contain very little carbonate (inorganic C) which would otherwise interfere with the

measurement of soil organic C (Miller 1968). In other countries it is often necessary to remove or

measure the inorganic C before measuring the soil organic C. The recommended method to

determine total C in New Zealand soils soil is high temperature combustion. High temperature

combustion of soils and measurement of CO2 evolved is a very reliable method and causes less

potential environmental pollution than chemical oxidation because no toxic chromium salts, or

boiling of highly concentrated acids are required. If high temperature combustion instruments

such as the Leco FP-2000 CNS Analyser are not available, then dichromate oxidation and

titration or spectrographic measurements are acceptable (Metson et al. 1979; Blakemore et al.

1987). These are specialist measures that need to be completed in a registered laboratory.

An approximate method to estimate soil organic matter is ―loss on ignition‖. In this method dried

soil is heated on a hot plate in the presence of air, and the organic matter in soil is gradually burnt

away. The loss in weight is taken as loss of soil organic matter from the sample and hence the

organic C content. However, this is a very approximate method and the temperature and time of

heating needs to be very carefully controlled to ensure only organic matter is oxidized. Thermal

19


decomposition of minerals in soil occurs on heating together with further loss of bound water from

the soil. If only the change in weight is recorded, this can result in large overestimates of soil

organic matter because water, as well as CO2 has been lost from the sample, and there will also

have been thermal decomposition of clays and minerals resulting in weight loss.

The high temperature combustion methods cited earlier also heat the soil, but under very

controlled conditions and the C content is measured directly as carbon dioxide (CO2) release,

rather than total loss in weight The current generation of high temperature combustion

instruments is sufficiently sensitive to be able to distinguish CO2 from the thermal decomposition

of minerals (such as carbonates) from CO2 derived from soil organic matter (see manufactures‘

specifications)

2.5.1 Soil sampling for carbon measurements

A more challenging task than analyzing the soil is to obtain a representative soil sample,

particularly if the total soil profile storage of C is required (Schipper et al., 2007; Parfitt et al.,

2007).

The main issues are:

Obtaining a representative soil sample

Depth of sampling

Measuring and adjusting for soil bulk density

Units used to show C content.

A representative sample of the area under considerations must attempt to capture all the variation

that is likely to influence soil C levels, by collecting and bulking sufficient numbers of soil cores to

obtain an estimate of the C level in soil. Single or only a few core bulked together will give greater

variability when assessing changes in soil C over time. Although soil C does not change a great

deal between different seasons within a land use and within a paddock, one must collect soil

samples approximately at the same season when comparing soil C on yearly intervals. Ghani et

al. (1996) found less than 10% variability in soil C contents in paddocks when they re-sampled

the same paddocks at 24 different farms within the Waikato area following the protocols shown in

Figure 1.7a,b. At the farm scale it might be necessary to break the farm into blocks based on soil

type and slope sampled separately and bulked (Figure 1.8). In grazed pastures, animal camping

areas where litter deposition is relatively high should be sampled separately.

20


(a)

(b)

Figure 1.7a,b: Methods of collecting representative samples within a paddock.

21


Figure 1.8: Examples of how different blocks of the farm can be sampled to get a

representative sample for assessing total soil C stock.

Soil organic matter shows considerable spatially variability. The depth of topsoil differs depending

on soil type and land use, adding further to the variability. The simplest approach to measure the

organic C content of the surface soil is to sample to a fixed depth. This is usually to a depth of 75

mm or 100 mm for pastures and 200 mm or more for cultivated and forest soils. Tube augers are

suitable sampling tools, a diameter of at least 20 mm is recommended, and multiple cores are

required to obtain a representative sample. Schipper and Sparling (2000) examined the variation

in C contents over a transect length of 30 m at 67 sites, mostly under pasture. They estimated the

overall coefficient of variation (standard deviation/mean) of the C content was 9.4%. This

variation was the sum of the effects of systematic errors (e.g. replicate analyses in the

laboratory), spatial effects (changes across distance) and land use (changes due to different land

use, e.g. row crops). Giltrap and Hewitt (2004) expanded on these data and recommended that

the sampling transect should be at least 100 m to obtain a reliable estimate of C content at the

paddock scale. The transects need not be straight, ―Z‖ or ―W‖ patterns may be used. In

comparison to pastures, variability was less on cropping sites, and about two times more variable

under indigenous forest. If it is intended that the sites be re-sampled, then some means of

relocating the sampling point(s) is needed. A locating stake is useful but sometimes inconvenient

for the land owner. Handheld Global Positioning Units (GPS) are a useful locating tool. For long

term monitoring the device should be able to define the original sampling point to within 1 meter.

22


To obtain an estimate of the mass of C in the soil profile or sample depth, the mass of soil in that

depth must be known. This requires a separate measure of the soil bulk density (g/cm3), which

allows a conversion from the weight of soil, to the volume of soil. Then the mass of C in that

volume of soil can be calculated. Such calculations are essential for C budgeting and estimates

of C stored in the soil profile.

Most laboratories report C contents as %C, or g/kg. Some laboratories use a standard scoop so

the %C is probably as percent weight per unit volume (%w/v), with a possible correction for the

weight of soil in the standard scoop. It is important to know the method and units used. The

necessities to sample to a fixed depth and to obtain soil bulk density values do not make this

measure easy or cheap. Consequently, much of the data in the literature tends to refer to %C

contents in surface soil (0-10 cm), and very little on total C storage in the soil profile (typically 0-

50 cm). For national C accounting in NZ, the IPCC internationally agreed depths of 0-10, 10-20

and 20-30 cm have been adopted (Tate et al. 2005), and data are reported as tonnes per ha to a

specified depth.

Measuring soil C in the surface horizons can give misleading indications of C changes deeper in

the profile. For NZ pasture soils, Schipper and Sparling (in press) re-examined data collected by

Jackman (1964 a,b) for C contents in soils after conversion from scrub to pasture. They found

that when the soil from 0-10 cm depth was examined, over some 50 years there appeared to be

significant increases in total C content after conversion (expressed on a tonnes per ha basis).

However, when the same soils to 0-30 cm depth were examined, there were fewer significant

increases in the total C contents during the period after conversion. Changes in C contents of the

deeper depth masked or negated the changes in the surface soil. In the USA, Blanco-Canqui and

Lal (2008) found that for corn-soybean rotations, zero tillage methods increased C contents in the

0-10 cm depth soil compared to ploughing, but when the soil profile 0–60 cm depth was

examined, there was no significant difference in soil profile C content between the two tillage

methods. In a wider review where other soils had been sampled to 50 cm depth or deeper,

Blanco-Canqui and Lal (2008) concluded that zero tillage increased C contents of the 0-10 cm

depth soil, compared to more intensive cultivation, but there was no firm evidence to show

increased C sequestration in the whole soil profile under zero till management. We are not aware

of any similar NZ studies to this depth of soil but the conclusion challenges whether or not we

have sufficient knowledge of soil and plant management to be able to increase C sequestration in

soil and maintain it in the longer term, and again emphasis that short term (1-5 years) changes in

C contents of the surface soil may not represent changes in the C stocks in the soil profile.

2.5.2 Preferred units

Reported %C figures must be checked for expression on a weight/volume bases (w/v) or a

weight/weight basis (w/w). Most soil laboratories report soil C content as %C (g C/100 g oven-dry

23


soil) or as g C/kg oven-dry soil. The g/kg units are preferred, because these units are specified by

scientific journals and if required, can be converted readily to % C on a weight/weight basis.

For comparisons within a single soil type, or monitoring trends on a single soil, and for land use

comparisons on the same soil, the % C is usually adequate. However, if we wish to know the

physical amount of C stored in the soil profile then those data need to be converted to a weight of

C per unit weight of soil in the profile. Determining soil bulk density to the depth of interest

enables the weight of soil in an area (usually 1 ha) to be calculated. This is combined with %C to

determine the amount of C stored in that volume of soil.

2.6 Characterising soil organic matter

Various methodologies to characterise soil organic matter have been used over the centuries

(Carter and Gregorich 1996). The complex and dynamic nature of organic matter composition

presents challenges for characterisation.

Most organic matter is in the form of complex aromatic (ring structured) and aliphatic (long chains

of condensed) polymers of high molecular weight that are not easily identified (Theng et al. 1989;

Baldock and Skjemstad, 1999). There is no single structure to soil organic matter because it is

derived from a wide range of complex biological compounds, and the organic matter has often

been reprocessed many times by soil organisms. Traditional organic matter classifications rely on

chemical or physical fractionation. For example, soil organic matter has often been separated

into fulvic acid, humic acid and humin fractions, depending on its solubility in water, alkali or acid

(Table 1.3). Carbon in the form of charcoal is found in many soils, it is not soluble, and is more

abundant where there has been regular burning of plant material.

Table 1.3. Classification of soil organic matter fractions based on their solubility in alkaline

and acid extractants.

Group of substance

Solubility in:

Water Alkali Acid

Fulvic acid Soluble Soluble Soluble

Humic acid Sparingly Soluble Insoluble

Humin Insoluble Insoluble Insoluble

Adapted from Vaughan and Ord 1985.

Because strong acids or alkali may modify the organic matter extracted from soil it is questionable

how useful such chemical extracts are, and the extracted fractions still have a very

heterogeneous composition (Theng et al. 1989; Carter and Gregorich 1996). More modern

approaches use less drastic methods such as physical separation into ―light‖ and ―heavy‖

fractions (using floatation in a high density liquid). The less decomposed material such as

24


relatively fresh shoot and root material is generally ―light‖ and floats, whereas older, more highly

degraded material is comparatively ―heavy‖ often being bound to soil mineral particles (Carter and

Gregorich 1996). The ―light‖ material is generally more readily decomposed and contributes to

nutrient flows. Modern analytical methods using spectographs and techniques such as pyrolosis–

mass spectrography have revealed a huge range of compounds in addition to the long-chain

plant polymers of lignin and cellulose origin. Additional compounds include lipids (fats and

waxes), those containing nitrogen (amines and amides) and complexes of these constituents.

Baldock and Skjemstad (1999) provided a classification of the various forms of soil organic matter

and their diagram is reproduced here as Figure 1.9.

Figure 1.9 Components of soil organic matter (from Baldock and Skjemstad, 1999)

The living organic matter in soil comes from plants, microbes and animals, and this component is

temporally changeable due to the patterns of growth and land management. After ‗death and

decay‘ this C forms the component of non-living organic matter.

Some fractions of soils organic matter have been identified as being more responsive to soil

management than total organic matter. In particular the living fraction of soil organic matter has

been suggested as being responsive to changes in management and possibly being and early

indicator of changes in total soil C content (Powlson et al. 1987, Sparling 1992). The hot water

extractable fraction of soil organic matter has also been shown to be a useful indicator of overall

soil C status and closely linked to soil N, P and S status (Ghani et al. 2000).

To further complicate the identity of soil organic C, most organic matter in soil is intimately mixed

with the soil mineral components, particularly clays, iron oxides and aluminium oxides and

hydroxides. These form ―organo-mineral complexes‖ which modify the behaviour of both the clay

and the organic matter (Huang and Schnitzer 1986).

25


2.7 Climate change and soil carbon

At present, knowledge about the effect of climate change on soil organic matter is uncertain, and

may well vary in different parts of the world (Lal 2004). The key to understanding what the long-

term changes will occur as a consequence of climate change will be through knowledge of the

changes in the balances between the inputs and outputs from systems. Warmer temperatures

may increase plant growth, but they will also hasten decomposition. Higher atmospheric CO2 may

increase plant productivity. Drier conditions may inhibit both plant growth and decomposition. A

laboratory study by Conant et al. (2008) showed that changed soil temperature did not affect

organic matter uniformly. When soil was warmed, the more resistant and recalcitrant components

of soil organic matter showed a proportionally greater rate of decomposition. This may infer

proportionally greater losses from deeper parts of the soil profile if soil warming occurs.

2.8 Soil carbon and soil quality

Soil organic C is frequently advocated as a soil quality indicator (Doran et al. 1994; Gregorich and

Carter 1997). The justification is the beneficial effects soil organic matter has on modifying the

physical, chemical and biological characteristics and their potential benefits to production and to

the wider environment (see both earlier and later sections). In terms of C sequestration, if the

goal is to store greater amounts of C in soil then the quality target must be that ―more is better‖

and that the soil with greater organic C will be considered of greater quality.

However, the situation is not so clear when only production criteria are considered. While there

are many examples of greater productivity or improved crop quality on higher organic matter

soils, such benefits are by no means always obtained or even a desired target (see section on

horticultural soils). Sojka and Upchurch (1999) in reviewing the soil quality concept, point out that

on intensively managed soils the benefits of increased soil organic matter are not obvious. This is

because any benefits accruing from increased organic matter were masked by the interventions

from the land manager, in adding artificial fertilisers, irrigation, selecting tolerant plant varieties,

with timely cultivation, and use of pesticides to control weeds and pathogens. Sojka and

Upchurch (1999) point out that greater quantity of pesticide are used on higher organic matter

soils, due to the sorption of pesticide on to organic matter, reducing effectiveness.

The tenet ―More is better‖ is also not true for vineyard crops where better fruit quantity and quality

is frequently obtained by suitable management on very low organic matter soils (see discussion

later in this review). In those cases, it is generally the desire of the vineyard managers to keep the

soil organic C content lower rather than higher.

26


Even where soil organic C has been suggested as a quality indicator, it is rare that any specific

targets are specified. This is problematic because soils differ in their ability to store organic

matter, and achievable targets for each soil type need to be specified (Sparling et al. 2003a).

Further, it is not necessary for a soil to always maintain a minimum soil organic C content.

Sparling et al. (2003a) suggested that soil organic matter content could be considered

sustainable provided any period of depletion (say under cropping) was countered by a

compensating period of accumulation (say permanent pasture under a lenient grazing regime).

Sparling et al. (2003a) pointed out that the recovery period was invariably longer than the

depletion period, and suggested a 25 year cycle could be considered, the idea being that within

any 25 year period, the soil could be restored to 80% of its original content under long term

pasture. They selected 25 years because this would meet the criterion of ―intergenerational

equity‖ where one generation does not deplete a resource to the expense of the following

generation.

The target for soil organic C content for production criteria depends on the soil type and

land use, and needs to be specified for individual sites.

If it is accepted that a lower atmospheric CO2 content is a desired target for

environmental objectives, then the target for soils would be to increase C storage to the

maximum amount.

2.9 Sustaining the soils carbon

The battery analogy

When managing organic matter it may be useful to consider the organic matter in soil as a

battery. Different soils have different sized batteries. Typically volcanic ash (Allophanic) soils

have lots of organic matter and hence a large battery. Other soils have much less organic matter

(smaller batteries). However, if the low organic matter soils are managed well then the organic

matter content can be sustained. Think of the battery in a cell phone – it‘s only small compared to

that in a car. But so long as the cell phone battery is regularly recharged then it can continue to

be used and perform well. In contrast, a big battery will last a lot longer and hold heavier loads,

but once discharged it is a major exercise to get it recharged. It is the same with low organic C

soils, the organic C can generally be restored over 50-60 years (Table 1.4), but the Allophanic

soil will take 150 years (Sparling et al. 2003a).

Because C accumulation at the later stages of recovery becomes much slower (Parshotam and

Hewitt 1995; Sparling et al. 2003a), it is common to specify 80% or 90% of the maximum C

content as the target value. Even so, it will still take at least 20-30 years for soils depleted by

normal agricultural management to be restored to that lower target level (see Table 1.4).

Following topsoil loss through surface erosion, Sparling et al. (2003a) estimated that soil C

content of sites on Wairarapa mudstone where the topsoil had been lost through landslips, would

27


take at least 34 years to recover to 90% of the non-slipped sites. Schipper et al. (2001)

investigated sites of different ages looking at the re-establishment of soil organic matter in the

topsoil. Following the Tarawera volcanic eruption, they estimated it took 130 years for the soil to

reform, longer than recovery times for other ecosystems probably because of the low nutrient

conditions and more extreme climate of the mountain region. Application of fertilisers to the

Wairarapa slips increased the pasture production, but in the shorter term (0-20 years) had little

effect on soil C content (Sparling et al. 2003b).

Table 1.4 Estimates of the amounts (tonnes/ha 0-20 cm depth) of soil C in different soils and the

time (years) required to restore organic C in a depleted soil after conversion back to long term

pasture (Adapted from Sparling et al. 2003a)

Soil Order

Maximum C

content

under long

term

pasture

Depleted

soil C

content

Number of

years to

decline from

max. C to

depleted

value

Rate of

accumulation

under pasture

(tonnes/ha/year)

Time

(years) to

recover to

maximum C

content

Recent 78 55 27 0.4 58

Granular 93 54 10 0.8 49

Melanic 102 67 26 0.6 58

Allophanic 134 104 69 0.2 150

The difficulty in restoring organic matter even in temperate farmland is well illustrated by long-

term trials from Rothamsted, England (Poulton 1995). The monitoring began in 1850. Plots (under

long term wheat) received fertiliser or 35 tonnes of farmyard manure each year. In 1980 the

manured plots were still slowly accumulating C and the C% had risen from around 1.1% to 3.4%

(Fig. 1.11). The chemical fertiliser plots showed little change. After the application of 4550 tonnes

of manure over 130 years, the %C had only increased by 2.3% (absolute), or a change of only

0.0005% in soil organic matter content per tonne of manure applied. Thus of the 4550 tonnes of

manure added (wet weight, already partially decomposed), after 130 years only about 46 tonnes

(about 1% of the original), had been retained to contribute to the soil C content.

A more modest example is available for C recovery in cropped New Zealand soils after

conversion back to pasture. Shepherd et al. (2000) showed that recovery in soil C after long-term

cropping was not readily noticeable until pastures had been re-established for some 10 years

(see Figure 1.12). The message is clear: Increasing soil organic C content even of depleted soils

is a long, slow process taking many years and needing high inputs of C to the soil.

28


Figure 1.11 The recovery of soil C content (0-30 cm) under continuous wheat at

Rothamsted, England, on plots receiving chemical fertiliser or 35 tonnes of farmyard

manure each year since 1850. Redrawn from Poulton (1995).

Figure 1.12 Recovery of soil C in formerly cropped Kairanga soils (From Shepherd et al.

2000)

Year

18401860

18801900

19201940

19601980

2000

Tota

l C (

%)

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Continuous manure inputs 35 t/yr

Manure inputs stopped

No manure inputsfertilised

29


2.10 Why is it so difficult to increase the soil carbon content over the long

term?

Most organic matter inputs decompose to gas as CO2 very quickly and do not become

incorporated into soil organic C (but see Sections on charcoal and biochar). Saggar et al. (1996)

and Parshotam et al. (2000) measured the decomposition and retention of ryegrass clippings in a

variety of NZ soils and found that in all cases, more than 50% of the added ryegrass C was

decomposed to CO2 within two months. This is consistent with overseas results. Very large

amounts of organic matter need to be added to soil to increase the remaining organic C content

in the longer term. These authors calculated the mean residence times of the added ryegrass to

be around 1-2 years, with the mean controlling variables being the amount and type of clay,

particularly the surface area of the clay and environmental factors particularly temperature and

moisture at the various sites around New Zealand. Soils with higher clay content with a high

surface area retained more of the added organic matter, as did those soils on the colder drier

sites as decomposition was slowed. However, considering the contrasting soils and climates

studied, there was remarkably little difference in the patterns of decomposition.

30


3. Carbon cycling in grassland/pasture systems

Summary

Pasture and grassland soils contain one of the highest quantities of C stock on the planet. Nearly

1/3 of the terrestrial C stock is stored in pasture and grassland soil therefore, internationally; it is

one of the preferred land use change options for improving sequestration of C in soils. Native

rangeland and grasslands generally contain only small amounts of soil C (1-5%) and so tend to

respond positively in terms of C sequestration to inputs such as fertilisers, irrigation and grazing

in the short to medium term. In comparison most land under pastoral agriculture in New Zealand

is relatively higher in organic matter (3-15% C) reflecting its initial soil development under a

forestry ecosystem and the maintenance of that organic matter content as part of pasture

management through inputs such as legumes, fertiliser, grazing, drainage and irrigation.

Therefore great care must be taken in extrapolating the C sequestration potential from native

grasslands and rangelands of other countries, to the potential C sequestration rates in New

Zealand pastures. Recent study in New Zealand indicates that in intensive lowland livestock

systems soils have lost C over the last approximately 2 decades, while in hill land soil C has

increased. Both of these current observations were unexpected, and further work is required to

identify the reasons for the measured losses of soil C under intensive livestock farming, to

establish if the losses are ongoing and explore options to arrest any more decline in soil C. The

challenge on many of our intensively farmed lowland soils might be to retain existing soil C levels,

while in hill land there may be still some opportunity to increase soil C levels. There are soils in

flat and rolling landscapes (i.e. recent soils, sands, gravels, pumice and semi-arid soils under

irrigation) which may still accumulate soil C under more intensive land uses.

Suggestions for increasing soil carbon under pastoral agriculture

The key to increasing the organic matter in soil is to ensure more carbon is being added than is lost. Practices that increase organic carbon additions, or reduce organic carbon losses, enhance soil organic carbon matter levels.

On most New Zealand soil types, the challenge in pasture systems is preventing the loss of

generally high soil carbon contents rather than increasing soil carbon levels.

To increase soil C inputs: Encourage organic matter recycling

Apply organic farm wastes, such as dairy-shed effluent, herd home and feed pad organic wastes

Add biochar, compost (humus), supplementary feeds and plant residues

Increase Biomass inputs

Maximise pasture growth (e.g. ensure adequate nutrients, grazing practice, etc) to help maximise carbon capture.

31


3.1 Soil carbon in grassland and pasture soil – A global view

Grasslands store approximately 34% of the global stock of C in terrestrial ecosystems while

forests store approximately 39% and agro-ecosystems approximately 17% (IPCC 2000). Unlike

forests, where vegetation is the primary source of C storage, most of the grassland C stocks are

in the soil. Areas converted from cultivation and maintained under grassland, rangeland or

managed as pastures have shown considerable potential to remain a long-term sink of C. Most of

the long-term increase in soil C measured under pasture land use is mainly in soils that initially

had low soil C contents (Conant et al. 2001). In these situations, better management including

fertilisation, irrigation, grazing management and sowing of better cultivars and introduction of

legumes, had positive effects on C sequestration into the soils organic matter fraction (e.g. Rixon

1966; Watson 1963; Metherell 2002; Conant et al. 2001).

There is considerable interest in the influence of land management practices on soil organic C in

New Zealand under pastoral land use. A key question is whether soil C is declining on pastoral

land. If it is, what are the implications for New Zealand under our climate change commitments

reporting soil C stock as per Intergovernmental Panel on Climate Change (IPCC) article 3.4 and

Remove management practices that restrict legume growth and nitrogen fixation to help maximise pasture growth.

Avoid water deficits by irrigation to increase pasture growth and carbon inputs. Irrigation may also increase the decomposition rate of organic matter

Reduce grazing pressure (at the extreme use pasture fallow) to increase carbon capture and potential carbon inputs into the soil.

Eliminate soil physical conditions that limit root and shoot growth (e.g., livestock treading and compaction) as these will reduce pasture growth and C inputs

Adopt practices that encourage deeper root growth. This creates a larger volume in which carbon is actively stored, with the potential to increase the amount of carbon stored.

Explore the influence of pasture type and the inclusion of tree pasture systems on C inputs

To reduce soil Carbon losses:

Minimise erosion

Reduce the risk of soil erosion and direct loss of soil and organic matter from the large tracts of hill land susceptible to soil erosion. Most of the organic matter is stored in the topsoil which is easily lost during high rainfall events.

Encourage growth of permanent species, especially trees with under-storey, on land with marginal capacity to sustain a pasture sward

Minimise soil degradation

Avoid soil damage and compaction and the loss of biological activity

Minimise leaching

Manage irrigation to minimise leaching of nutrients and dissolved organic carbon. Optimum irrigation also maximises root development and increases organic matter.

Manage decomposition rates

Encourage soil organisms (worms and insects etc) to enhance the burial and incorporation of plant litter into soil aggregates. Until buried and incorporated into soil aggregates the C in plant litter is susceptible to decomposition and loss in microbial respiration.

32


what are the long-term implications to the wide range of soil services linked very closely to soil C?

Are current farming practices sustainable in maintaining soil C stocks in New Zealand?

Pasture is the dominant land use in New Zealand with grasslands accounting for over 50% of the

soil organic C stocks (Table 1.2, Tate et al., 2005). Small changes in pasture land C density

could be significant in relation to our Kyoto Protocol commitments (Baisden et al., 2001) and a

range of ecosystem services.

3.2 Carbon cycle in grazed pastures

The pastoral C cycle includes a large number of fluxes, including photosynthesis and respiration

by pasture plants, pasture senescence and root death, animal consumption, pasture conservation

and animal supplementation, respiration and methanogenesis by ruminant animals, animal

excretion, organic matter decomposition, microbial and macro-faunal consumption and

respiration, mineralisation and immobilisation of soil organic matter, and leaching of dissolved

organic matter (Figure 2.1). The level of soil organic C is determined by the overall balance of

the C inputs and outputs. Under a stable environment and management regime, C inputs and

outputs will become similar and soil organic C levels will trend to quasi-steady state levels (Cole

et al. 1993, Saggar et al. 1997).

A large number of pasture and soil management and environmental factors including moisture,

temperature, fertiliser inputs, irrigation, and cultivation and grazing intensities affect C inputs and

the turnover rates of the C cycle and stabilisation of soil organic matter. The net effect of these

factors on soil C storage and the rate of storage depend on the baseline C contents in soils.

Pasture production is the primary source of C inputs. Net primary production (NPP), which is

defined as the net flux of C from the atmosphere into green plants per unit time or the balance of

photosynthesis minus respiration, in pastures will be affected by light interception, moisture,

nutrients, temperature and plant genetics (Metherell et al. 2008). Grazing management,

irrigation, fertiliser, pasture species and weed and pest control will all affect these factors which

drive net primary production.

33


Figure 2.1 Input and output fluxes of C in grazed pasture.

In addition, these same management factors will affect the allocation of C above and below

ground. These effects are very important as root C inputs are a major contributor to soil organic

C (Stewart and Metherell 1999). The effects of moisture, fertility and grazing management on C

allocation have been demonstrated in New Zealand pasture ecosystems (Saggar et al. 1997;

Stewart and Metherell 1999; Metherell 2002). Plants have a proportionately greater allocation of

C to roots in dry and low fertility environments and with high grazing intensity and continuous

grazing. Plant components other than roots also contribute to below-ground production. In a

grazed pasture, tillers and stolons are frequently buried by treading or earthworm activity

(Matthew et al. 1989). Hay et al. (1983) found 95% of white clover stolons were buried in early

spring.

In intensive pastoral systems a greater proportion of net primary production is consumed by

animals, with more of the organic matter returning as dung. Pasture utilisation also determines

how much above ground production senesces and contributes to the plant litter pools. It will also

have an impact on root turnover (Matthew et al, 1986) and subsequent pasture production.

Moisture, nutrients, grazing management and pasture species all affect litter quality (Metherell

2002) which impacts on decomposition processes. Only a small proportion of C consumed

contributes directly to animal production. Most of the C consumed by animals is respired as CO2

34


or lost as methane. The indigestible component, primarily dependent on pasture quality, is

excreted as dung, a significant contributor to the litter pools.

Soil organic matter is a heterogeneous mixture of organic pools with different physical and

chemical properties, and turnover rates. Within soil the decomposition of litter, roots and dung,

and the mineralisation of soil organic matter are all mediated by micro-organisms and macro-

fauna with losses of C as CO2 or sometimes methane. Irrigation, fertiliser, drainage, grazing

management and cultivation all impact on the soil conditions which affect the turnover rates, C

losses and stabilisation of C in the soil. The rate of microbial activity will be greatest when the soil

moisture is maintained near to field capacity, while a high fertility status will remove nutrient

limitations to microbial activity. Grazing may result in soil compaction or pugging which changes

soil aeration and moisture relationships, which in turn affect microbial activity. It also impacts

negatively on macro- and meso-fauna, important in the removal and incorporation of plant litter

from the soil surface into the soil and with the mixing of the litter with the mineral soil. Cultivation

will reduce the physical protection of soil organic matter in soil aggregates, thereby increasing the

rate of mineralisation.

Dissolved organic matter losses are greatest under dung and urine spots and are likely to

increase with stocking rate (Ghani et al. 2007). Poor draining soils tend to lose more dissolved C

through leaching because anaerobic conditions results in the production of more soluble C which

can be lost through leaching beyond the active root zone. In lysimeter studies, 400-1000 kg

C/ha/yr of leaching losses of DOC have measured by Ghani et al. (2008a).

3.3 What are the levels of soil carbon in temperate pastures?

In temperate regions, grasslands and well managed pastures have one of the highest stocks of

soil C in the top 100-200 mm soil (Conant et al. 2001). The level of C under pasture is generally

higher than under most other land uses (Table 2.1). This is because soil C levels are generally

greatest under long term pasture, the most productive pastures are generally on volcanic ash

(Allophanic) soils that are naturally high in organic C, and pastures are only cultivated

occasionally. Note from Table 1.2 that the amount of C in pasture soils is not far different from

indigenous forest soils. This suggests that many NZ pasture soils may be near their maximum

capacity to store C.

The net accumulation rate of C in pasture soils is dependent on the net inputs minus removal of C

in products and loss of C through decomposition processes. The net accumulation rate of C in

soil depends on the base values of C in soils when land use change occurs to pastoral

agriculture. Based on a number of published results from around the world, Lal (2004) suggested

conversion of marginal land and underdeveloped rangeland in both tropical and temperate

35


regions as a means of accumulating soil C. There is a limited opportunity in NZ on soils with low

initial soil C levels (Fig. 1.3) under the right management conditions

3.4 Soil carbon changes, turnover and half-life in NZ pastures

O‘Brien and Stout (1977) estimated the average age of soil organic C in the whole profile of the

Judgeford pasture soil to be around 1500 years with an average turnover time of around 70

years. They fractionated the organic matter into ―old‖ and ―modern‖ components. The old C had

been in the soil for thousands of years, whereas the modern C was less than 100 years old. The

old C (about 16% of the total) was distributed evenly through the soil profile (to 94 cm), whereas

the modern C was concentrated in the surface layers of soil and decreased exponentially with

depth. This is the expected pattern if the modern C was derived after conversion of the old growth

forest to pasture. The modern C moved down the soil profile at some 13 cm per year, a mixture of

diffusivity of soluble components and mixing of the soil by earthworms.

Figure 2.2: Changes in soil C and N in pasture soils (0-100 cm depth) over a period of 20 to

30 years? Adapted from Schipper et al. (2007)

The effect of pasture land management on the C content of pastures was investigated by

Schipper et al. (2007). Sites that had been described some 20–30 years earlier and soils

analysed for New Zealand soil surveys were identified and relocated. The same sites were then

re-sampled to see what changes had occurred to the soil. In addition, the authors were able to

obtain original soil samples from the NZ National Soils Archive formerly maintained by DSIR and

Change in total carbon (t ha-1

yr-1

)

-10 -8 -4 -2 0 2 4

Ch

an

ge

in

to

tal n

itro

ge

n (

kg

ha

-1 y

r-1)

-400

-200

0

200

400

36


currently by Landcare Research. By re-analyzing the archive soil samples the researchers were

able to avoid any systematic differences caused by changed laboratory methods in the

intervening years. On average the soils had lost 2.1 kg-C m-2

since the initial sampling. This

translates to a linearly interpolated loss rate of 106 g-C m-2

yr-1

, or about 1 t-C ha-1

yr-1

(Figure

2.2).

Sheep/beef on hills Sheep/beef on flat land

Dairy on Allophanic Soils Dairy on non-Allophanic soils

Figure 2.3 Percent of changes in soil C on hill and flat land under sheep and beef farming

and allophanic and non-allophanic soils under dairy farming. (Adapted from Parfitt et al. 2007)

Parfitt et al. (2007) extended the sampling of Schipper et al. (2007) to 55 sites (Fig 2.3). They

found losses of soil C (0-100 cm) to be -1.5±0.4 t C ha-1

yr-1

for dairy pastures on allophanic soils,

-0.2±0.4 (not significant at P <0.005) for dairying on allophanic soils and -0.7±0.3 (not significant

at P<0.005) for dry stock on flat land. For all flat land the average loss were -0.8±0.2 t C ha-1

yr-1

,

which reconfirmed Schipper et al. (2007) earlier finding. Parfitt et al. (2007) found that in hill soils

under dry stock farming C had accumulated at a rate of 1.4 t-C ha-1

yr-1

. The reasons and

mechanisms causing these changes are not yet fully understood, nor likely future trends.

However, possible causes for these changes in soil C and N that are been currently considered

by Sustainable Land Use Research Initiatives (SLURI) team are listed below:

0

20

40

60

80

100

120

-2 0 2 4 6

Carbon Stock Change (Percent Change Per Year)

So

il D

ep

th (

cm

)

0

20

40

60

80

100

120

-2 0 2 4 6


So

il D

ep

th (

cm

)

0

20

40

60

80

100

120

-2 0 2 4 6


So

il D

ep

th (

cm

)

0

20

40

60

80

100

120

-2 0 2 4 6


So

il D

ep

th (

cm

)

37


1. Enhanced C leaching and degradation occur in concentrated zones of urine deposition.

2. Intensified N cycling increases N losses, and N is required to store C at fixed C:N ratios.

3. Below-ground allocation of C and N has decreased.

4. Leaching of dissolved organic matter is increasing.

5. Grazing harvest has increased, carrying C and/or N off site.

6. Change in plant, faunal and microbial numbers and species (diversity) and functional

groups responsible for litter removal and incorporation into the soils profile.

7. Macroporosity is being lost, affecting the C cycle.

8. Climate change has enhanced soil respiration more than C inputs.

9. Overland erosion.

10. Physical disturbance (cultivation, renovation or pugging) associated with maintaining and

increasing biomass production to support intensification of livestock systems

Four of the possible mechanisms (1, 3, 4, 6) could help explain some of the recent observed

changes in the C stock on pasture soils are being investigated by the SLURI research teams

within New Zealand.

3.5 Effects of livestock on soil carbon

There is very little published information on the impact of animal grazing on soil C stocks in

permanent pastures. In most situations, intensive grazing may lead to either a potential gain, no

change or net decline in the soil C stocks (Bruce et al. 1999).

Total C fixed through photosynthetic processes in pasture plants equates to approximately 40%

of total dry matter (DM). In a pasture with an annual yield of e.g. 10t DM ha-1

yr-1

(i.e. 10t DM

harvested as intake by animals), the amount of C harvested is therefore c. 4t C ha-1

yr-1

. The total

amount of C fixed from the atmosphere in photosynthesis is considerable, and has been

measured e.g. as c. 16t C ha-1

yr-1

, of which some 40-50% (6.4-8t C ha-1

yr-1

) is returned to the

atmosphere in plant respiration. This includes the respiration involved in the synthesis of shoot

and the maintenance of shoot tissues. This figure also includes respiration from the synthesis and

maintenance of roots (some of which is expended via the shoot). Of the remaining c.8t C ha-1

yr-1

(‗NPP‘ or ‗gross tissue production‘) in new shoot and root tissues, only about 50% is typically

harvested (hence the 4tC ha-1

yr-1

harvested), and the remainder of the plant tissues turnover and

senesce to form shoot and root litter. Shoot and root litter contribute C ultimately to either

respiration from the soil (and soil surface), from the microbes that consume the litter, or contribute

to a potential increase in C sequestered in the soil. Management (both fertiliser inputs and/or

changes in grazing intensity) alter all of these fluxes. In general, increasing the intensity of

utilisation (e.g. increasing stocking rate per se) will reduce all the fluxes, simply because it

reduces vegetation cover (leaf area and so photosynthesis) although there is an optimum grazing

38


intensity at which the amount harvested is maximised. Even at this optimum only about 50% of

the gross production of DM is harvested because removing a greater proportion would adversely

reduce light interception and photosynthesis. These principles are described in detail in Parsons

and Chapman 2000 (and see references to original papers therein), and a detailed review of the

understanding of the dynamics of C flows in grazed pastures, see Parsons (1994). Based on

theses flows of C in the grazed pastures Clark et al (2001) summarised C flows in New Zealand

pastures (http://www.maf.govt.nz/mafnet/rural-nz/sustainable-resource-use/climate/green-house-

gas-migration/ghg-mitigation-06.htm).

Contrary to common perception, increases in pasture and farm productivity do not necessarily

results in increased soil C. Long-term studies on in hill country and one in high country in New

Zealand (Ballantrae, Tara Hills, respectively) have shown a negative impact of increased stocking

rates on soil C stocks (Lambert et al. 2000; Metherell 2002). Main reasons for such observations

are increased stock carrying capacity over time, with an increased proportion of net primary

production being consumed by the animal and lost to the atmosphere through respiration and

methanogenesis (Clark et al. 2001). Another possible explanation could be less root biomass

inputs in the rhizosphere when soils are well fertilised to enhance dry matter production. This

generally promotes greater transfer of the fixed C to shoot and very little to roots, which is also an

important pathway for C sequestration in soils (Saggar et al. 1997).

Table 2.1 The effect of sheep stocking rate at Tara Hills on soil C (1997), pasture

production (1981-1984) and root measurements made under tussock and inter-tussock

vegetation.

Adapted from Metherell 2002

3.6 Effects of irrigation

Irrigation in poorly managed pastures can result in a greater dry matter production which in turn

may increase C in soils (Rixon 1966; Murata et al. 1995). In well established pastures, gains in

soil C from irrigation are less convincing. One of the long-term irrigation trials under pasture land

use (Winchmore irrigation Research Station in mid-Canterbury, New Zealand) showed that in the

first 15-20 years irrigation had a positive effect on soil C, irrespective of P or S fertilizations

(Nguyen & Goh 1990). In the longer-term, the irrigated plots have shown less C sequestration

http://www.maf.govt.nz/mafnet/rural-nz/sustainable-resource-use/climate/green-house-gas-migration/ghg-mitigation-06.htm

http://www.maf.govt.nz/mafnet/rural-nz/sustainable-resource-use/climate/green-house-gas-migration/ghg-mitigation-06.htm

39


compared with non-irrigated plots (Methrell 2002; Srinivasan & McDowell, 2009). Table 2.2 shows

that plants under moisture and nutrient stress have larger root systems and turnover rates of their

organic matter is slower than in the irrigated plots (Metherell 2002).

3.7 Effects of effluent application

The effects of long-term application of dairy factory effluent to pastures on Horotiu and Te Kowhai

soils were reported by Sparling et al. (2001). After 22 years of effluent application every 2 weeks,

there was no effect on C content of the Te Kowhai soil, but an apparent decline in C content of

the Horotiu soil. Further investigations by Degens et al. (2000a) showed that the decline occurred

only in the surface soil and that lower in the profile there had been a compensating accumulation

of C. Long-term effluent application seemed to have speeded the movement of C down the

profile. Over the soil profiles to 50 cm depth effluent application had not resulted in any change in

soil C content. This again emphasizes the importance of considering the changes over the whole

soil and not just the surface layer.

Table 2.2 The effects of irrigation frequency and fertiliser rates at Winchmore on soil C

(1997), pasture production (1952-2002), C allocation to roots, root production, mass and

turnover, and shoot and root N and lignin contents.

Adapted from Metherell (2002)

Application of other effluents such as farm dairy effluent and meat processing effluent on pastoral

soils has shown either small or no sequestration of C in soils (Russell 1986; Speir et al. 1987;

Yates 1976). Most of the studies monitored soil C at 0-75 or 0-150 mm depths. One of the main

reasons for poor sequestration of C is that most of the C in effluents is present in an easily

metabolisable form. This is respired quickly by microorganisms and therefore only a small fraction

of applied C is likely to be stored in soils. Excessive application of effluents on soils may also

40


result in leaching losses of dissolved nutrients and dissolved organic C. Most of the effluent

studies were relatively short-term and were designed to capture water and nutrients from

effluents and not specifically designed to monitor C sequestration in soils. In most cases

observations were made over 2-5 years which is a relatively short-term observation period in the

context of observing significant differences in C levels in soils that have large background levels

of native C.

3.8 Effects of fertilisers

There are a number of studies predominantly under rangeland and poorly managed grassland

land use that have shown increases in soil C in the early stages of development with increased

fertiliser inputs, resulting from increased productivity (Conant et al. 2001). However, in well

managed long-term temperate New Zealand pastures, opportunities for increasing C

sequestrations through fertiliser inputs and increased productivity are limited.

Supplementary data on fertiliser input history from 14 of the lowland dairy pastures in the

Schipper et al. (2007) study (Fig 2.4) showed no significant relationship between the amount of N

or P input and the observed changes in soil C and N, suggesting that fertiliser input was not a

factor driving the observed decadal changes. Greater fertiliser inputs result in less translocation of

photosynthetic C to roots (Stewart and Metherell, 1999; Saggar et al. 1997) which is important

given that root biomass and root exudates are important inputs of C to soils that result in

sequestration of C. Greater fertiliser inputs are generally also associated with increased stocking

rates, having the effect of mitigating greater net primary production as shown in the previous

section.

41


Fig. 2.4 Relationship between N and P fertiliser inputs and decadal soil C and N changes in

dairy pastures (provided by M. Dodd 2009 – personal communication)

3.9 Effects of drainage of wetlands

Of the 5 million ha with slopes <12° in New Zealand, 2 million ha are poorly (water at or near the

surface for extended periods) or imperfectly (soils remain wet for extended periods) drained. The

purpose of drainage is to assist the soil remove water found in large pores (>30 µm) quickly after

rainfall. There are remarkably few data on the long-term effects of drainage on soil C levels.

There is evidence from overseas to show that drainage of wetlands results in a decline in soil C

levels (Sigua et al. 2004). Draining wetlands could lead to significant losses of C (400-1800 kg

C/ha) in dissolved organic form in the drained water (Ghani et al. 2008a).

42


Some of the early experiences in this country with the drainage of peats and before the

introduction of water table control found that the loss of large amounts of organic matter following

the drainage of peat soils was not uncommon. The impact of drainage on soil C depends on the

initial soil C content. In some soils drainage, by improving aeration and biological activity could

cause a decline in soil C, while in others soils drainage, by improving plant production and rooting

depth may result in greater C sequestration.

3.10 Effects of farm systems (organic, biodynamic, conventional)

Organic agriculture is advanced as a viable and sustainable option that reduces the

environmental impacts of agricultural practices, and provides ―clean, green‖ produce to the

population. Despite the continued interest in the potential opportunities from increased certified

organic production from all primary sectors, with the current demand for organic milk and lamb

beyond the current organic supply base, only a very small percentage (<1%) of New Zealand‘s

pastoral agricultural land is under organic management.

The majority of published literature on the performance of organic livestock systems is drawn

from comparisons of commercial farming operations (e.g. Kristensen and Kristensen, 1998; Ogini

et al. 1999, Richardson and Richardson 2006, ARGOS), or from units established on research

facilities that are compared with the industry standard (MacNaeidhe and Fingleton, 1997). Few

robust livestock-systems studies, comparing organic systems under controlled experimental

conditions, have been undertaken (Kristensen and Kristensen, 1998, Mackay et al. 2006, Kelly et

al. 2008).

Characteristics of an organic pastoral system include an emphasis on a legume-based pasture

forage supply base, enhanced soil biological activity, crop rotations, mixed livestock, nutrient

inputs limited to the use of natural unprocessed products (i.e. reactive phosphate rocks), strict

limits on imported feeds from beyond the farm gate, and a probation on the routine use of

drenches, vaccines, antibiotics, dips and other chemical remedies unless an individual animal

suffers or shows signs of ill thrift. Under organic production specifications soil health is predicted

to be improved, reflected in greater amounts of SOM, biological activity, and greater resilience to

pests and extremes of climate. Increases in SOM and enhanced activity of soil invertebrates

including earthworms, mesofauna and nematodes, result in benefits to soil services such as

nutrient cycling (Cole et al. 2004; Fonte et al. 2007). A number of studies have found SOM

contents and soil fauna positively influenced by organic management in permanent pasture

(Reganold et al. 1993; Yeates et al. 1997; Mulder et al. 2003). Reganold et al. (1993) comparing

a range of biodynamic and conventional farm systems, including livestock, found more SOM,

thicker top soils and more biological activity under biodynamic management. Yeates et al. (1997)

43


found a consistent increase across three soil textures in the abundance of fungal-feeding

nematodes with organic management, which is likely to reflect changes in nutrient cycling.

In studies by Mulder et al. (2003) and Yeates et al. (1997) the organic systems had lower

ruminant stocking rates than conventional systems, in addition to the absence of chemical

fertilisers. Parfitt et al. (2005) reported no measurable benefits of organic management on either

SOM or soil fauna when they compared organic system with conventional systems at the same

stocking rate and under the same nutrient management.

One of the challenges when comparing organic and conventional pastoral systems is that

associated with the shift to organic production which involves multiple changes in management

practices, all of which influence the quantities of C entering the SOM and soil fauna.

Consequently one must be very careful in any prediction of the potential changes in soil

invertebrates associated with the shift from conventional to organic practices. In the only

replicated experimental farm systems comparisons of organic and conventional sheep and beef

production in New Zealand (Mackay et al. 2006), a legume-based pasture grazed in situ by

animals year round was used in both systems.

Figure 2.5 Soil C content (%), averaged over 4 fixed transects in each of two organic and

conventional replicates. One of the organic farmlets was registered with BIO-GRO in 1988

and the other in 1997 (Mackay A.D unpublished).

Livestock type and stocking rates were also the same, as was grazing management. Both the

conventional and organic systems received the same type and rate of reactive phosphate rock/

Elemental sulfur each year, and their pasture productions were also very similar. In that study no

difference in topsoil C (0-75mm) between conventional and organic systems was found (Fig. 2.5)

in the 6 years measurements made. The comparison started in 1997. The absence of any

measurable difference in topsoil C content, was not surprising given that animal stock type and

stocking rate, nutrient and grazing management and nutrient inputs, were very similar between

0

1

2

3

4

5

6

7

2000 2001 2002 2004 2005 2006

Soil

C c

on

ten

t (%

)

YEAR

Conventional

Organic

44


the two systems. Some differences in soil fauna were found in a survey of the soils in the two

systems at the end of the study in 2007 (Schon, N. pers. comm.), possibly reflecting the impact of

the differences in agro-chemical use and the slightly higher pasture residuals on the organic

farmlets.

3.11 Total Vs. labile carbon as indicator of early changes in soil carbon

Pasture management practices may lead to changes in SOM contents. However, these changes

often occur gradually. Against the larger background of total C already present in pasture soils,

subtle changes in total C pool are difficult to detect in the short or medium-term (Ghani et al.,

1996). From the environmental performance-monitoring viewpoint, short-term sensitivity of a

measurement is desirable for its use as an indicator. There are a number of labile fractions of

SOM that have shown to be sensitive to short to medium-term changes (1-5 years) in soil

management e.g. soil microbial biomass-C, light fractions, low density organic matter, hot-water

extractable C. Ghani et al. (2000) compared some of these labile fractions and concluded that

hot-water extractable C (HWC) was good measure in showing difference between a range of

treatments under pasture agriculture in New Zealand (Fig 3.6).

Figure 3.12 (A) Effect of grazing intensities on HWC in soils. Error bars are standard error of difference of means, (B) Effects of land use on HWC in soils, (C and D) Effects of N or P applications on HWC. Error bars are standard errors from the mean values. Adapted from Ghani et al. (2000)

1000

1500

2000

2500

3000

3500

0 200 400

HW

C (

µg

C/g

so

il)

Rates of N (kg N/ha/yr)

C

1000

2000

3000

4000

5000

0 30 50 100

Rates of P (kg P/ha/yr)

D

0

1000

2000

3000

4000

Market Garden Cropping Dairy Pasture Native

B

1000

1500

2000

2500

3000

3500

4000

Sheep/Beef Dairy

HW

C (µ

g/k

g s

oil)

*A

45


4. Carbon cycling in horticultural systems

Summary

Horticultural production systems are managed to enhance the production of the fruiting part of the

plant, rather than the vegetative component. Thus free-draining soils that typically have limited

fertility, therefore those that are low in soil C generally are often used for horticulture, especially

viticulture, where management practices can then be used to ensure quality production of fruit.

Horticulture is typically concentrated on recent coarse-textured alluvial soils such as in Hawke‘s

Bay, central Hawke‘s Bay, Nelson, and Canterbury, as well as the arid soils of central Otago.

Because of naturally low vegetative vigour, pruning practices, and high harvest index,

horticultural systems return low amounts of C to the soil. As a result, horticultural soils have been

observed to be losing soil C. Because soil C is critical for soil functioning and health, especially

water-holding capacity, aeration and nitrogen mineralization, it is imperative that C levels be

maintained, even if it is at levels lower than pastoral soils. In orchards and vineyards this can be

achieved through mulches and composts, and in the future there is the possibility that biochar

could be used.

Suggestion for improving C in soils under horticulture land use

Horticultural systems generally have a high harvest index, meaning much of the captured carbon (up to 70-80%) is exported in the product. In fruit systems

To increase soil carbon:

Encourage organic matter recycling:

Maximise the incorporation of the prunings, residues and re-importation of processing wastes such as pomace and marc each year.

Compost prunings and wastes, and use as surface mulches to build soil carbon, without increasing vegetative vigour excessively. Composting of residues and mulching is more effective at raising soil carbon, than litter decomposition in situ. Because of the high harvest index, there is insufficient material to cover the entire orchard in mulch, thus outsourced clean composts can be used to achieve coverage. Organic systems already use this approach.

Soil carbon is buffered, and changes can take a long time (> 10 years), but mulching can return soil health benefits much quicker.

Orchard redevelopment for planting of new cultivars means the standing biomass C in the existing tree is removed, this carbon can be used as a feedstock for composting or biochar to be incorporated into the soil during redevelopment. This might even help, we speculate, to reduce specific apple replant disorder (SARD). This dual value from biochar would be worth exploring.

Grafting onto the existing rootstock, if possible, would keep the sub-soil root carbon store by and large intact. Likewise, the prunings from living shelter can be recycled as mulch, or compost.

To increase biomass inputs:

Capture more carbon by increasing biomass production in components of the orchard system other than the tree canopy. Excessive vegetative vigour is generally avoided in fruit production.

Use irrigation to encourage more root growth, especially by using regulated deficit techniques, and in particular partial rootzone drying which seeks to cycle root growth between alternately wetted and dry-down zones.

46


4.1 Soil carbon in horticultural soils – A world view

The literature on the levels and function of C in horticultural soils is small in contrast with that on

soil C in agricultural soils. Using Elsevier‘s literature search engine SCOPUS

(http://www.scopus.com/home.url ) with the search criteria of ‗carbon‘ and ‗agriculture‘ returns

4,437 hits to scientific papers. Replacing the ‗agriculture‘ with ‗horticulture‘ yields only 111

articles in scientific journals. Soil C in horticultural systems has received less attention probably

because of the ‗inverse‘ relationship between soil fertility and horticultural production. Whereas

for agriculture ‗more is better‘, horticulturalists often to grow their crops on free-draining soils

which typically have lower levels of C.

Because the product exported from the orchard is the floral part of the plant, horticultural

enterprises tend not to select soils with high levels of fertility as this would only favour the growth

of the vegetative parts of the plant. The origins of many of our horticultural crops are in the

Middle East, the Mediterranean, Asia, or Central America. Apples have their origin in the

Caucasus, avocadoes in the Mexican highlands, olives in the Mediterranean, and kiwifruit and

citrus in China. These areas typically have soils that are generally lower in soil C (Jones et al.

2005).

4.2 Soil carbon in horticultural soils – A New Zealand view

In 2008, horticultural exports were $2.95 b, being 7% of New Zealand‘s export revenues. Kiwifruit

($870 m), wine ($794 m) and apples ($345 m) are the big three, making up nearly 70% of our

Minimise the size of the bare-soil strip to maximise biomass as far as is practical. Minimise use of herbicides and encourage incorporation into the soil of carbon from residues and mulches.

Use deep-rooted multi-species vegetation as the understorey to encourage deeper storage of carbon in the soil of the inter-row.

Utilise living shelter to encourage net biomass growth in the orchard. This can also provide the opportunity for additional prunings and mulching materials.

Increase soil microbiological health in general by avoiding soil compaction from excess traffic and to enhance incorporation of biomass carbon into the more resistant carbon pools.

To decrease Carbon Losses (In vegetable production):

Manage erosion & tillage

Adopt multiple-crop bed techniques which reduce tillage frequency. In vegetable growing, tillage is frequent, encouraging both erosion and respiration of soil carbon.

Include frequent periods of pastures growing, if possible, to help to restore soil carbon levels, and improve soil structure and health.

Minimise leaching

Manage irrigation to apply only the minimum amount of water required. This will limit leaching and loss of dissolved organic C from the root-zone.

Managing decomposition rates

Encouraging soil health and microbial activity will enhance the incorporation of litter residues into soil C pools. This can be achieved through good irrigation management, maximising the area in understorey cover crops, maintaining soil structure, limiting compaction, and using the minimum number of sprays.

Living shelter with deep roots will capture and sequester carbon at depth.

http://www.scopus.com/home.url

47


horticultural exports. Winegrapes cover 30,000 ha, kiwifruit some 13,250 ha, and apples are

grown on 9,250 ha, making up 75% of New Zealand‘s horticultural plantings of 68,297. The

prime horticultural regions are the Marlborough (17,600 ha), Bay of Plenty (13,000 ha), and

Hawke‘s Bay (11,885). Other regions having significant horticultural plantings are Canterbury,

Central Otago, and Tasman-Nelson.

New Zealand‘s horticultural enterprises are located on free-draining soils: the volcanic loams

(yellow-brown loams) and pumice (yellow-brown pumice) soils in the Bay of Plenty, on recent

alluvial soils (recent soils from alluvium) in Hawke‘s Bay, Marlborough and Canterbury, plus semi-

arid soils (brown-grey earths) in Central Otago. The terms in brackets are the common names in

the New Zealand Soil Classification. The total C contents of New Zealand‘s soils are shown in

Figure 1.2, which was taken from Landcare Research‘s Fundamental Soil Layers.

Indeed, with the recently rapid expansion of viticulture, there has been a focus on soils with low C

contents. So valued are these soils they have been given terroir appellations such as the

Gimblett Gravels in Hawke‘s Bay, the Terraces near Martinborough, and Gibbston Valley in

Central Otago. The growth of viticulture in Marlborough has been focussed on the stony recent

alluvial soils of the Wairau and Awatere valleys. The fine-earth fraction of the Gimblett Gravels

has a C content of 1.5-2%. However in the top 100-150 mm the Gravels are about 30% stones,

so the effective C content there is at maximum 1.4%. Below 150 mm, the stone fraction is at

70%, thus the volumetric C content is just 0.6%. This lack of C confers little fertility or water-

holding capacity to the soil, which enables the winegrape growers to use cultural practices

through managed irrigation and fertiliser use to maximise the quality of the grape berries.

Scientists and growers are developing orchard and vineyard practices that are targeted to

improve the quality of the fruit products. This could be, say, through the tactical application of

surface mulches of composted marc in an attempt to raise N mineralisation levels to achieve

appropriate levels of yeast available N (YAN) in the berries to avoid stuck ferments.

Orchard practices might actually reduce levels of soil C because there are generally low rates of

C capture by the plants, and high rates of C export in the fruit.

4.3 Land use and land-use change and impacts on soil carbon

Land use, and land-use change can have impacts on the levels of C in the soil through alterations

in the balance of the soil-plant C cycle (Lal, 2009). Plant and Food Research have developed a

mechanistic modelling scheme of C flows in the soil-plant-atmosphere system. The model is

called SPASMO (Soil Plant Atmosphere System Model). This model has a plant growth sub-

model that captures C through photosynthesis and then allocates it to plant parts, some of which

end up as little through deciduous leaf fall, or by pruning. The structure of the model is shown in

Figure 4.3, where the creation of new carbon Cnew by the plant depends on solar radiation (R),

soil nutrients (N), water (W) and temperature (T). Later, in Section 4.5 we will use SPASMO to

48


explore the different patterns of C cycling for different horticultural crops, as well as to examine

the impact of understorey management of soil C storage in Section 4.6. Here we simply use it as

a graphic (Figure 4.3) to highlight how C is captured by horticultural plants, allocated to the plant

parts of roots (Croot), leaves (Cleaf), shoots (Cshoot) and fruit (Cfruit) , some of which are exported off-

orchard, and some of parts of which are deposited in the soil, or onto the soil as leaf fall, thinned

fruit, or prunings.

Land-use change from one form of farming to another, or to changed practices within a farming

system will also change the levels of soil C by changing flows in the C cycle. Tate et al. (2005)

used a generalised linear model to determine the land-use effects (LUE) on soil C of various land

uses relative to that of grazing land.

Table 4.1 Impact of land use on soil C stocks relative to grazing land. From Tate et al.

(2005)

In Table 4.1 is shown the impact of LUE on soil organic C stocks in the top 30 cm of soil.

Horticulture is predicted to result in a loss of about 9 Mg-C ha-1

if an orchard were to be

established from a pastoral farm. Tate et al. (2005) established that between 1990 and 2000,

there was a decrease in grazing land due to the growth in exotic forestry and cropland. The 3000

ha yr-1

growth in cropland would result in a change in New Zealand‘s stock of soil C of -0.03±0.02

Tg-C yr-1

, which is small relative to the 43,000 ha yr-1

growth in exotic forestry which would result

in a -0.6±0.2 Tg-C yr-1

loss in New Zealand‘s soil C stocks. A teragram (Tg) is 1 gram with 12

zeros after it, and is equivalent to one million tonnes.

49


Horticultural systems are diverse and the amount biomass growth and root penetration varies

considerably between types of orchards. Kerckhoffs and Reid (2007) determined the standing

biomass of various horticultural crops: grapes were about 5-10 t-DM (dry matter) ha-1

, kiwifruit,

avocadoes and apples 20-35 t-DM ha-1

, with almonds and walnuts 75-100 t-DM ha-1

. Dry matter

is about 47% C.

Figure 4.1 A typical avocado orchard in Northland showing the grassed inter-row. The

trees can reach heights of 10-15 m.

To highlight the difference between the C capture between horticultural systems, we show in

Figure 4.1 a typical avocado orchard, where the standing biomass will be well over 5 times that of

grapes (cf. Figure 4.7). The ability of these two different systems to capture C, and return it to the

soil by root decay, prunings and leaf fall will be very different.

Not only are horticultural systems different in the make-up of their canopies, they are also quite

different underground in relation to their root systems. The export of C into the soil from root

systems is critical in determining the level of C in the soil. Jones et al. (2005) discussed C flows

in the rhizosphere from the perspective of C-trading at the soil-root interface, and they note that C

and N flows in the rhizosphere are extremely complex, being highly plant and environmentally

dependent. With perennial horticultural crops the system is even more complicated for there is a

steadily growing woody network of roots, from which fine roots grow and then die in a

complicated pattern, both spatially and temporally.

The rooting strategies and pattern dynamics of rooting vary greatly between horticultural crops.

Hughes et al. (1995) examined the pattern of rooting in apple, kiwifruit, peach, Asian pear and

grapes. Their key results in terms of root length density are presented in Table 4.2.

50


Table 4.2 The characteristic patterns of root length density between five horticultural

crops (from Hughes et al. 1995).

Species Number of sites Mean root length density

(104 m m

-3) and SE

Kiwifruit 18 0.93 ± 0.04

Peach 3 0.93 ± 0.15

Apple 5 0.15 ± 0.04

Grape 2 0.09 ± 0.02

Asian pear 2 0.09 ± 0.01

Thus the spatial density of plant-based C in the soil is highest in kiwifruit and peaches, and lowest

in grapes and Asian pear (Table 4.2 and Fig. 4.2). What will matter for C flows in the root zone,

will be the different exploration and exploitation strategies used by the rooting systems of these

crops. Hughes et al. (1995) developed indices of exploitation and exploration.

Figure 4.2 The pattern of root exploration and exploitation by a young kiwifruit vine. The

vine is on the left, and the exploration front of white roots can be seen in the middle of the

photo, behind which is the zone which is exploited.

The exploration-exploitation rooting strategy of plants will control C flows from the plant to the

soil. For peaches they found an exploration index of almost 1. So that means that 100% of the

volume of soil that was explored by roots, contains some roots. Other species had an exploration

index of about 0.8. Thus within the extent of explored rooted volume, there was about 20% of the

space that contained no roots. So, one would imagine that C flows into the rooted volume of

peaches would be at a higher spatial intensity than the other species. The exploitation indices for

peaches and kiwifruit were 0.45. In other words, nearly half of the explored volume had rooting

densities that were quite high. For apples, pears and grapes, these were only about 0.05. So

51


their rooting pattern only had pockets, about 5% of the explored volume, where roots were at a

high density. Thus different rooting strategies result in different exploration and exploitation

patterns, and so the delivery of C to the soil roots will be quite different between species. Little

however is known about how these strategies relate to the flows and stocks of C in the root zone

of horticultural crops.

There is a general lack of data on the C stocks and flows in diverse horticultural systems, and

here we report on results which have been obtained in the three major horticultural crops:

kiwifruit, wine grapes and apples.

4.4 Carbon stocks and flows: The Big Three - Kiwifruit, Grapes, and Apples

The soil‘s C content reflects the balance between C inputs and C losses (Figure 4.3). Therefore

a loss of soil C can be counteracted by continuous C inputs from the addition of organic matter

residues such as prunings, leaf-fall, compost or decaying root biomass. The addition of organic

matter residues via prunings, leaf-fall, compost in organic systems, or decaying root biomass will

vary greatly between different horticultural systems, driven in large part by primary C capture by

the tree.

4.4.1 Kiwifruit

The annual net primary production of leaf, shoot and fruit totalled about 15 t ha-1

. The fraction of

C in woody tissue is approximately 47%, so the vine‘s canopy has captured about 7 t-C ha-1

yr-1

.

We have not considered annual growth in the root system rather we have assumed it to be in

equilibrium, with root growth balancing root death. Linda Boyd (pers. comm.) has carried out vine

excavations and for ZESPRI GREEN kiwifruit; she found that of the 29.5 kg-DM per vine, some

14.6 kg-DM was in the structural and fibrous roots. This fraction of about a half was similar for

the more vigorous ZESPRI GOLD, namely 21.4 kg-DM of roots relative to the 48.2 kg-DM for the

total vine. The roots of kiwifruit can penetrate to great depths, especially in the deep volcanic

soils of the Bay of Plenty, although the root length density is greatest near the surface. The

pattern of C in the soil reflects this, as Dr Sivakumaran (pers. comm.) has found under kiwifruit

near Te Puke. Elevated levels of C can be found down to nearly a metre (Figure 4.4). To

understand changes of soil C is such deep-rooted plant systems will require some considerable

sampling effort in the field.

52


Figure 4.3 The net dry matter production of kiwifruit growth allocated between fruit, shoots

and leaves. The lines are predictions by the SPASMO model run using 30 years of weather

data (1972-2003): DMFT – dry matter foliage at time t, DMST – dry matter shoot at time t,

DMCT – dry matter crop at time t. The points are measurements made during 2004-05

(Green et al. 2007).

In Figure 4.5 it can be seen that of the 7 t-C ha-1

yr-1

captured by the vine, nearly 70% leaves the

orchard in the form of the fruit. Thus a large fraction of the net primary production is exported

from the orchard. This high harvest index means that there is only a small proportion of captured

C returned to the soil through prunings and leaf fall. Furthermore, trunk girdling is being used to

limit the transfer of C down to the roots, and enhance the harvest index.

The two major kiwifruit varieties are GREEN and GOLD, and they can be grown under integrated

management or organic. Dr Tessa Mills (pers. comm.) has measured the total C content of the

top 20 cm of soil in 5 integrated orchards and 5 organic organic orchards, with there being a mix

of GREEN and GOLD in both systems. Under organic practices, applications of chicken manure,

chicken compost and vermicomposts are used, typically at a rate in excess of 10 t ha-1

. Dr Mills

found the total C in the integrated orchard soils to be 4.5 ± 0.3%, and 6.0 ± (0.3)% in the organic,

reflecting the increased input and soil storage of C under organic practices.

Cnew=f(R,T,W,N)

Croot

Cleaf

Cshoot

aR

aL

aS

Cfruit

aC

YR

YS

YL

YC

Clitter

Cnew=f(R,T,W,N)

Croot

Cleaf

Cshoot

aR

aL

aS

aR

aL

aS

Cfruit

aCaC

YR

YS

YL

YC

Clitter

YR

YS

YL

YC

YR

YS

YL

YC

Clitter

0

2000

4000

6000

8000

10000

12000

1-Jan 2-Mar 2-May 2-Jul 1-Sep 1-Nov 1-Jan

Dry

matt

er

[kg

/ha]

DMFT Leaf DMST Shoot DMCT Fruit

53


Figure 4.4 The measured profile in total C % under kiwifruit in an orchard on a deep

volcanic loam near Te Puke.

Carey et al. (2009) carried out a comparison of soil quality and nutrient budgets between organic

and integrated kiwifruit orchards. They found that the soil (0-15cm) under organic GREEN

kiwifruit had a significantly higher microbial-C biomass (427 mg-C kg-1

) than that under integrated

GREEN at kiwifruit (356 mg-C kg-1

). As above, this probably reflects the use of composts under

organic practices. Also they found that worm numbers were greater under the organic systems

than either of the two integrated systems. Parfitt et al. (2007) found a positive relationship

between worm number and changes in soil C. The microbial C biomass under integrated GOLD

kiwifruit at 384 mg-C kg-1

was significantly higher than that under GREEN, but not different from

the organic GREEN. That the GOLD and organic were the same could reflect the higher net

primary productivity of the GOLD kiwifruit.

Kiwifruit growers are using a range of management techniques to enhance the harvest index of

the vine, and these practices in consequence limit the return of plant-captured C to the soil. We

have setup sites which we will be able to revisit in the future to determine whether or not levels of

C in the soils are changing.

4.4.2 Grapes

Similarly we have no time records of the changes of soil C in vineyards. As noted above,

viticulturalists have sought out free-draining soils of low C status so that they can manipulate vine

growth to favour the growth of quality berries. Green et al. (2007) used SPASMO to predict C

capture and allocation by grapes growing on the Gimblett Gravels. The net primary production of

grapes on the Gimblett Gravels was found to only 6.75 t ha-1

, of which half, about 3.5 t ha-1

, is

harvested in the fruit. There is only a return of about 3.25 t-DM ha-1

to the soil via pruned shoots

and leaves, or just 1.65 t-C ha-1

, if as is usual the alleys are essentially bare.

0

20

40

60

80

100

120

140

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

% Carbon

Dep

th [

cm

]

54


Although we do not have any time records of changes in soil C under grape growing, Greven et

al. (2007) examined soil C in different-aged vineyards to determine the impact of viticulture on C

stocks. They used the C stock in the top 15 cm of the headlands of vineyards of different ages as

a reference. They compared this with the levels of soil C in the top 15 cm of the row and inter-

row of the vineyards, and then averaged these on the basis of the fractional areas of the rows

and inner-rows. They compared vineyards that had just been established with those that were 2

and 15 years‘ old, and their results are presented here as Figure 4.5. Not surprisingly with such a

small amount of C return, compounded by low worm numbers naturally, and worsened through

the use of pesticides, soil C levels have declined under viticulture.

Figure 4.5 The measured decline in soil C (0-15cm) with vineyard age (Greven et al. 2007).

Viticulturalists, especially those growing grapes on very stony soils are concerned about low

levels of yeast available nitrogen in the berries which can cause stuck ferments. To overcome

this, and improve general soil health, mulches from composted pruning‘s and marc (crushed

grape skins) are sometimes add to the soil of the rows to increase soil fertility slightly, without

encouraging vegetative vigour. Greven et al. (2007) found that levels of soil C under mulches

nearly twice that for the bare soils

Whereas grape growers might not be too interested in increasing levels of soil organic matter for

fear of encouraging vegetative vigour, there might well be the possibility of sequestering C in

these soils in the form of biochar, or tactically using surface mulches to improve soil functioning

and soil health in the surface zone of the soil without an overall increase in vigour.

4.4.3 Apples

Yet again, we have no long-term records of changes in soil C in apple orchards. However, from

our paired study of 12-year old integrated and organic orchards in the Hawke‘s Bay we can use a

comparison to infer changes in soil C due to apple growing.

We have estimated the change in soil C stocks in these two comparable apple-orchards in

Hawke‘s Bay under integrated, or organic, management on an alluvial silt loam soil with respect

to a permanent-pasture reference taken as being the alley (Figure 4.6). So we can estimate how

0

2

4

6

8

10

0 2 15

years under vineyard

Carb

on

[kg

/m2]

55


much the soil C stocks would change if the land-use changed from a permanent pasture to an

apple orchard.

We used the soil under the permanent grass in the alley of each orchard as the reference. We

considered the soil C stocks in 0-0.3 m depth. More details on the soils, and orchard

management are given elsewhere (Deurer et al. 2008b).

Figure 4.6 Two neighbouring apple orchards near Havelock North. Left: The integrated

orchard. Note that the strip under the apple trees is free of vegetation. Right: The organic

orchard. Note that the strip under the apple trees is grassed.

Over 12 years (1994-2006), the row in the organic apple orchard lost about 1.7 ± 3.1 kg-C/m2

(Table 4.3), in relation to the permanent-pasture reference in the alley. Simply by assuming that

half the total area of the orchard is managed as a row, and the rest is permanent pasture, this

equals a net decline in the C stocks by 8.5 ± 15 t-C/ha. This value is close to the estimated loss

of 9 ± 7 t-C/ha of Tate et al. (2005). We note also that because of the high standard-deviation of

the measurements, the change in soil C could actually be either zero, or positive. Greater

definition is obviously needed.

Over the same time, the row in the integrated apple orchard lost about 2.2 ± 1.3 kg-C/m2 (Table

4.3) relative to the permanent-pasture reference. Assuming again that only half of the total area

of the orchard is managed as a row, and that the rest remains as permanent pasture, this equals

a decline in the C stocks by 11 ± 7 t-C/ha. This value is somewhat higher than the estimated loss

of 9 ± 7 t-C/ha of Tate et al. (2005). It equals the estimated loss of 11 ± 8 t-C/ha that Tate et al.

(2005) estimated for the conversion of permanent pasture to cropland.

56


Table 4.3 Average C stocks to 0.3 m depth and the calculated change in two apple-orchard soils (organic, integrated) in Hawke’s Bay. The alley in both systems is permanently covered with grass, and served as the reference.

Depth

[m]

Organic – row

[kg-C/m2]

Organic – alley

[kg-C/m2]

Integrated – row

[kg-C/m2]

Integrated – alley

[kg-C/m2]

0-0.3 7.8±1.9 9.5±1.2 5.9±0.6 8.1±0.7

Estimated change in soil organic stocks over 12 years [kg-C/m2 year]

-0.14±0.3 0 -0.18±0.1 0

Where did the (net) loss of C go? John Palmer of Plant & Food Research has recently published

a paper in Acta Horticulturae on ―Changing concepts of efficiency in orchard systems‖, and he

noted that the need for traditional ―… whole plant physiology has never been more important than

today as we seek to understand C flows within the orchard.‖ Palmer (2009) commented that the

fruit tree grower has the advantages of a perennial tree in undisturbed soil.

Trees capture C from the atmosphere, and some of this ends up as C in the tree‘s standing

structure, leaves and apples. The standing woody tissue represents a considerable quantity of

stored C. At the end of eight years, the mean standing dry weight of ‗Braeburn‘, ‗Royal Gala‘ and

‗Fuji‘ apple trees on M.9 rootstock at 3.5 x 1.3m, grown as slender spindles, was about 17 t (dry

matter DM) ha-1

, with an annual increment of 2.2 t-DM ha-1

(Palmer et al. 2002). The size of the

annual increment comes from net diameter growth in roots, trunk and permanent branches. This

growth is not immediately obvious for the mass of a trunk grows at the square of the radius, and

so a small diametric growth comprises a significant increment in biomass

Taking the data from the trees on M.9 and extrapolating out to 12 years would give a standing dry

weight of 26 t-DM ha-1

. Walton et al. (1999) determined the fraction of C in the woody tissue of

an apple tree as 47%, which would make the standing tree material be 12 t C ha-1

, which is not

too dissimilar from the 12-year loss of soil C in the herbicide strips of 11 t C ha-1

we found above.

Further, circumstantial evidence hinting at this counter-balancing C accumulation in the standing

biomass would be that organic trees tend to be less vigorous and have less standing biomass.

So the lower standing biomass in the organic orchard is coincident with the lower decline of soil C

(8.5 t-C ha-1

) there, whereas for the more vigorous integrated orchard, with presumably its higher

standing biomass we estimate the soil losses to be 11 t-C ha-1

.

Thus it could be that the C decline we have observed in the soil has as the net result of a

complex of dynamic and interacting processes been counterbalanced by growth in the standing

biomass of the tree (Palmer, 1988). If this were so, then in a net sense, the changes in soil

emissions and gain in biomass would cancel each other out at an orchard level, as long as the C

stored in the tree remained captured.

57


4.5 Understorey management

Typically for integrated orchards in New Zealand, the tree-row is sprayed to create bare soil (see

Figure 4.8), or the trees are evergreen and create sufficient shade so that there is no understorey

in the tree row (Figure 4.2). The inter-row, or alley, is generally grassed. This grassed inter-row

acts to protect the soil from compaction during trafficking operations, and it provides some

measure of frost avoidance in relation to bare soil. The alley is not generally irrigated, and so as

summer progresses drought will reduce the growth of the grass and limit C capture in the inter-

row. As typically horticultural soils are light with low water holding capacity, this can mean that

there is no growth in grass in the inter-row during a substantial part of the year (Figure 4.10).

We have modelled the orchard-wide change in soil C as a function of inter-row cover using our

SPASMO model for a kiwifruit orchard in the Te Puke region. Not surprisingly given the high

harvest index of kiwifruit, in the absence of a cover crop, soil C stocks are predicted to decline by

about 20 t-C ha-1

over 20 years. If there were a cover crop with 50% ground cover, then soil C

stocks are predicted to lose only about 8 t-C ha-1

over 20 years. If there were greater cover, then

we predict a rise in soil C levels.

Soil C levels in integrated orchards can be affected by understorey management. However,

horticulturalists are presently focussed on managing the inter-row for trafficking, frost avoidance

and aesthetics, rather than C stocks.

4.6 Irrigation

Because horticultural crops are preferentially grown on light, free-draining soils, there is

widespread use of irrigation (Figure 4.7)

Figure 4.7 The typical pattern of drip irrigation in a vineyard.

58


Irrigation is however used very tactically and water is sparingly applied to the bare soil of the row

to ensure good connection with the tree, or vine, and to limit wastage on the vegetation of the

inter-row and avoid need for additional mowing of the grass strip. Overall this pattern of irrigation

limits excessive vegetative growth, and hence there is a moderate increase in C capture by the

plants.

Increasingly deficit irrigation techniques are being used to enhance the quality of the fruit and

ensure good sunlight penetration to the fruit by limiting vegetative vigour. For example in

viticulture, the vine‘s needs for water are fully met up until flowering to ensure successful

pollination. After that, through until veraison (grape softening), increasing plant water deficits are

established, and deficit irrigation used from veraison to harvest to ensure better sugar allocation

to the berries. By limiting vegetative vigour, the grower needs to carry out less leaf plucking and

hedging, which is a cost saving. So leaf growth, and hence C capture, is not greatly affected by

irrigation is many horticultural systems.

4.7 Cultivation

With the rapid development of new cultivars, the life of the orchard trees and vines might indeed

only be 15 years, or so. In the apple industry there is a reasonably rapid flow of new cultivars. In

the kiwifruit industry there has been a move to GOLD, and there is the prospect of a new red

kiwifruit in the future. It is understood that 5% of orchards are, at any time, undergoing such

redevelopment. Redevelopment, as well as new developments, often involves tree removal, plus

tillage and cultivation.

Ussiri and Lal (2009) have shown how tillage, and the type of tillage, can affect losses of C from

the soil. The long-term impact, over 43 years, of these two different forms of cultivation on total C

stocks was found to be dramatic as the soil organic C stock to 30 cm under no-till was 80 Mg-C

ha-1

, yet just 45 under mouldboard plough tillage. Tillage, even just once every 15 years or so,

can result in a large one-off loss of soil C.

So if prior to replanting the new cultivar, the soil were then tilled and say fumigated to prevent

specific apple replant disorder (SARD), soil C would be respired by this tillage. Furthermore if,

during orchard redevelopment, the C captured and stored in the trees‘ standing biomass were

then burnt that would negate the gains of the 15 years of capture.

There are options for conserving soil C during orchard redevelopment. If there were no need to

change the rootstock, then grafting onto the existing rootstock would be advantageous from a C

stocks viewpoint. However, in both the cases of new planting and even grafting, some part of the

standing biomass of the old trees would need to be removed. This wood, could possibly be used

59


to substitute for primary material in another process, or the wood could be biocharred. The

biochar could then be incorporated back into the soil during redevelopment. It would even be

worthwhile assessing whether biochar has any value in preventing SARD.

4.8 Production systems: Integrated and Organic

The only completed inter-comparisons of an integrated orchard with an organic one are those

published by Cary et al. (2009) for kiwifruit, and Deurer et al. (2008b) for apples. We have

discussed the key results from Carey et al. (2009) above, and here we provide more detail about

the results from Deurer et al. (2008b).

Figure 4.6 show photographs of the integrated and organic apples orchards studied by Deurer et

al. (2008b). This case study involved an organic and a neighbouring integrated apple production

system in the Hawke‘s Bay. The neighbouring orchards have the same general soil

characteristics. The soils are Fluvisols and have a silt-loam texture. The organic orchard system

had been under organic management (BioGro) since 1997. The apple trees in the orchard were

13 years old. The apple variety was ‗Braeburn‘, and the rootstock variety was ‗MM.106‘. Green-

waste compost was applied to the topsoil of the tree rows in the organic orchard once a year at a

rate of 5 to 10 t/ha, and lime was added at a rate of 300 kg/ha every 4 years. Lime-sulfur and

copper were used as fungicides if needed.

The apple trees in the adjacent integrated orchard system were 12 years old. The apple variety

was Pacific Rose™, and the rootstock variety was MM.106. A 0.5-m wide strip under the trees

was kept bare by regular herbicide applications. The apple trees were drip-irrigated during the

vegetative period. The irrigation, nutrient, and pest management followed the guidelines of

integrated fruit production.

Organic Integrated

Figure 4.8 The total soil C depth wise in the tree-row and alley of an organic apple orchard

(left) and an integrated orchard (right). From Deurer et al. (2008b).

60


The key results from Deurer et al. (2008b) are shown in Figure 4.8 or total soil C. The amount of

C under the tree row was significantly lower than under the alley of the integrated orchard. The

tree row without pasture received little input of root-biomass C and no input of C via compost.

Additionally, the drip irrigation in the tree row led to continuously favourable moisture conditions

for C mineralization and might promote the leaching of dissolved organic C. By contrast, the use

of pasture as understorey vegetation for C conservation is avoided as it competes with the crops

for water and nutrients. The pasture and regular compost applications in the tree row of the

organic system conserved C. Averaged, the C stock in the integrated orchard is 2.6 kg-C m-2

,

whereas it is 3.8 kg-C m-2

in the organic orchard.

Deurer et al. (2009) have examined the changing pattern of the difference in anecic worm

populations between the integrated and organic apple orchards. Their findings are presented in

Figure 4.9.

Figure 4.9 The pattern throughout the year of the density of anecic worm numbers

between the integrated and organic orchards (from Deurer et al. 2009).

On average the number of worms in the organic orchard was significantly higher, 154 ± 47 g m-2

,

in the organic orchard than in the integrated orchard, 85±53 g m-2

. This further corroborates the

findings of Parfitt et al. (2007) in relation to the positive link between worm numbers and C stocks.

4.9 Soil carbon and its impact on soil functioning and soil quality

Soil C is critical for the biophysical functioning of soil, and it is a measure of soil quality. Deurer

et al. (2008b) have developed a framework that can be used to quantify the impact of soil C

management on the biophysical properties of soil, and they showed how soil C controls soil

macroporosity and aeration, nitrogen mineralisation, and water holding capacity.

Deurer et al. (2008b) used this framework to compare and contrast the different practices of C

management in integrated and organic orchards to determine the impact on microbial biomass,

0

100

200

300

400

January April July October

Ea

rth

wo

rm F

W [

g/m

2]

Organic

Integrated

61


basal respiration, dehydrogenase activity, the respiratory quotient, aggregate stability and the

pore diameter near saturation. They concluded that:

the impact of C management extended further down the soil profile in the integrated, than

in the organic orchard.

the lower soil C status in the tree row of the integrated orchard resulted in a decrease in

microbial activities.

the lower soil C status in the tree row of the integrated orchard led to a decrease in

aggregate stability. The soil C conservation in the organic orchard improved the

aggregate stability.

for soil functions that are sensitive either to a decrease in microbial activity or aggregate

stability the soil management was sustainable in the organic system but not in the

integrated system.

Deurer et al. (2009) used 3-D X-ray computed tomography to examine the impact of soil C

management on the porous structure of the soil from the integrated and organic orchards.

Images of the two soils are presented here in Figure 4.10 and there it is possible to see the

differences in the sizes and connectedness of the pores in the two different orchard systems.

From these images it is possible to extract pore-size characteristics, and the spatial distribution in

these characteristics. Macropores are defined here as being pores with a radius greater than 0.3

mm. For the integrated system the macropores occupied 2.9% of the soil‘s volume, and had a

mean radius of 0.38 mm. The soil of the organic orchard had 32% more soil C, and here the

macropores occupied 8.3% of the soil‘s volume, and the mean macropore diameter was 0.41

mm.

Figure 4.10 The macropore networks as ‘seen’ by 3-D x-ray tomography for the integrated

apple orchard (left) and the organic orchard (right). The soil on the left has a microbial C

biomass of 73 g-C m-2

and that on the right has 143 g-C m-2

.

62


The impact of this enhanced macroporosity on soil aeration can be determined by using these

pore size characteristics in a physical model of diffusion in porous media. Deurer et al. (2009)

found that the relative diffusion coefficient of air in the soil (relative to free-air) was over fourfold

higher in the organic soil than in the integrated orchard. This macroporosity is sustained in part by

the higher numbers of worms (Figure 4.9), and also through the higher microbial biomass-C

(Figure 4.11).

Figure 4.11 The seasonal pattern in a microbial biomass-C in the organic orchard and the

integrated orchard.

The biophysical attributes of the soil are enhanced in the soil with a higher level of soil C. Kim et

al. (2009) examined these two soils to determine their biochemical functioning, in particular in

relation to nutrient provision through nitrogen mineralisation. Their data are presented in Figure

4.12 and show the clear relationship in these orchard soils between the labile C fraction of the

hot-water extractable C (HWC).

0

50

100

150

200

250

January April July October

Mic

rob

ial

bio

ma

ss C

[g

/m2]

Organic

Integrated

63


Figure 4.12 The relationship between labile soil C (HWC) and the net rate of N

mineralisation in the soils of the integrated and organic orchards (from Kim et al. 2009).

Figure 4.11 demonstrates how soil C, and in particular the labile fraction of the soil‘s C can affect

the ability to supply nutrients to plants. This relationship is vitally important for organic systems,

for their use of imported C in composts is primarily to meet the nutrient needs of the plant.

As well, soil C through its influence on soil structure can influence the water holding

characteristics of the soil. There are no definitive data on this from New Zealand soils, so we

refer the analysis carried out by Rawls et al. (2003) using the US National Soil Characterisation

database. Their analysis revealed the increase in soil water retention per 1% change in soil C for

soil at the pressure potential of -33kPa. This increase was given as a function of clay and sand

content, for three initial levels of soil C. At low organic C contents the sensitivity of the water

retention to changes in organic matter content was highest in sandy soils. An increase in organic

matter content led to an increase of water retention in sandy soils, and a decrease in fine-

textured soils. At high organic matter contents, all soils showed an increase in water retention,

with the largest increase being in sandy and silty soils.

Given that most horticultural soils are sandy, and generally low in soil organic matter, any

increase in soil C is likely to mean a greater water holding capacity to the soil. This might in turn

result in unwanted vegetative vigour for horticultural crops, and reduce the ability of the grower to

manipulate plant and fruit growth through the use of tactical irrigation practices.

64


5. Arable Soils

Summary

Levels of C in arable and, especially, vegetable-producing soils can be much lower than in

pastoral soils. To raise soil C levels, it is necessary to either increase C inputs from crop residues

or decrease the rate of decomposition. Managing crops to maximize yields (e.g., by providing

adequate nutrition for the crop) should also maximize C returns in post-harvest residues. Crop

type has a strong influence on C returns in plant residues, with perennial grasses (pasture)

returning largest amounts, vegetable crops the least, and small-grain cereals being intermediate.

In the long-term, burning of crop residues can cause depletion of soil C. Switching from intensive

cultivation to low-disturbance tillage may be beneficial in increasing C, though further work is

needed to quantify the C sequestration potential of no tillage under New Zealand conditions.

Although a reduction in tillage intensity can sometimes increase C inputs (by improving yield), the

effect of tillage on C soil is mainly related to its influence on the rate of decomposition.

Suggestions for increasing soil Carbon under cropping

Cropping systems have a high harvest index, meaning much of the captured carbon (up to 50%) is exported in the grain. Tillage practices expose and promote mineralisation of soil organic matter. In combination, these are constant drains on soil carbon stocks.

To increase soil carbon:

Encourage organic matter recycling:

Adopt residue management practices that maximise carbon returns to the soil. About half of the above-ground crop C (~ 2-5 t C/ha, depending on crop type and yield) will usually be present in post-harvest residues.

Avoid burning straw where possible (an important residue management practice for cereals, especially in Canterbury) but note that field experiments have been inconsistent regarding the benefits of straw retention for soil carbon.

To be detectable against background variability, increases in soil C may need to be greater than 4 or 5 t/ha. As the annual gain in C due to residue retention may only be a few hundred kilograms her hectare, it may be 10 or more years before an increase in soil can be measured.

To increase biomass inputs:

Eliminate factors that restrict crop growth (e.g., inadequate fertilisation) to help maximize carbon inputs in crop residues. Although avoiding water deficits by irrigation may increase carbon inputs, irrigation may also increase the decomposition rate of organic matter and the net effect on soil carbon is difficult to predict

Remedy soil physical conditions that limit root growth (e.g., soil compaction) as they reduce crop carbon inputs.

In many annual crops, roots make up a small fraction (~10%) of total plant biomass. Selective breeding for cereal crops with larger root biomass may in the long term provide arable farmers with soil carbon enhancing cultivars.

Grow cover crops rather than leave land fallow over winter to add carbon to the soil. If grazed in situ, a significant proportion of crop carbon will be returned to the soil in dung.

65


5.1 Carbon cycling in arable soils

Carbon levels in soils are a function of inputs from plant residues (above-ground residues, roots,

and root exudates) and outputs of CO2-C derived from the microbial decomposition of organic

matter (including plant residues). The C content of the soil will change if the balance between C

inputs and C outputs is altered.

Inputs of C depend on crop yield, the harvest index (the proportion of above-ground biomass that

is harvested and removed), and the mass of roots that the crop produces. Yield is, of course,

strongly influenced by water and nutrient availability and good management of both water and

fertiliser is essential to maximize C inputs to the soil.

The harvest index of small grain cereals is generally about 0.5 (Table 5.1), so that a crop yielding

10 t/ha of grain would produce a similar amount of above-ground residues (straw). C usually

makes up 40-45% of residue dry matter. Therefore, a 10 t/ha cereal grain crop would return 4 to

4.5 t/ha of C to the soil in straw. If the entire crop is harvested (e.g, for silage) essentially all of the

above-ground biomass is removed and the C input will be small. Similarly, above-ground C inputs

may be small when straw is burned to facilitate the establishment of the next crop. There is good

evidence that the harvest index of some arable crops has increased during the past century as a

result of plant selection and breeding (Evans, 1998). Although the proportion of

photosynthetically-fixed C that is returned to the soil may have diminished over time, the actual

Including a pasture phase in arable rotations can increase soil carbon. Perennial grasses return large quantities of carbon to the soil because (1) they have a large root mass and (2) most of the above-ground dry matter consumed by grazing animals is returned to the soil in dung.

To decrease carbon losses

Manage tillage practices

Adoption of no-tillage is often advocated as a way of increasing soil C. While no-till

soils often have higher C levels near the soil surface (top 7.5 or 10 cm) than

ploughed soils, the reverse may be the case deeper in the profile. Lal (2009)

recently suggested that the seemingly higher C concentrations under no-till could

be due to shallow depth of sampling.

Avoid soil erosion:

Implement appropriate soil conservation practices to avoid erosion of carbon-rich surface soil where there is a risk of water or wind erosion.

Minimise leaching

Avoid excessive irrigation to limit leaching of dissolved organic carbon. But note, leaching of dissolved organic carbon is likely to be relatively small in arable cropping systems, particularly in drier areas such as Canterbury.

Manage decomposition rate

The decomposition rate of organic matter depends to a large extent on temperature and soil moisture, but it can also be affected by the type of cultivation.

66


quantity of C added to the soil on above-ground residues is likely to have increased because of

the major yield improvements that have been achieved.

Whereas we have good data for above-ground C returns, information on root-C inputs is sparse

because of the difficulty of quantitatively extracting roots from soil. Information on the root to

shoot ratios is lacking for most arable crops grown in New Zealand (note: shoot = above-ground

biomass). It is well recognized that the proportion of plant C allocated to roots can vary

depending on the conditions in which the crop is grown, with crops in water-limited environments

having a relatively large root mass (Campbell and de Jong, 2001). Calculations using

unpublished data (G.S. Francis, personal communication) for barley and wheat grown in

Canterbury (Lincoln) suggest that the average root-to-shoot ratio is ~0.08 for barley and ~0.12 for

wheat. From a compilation of studies in Europe and North America, Williams (2006) showed that

the root:shoot ratio in small grains tends to decline as shoot biomass increases. Root mass (y)

was linearly related to shoot dry matter (x) by the equation:

y = 0.064 x + 0.34 (R2 = 0.84)

where root and shoot mass are in t/ha of dry matter. For crops producing 10 and 20 t/ha of

above-ground biomass, the root mass is estimated from this equation to be 1.0 and 1.6 t/ha,

respectively (i.e., C inputs of ~0.4 and 0.6 t/ha). Root mass in cereals often peaks at about

anthesis and then decreases as roots die off and decompose; root:shoot ratios reported above

are based on root mass measurements at anthesis and shoot measurements at, or near,

maturity. As they grow, plants release a variety of exudates into the soil. The quantity of C added

to the soil in root exudates is not known with any degree of certainty. It is thought that exudates

are decomposed very rapidly and contribute little to long-term soil C storage

C is released from the soil (as CO2) by the action of the microbial biomass in decomposing

indigenous soil organic matter and freshly added crop residues. The decomposition process is

strongly influenced by environmental conditions that affect microbial activity (temperature,

moisture, aeration), i.e., decomposition is most rapid under warm, moist conditions. Post-harvest

management can also have a strong influence on the rate of straw decomposition. Straw

remaining on the soil surface (no-till situation) will decompose at a much slower rate than straw

that is incorporated into the soil.

5.2 Soil carbon in arable soils – world view

Under arable cropping in NZ, C commonly makes up 2-3% of topsoil (0-15 cm) mass. The total

amount of C in the top 30 cm of arable land has been estimated for several regions in New

Zealand (Fig. 4.1). Regional median values range from about 80 to 100 t/ha, with considerable

67


variability within each region due to factors such as management history, soil type (clay soils tend

to be higher in C than sandy soil), and climatic differences.

C levels in NZ arable soils appear generally similar to those found in temperate regions

elsewhere. The average C content of arable soils in England (5.2 million ha) is about 2.5% in the

top 15 cm (King et al., 2005). Much of Canada‘s wheat is produced on land that has about 2-3%

C in the topsoil.

5.3 Soil carbon in NZ arable soils

Soil C levels change when the input of C into the soil differs from the amount of CO2-C released

when organic matter (including fresh plant residues) decomposes. On an annual basis, changes

in soil C are usually small (often much less than 1 t/ha under arable cropping) because inputs

and outputs approximately match. Although high-yielding crops may return over 5 t/ha of C to the

soil, most of this C is in compounds (cellulose, hemicelluse) that are easily decomposed. It is

often observed that about three-quarters of cereal straw decomposes in the year following

incorporation. The decomposition process is strongly temperature dependent (Douglas and

Rickman, 1992) and, under NZ‘s generally warm conditions (soil temperature usually > 5oC),

decomposition can usually proceed throughout the entire year. Studies in Canterbury (Lincoln)

have shown that only 21-39% of wheat straw remained in the soil 10 months after incorporation

(Curtin and Fraser, 2003). Assuming that 70-80% of straw decomposes within a year, less than 1

t/ha of residue-C would remain in the soil one year after incorporation of straw from a high-

yielding crop (4-5 t/ha of C added in straw). Decomposition of indigenous soil organic matter will

also release C, though its rate of decomposition is much lower than that of fresh plant residues.

As annual changes in soil C are small compared to the total quantity of C in arable soils (often 80-

100 t/ha to 30 cm), several years are usually needed to identify a trend in total soil C. Even within

a uniformly managed paddock there can be substantial spatial variability in soil C and changes

may need to exceed 3-4 t/ha to be detectable against background variability.

Straw management (removal vs. retention) is potentially an important factor affecting C levels in

arable soil. Burning continues to be a predominant residue management practice, particularly in

Canterbury. Currently, about 60,000 ha of crop residues are burned annually (53,000 ha in

Canterbury). The effect of straw management on C content of NZ arable soils is not known with

certainty. In a six-year trial in Canterbury, there was no significant difference in C between plots

where all of the straw was returned and plots where it was burned or removed (Curtin and Fraser,

2003). Under cooler conditions of northern Europe, increases in soil C due to straw retention

have been observed in long-term trials (Smith et al., 1997). The annual gain in soil C due to straw

retention has been estimated at 0.7 t/ha in Europe (Smith et al., 2005).

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5.4 Effect of cultivation practices

Cultivation mixes plant litter into the soil and improves the contact between residues and soil,

which facilitates microbial colonization. Due partly to more favourable moisture conditions for

microbial activity, soil-incorporated residues decompose at a faster rate than residues on the soil

surface. Cultivation can also disrupt soil aggregates, exposing organic matter that was previously

protected in the interior of aggregates to microbial decomposition. Elimination of tillage (adoption

of no-till) can alter the distribution of C in the soil profile. Under no-till, C tends to accumulate

close to the soil surface and is moved downwards only slowly by processes such as earthworm

activity. In intensively cultivated soil, C is uniformly distributed through the plough layer (top 20

cm). Because decomposition may occur more slowly under no tillage, there can be an increase in

soil C; however, the amount and rate of change following adoption of no-tillage can vary

depending on climatic conditions, soil type and the starting soil C level.

The effect of no-tillage on soil C has been studied extensively overseas. From a global database

of 67 long-term experiments, West and Post (2002) estimated that a change from conventional to

no-tillage may increase soil C by an average of 570 kg/ha per year. Increases in soil C following

conversion to no-tillage was estimated to peak in 5-10 years, with soil C reaching a new

equilibrium in 15-20 years. There is evidence that the soil C gain under no-tillage can differ

regionally, depending on climate, crops grown, residue quality, and possibly other factors

including earthworm activity (Gregorich et al., 2005). For example, cropping soils in semiarid

western Canada often show increases in C after switching to no-tillage (Campbell et al., 1996)

whereas in humid eastern Canada no-tillage does not appear to have a beneficial effect on soil C

(Gregorich et al., 2005). Recently, Blanco-Canqui and Lal (2008) have questioned the view that

no-tillage can increase soil C. They suggested that many studies reporting differences between

conventional and no-tillage based on shallow sampling need to be re-evaluated. In studies in

Ohio, Kentucky and Pennsylvania, soil C (0-10 cm depth) was higher under no-tillage than under

conventional tillage in five of eleven comparisons, but total C to 60 cm was not significantly

affected by tillage intensity (Blanco-Canqui and Lal, 2008).

It is estimated that about 20% of all seeding in New Zealand is currently done by no-tillage

compared with <4% in 1990 (C. Ross, Landcare Research, pers. comm.). The impact of the

expansion of no-tillage on soil C is still open to debate. Several studies have confirmed that no-till

soils can have higher concentrations of C near the soil surface (0-10 cm; Aslam et al., 1999,

2000) but published assessments that included the full depth of cultivation (preferably

measurements to 30 cm) are not available.

69


5.5 Effects of irrigation

Irrigated crops can produce substantially higher yields of both grain and crop residues than

dryland crops. However, it is not known if irrigation alters C levels in arable soils (long-term

experimental data are lacking). Because straw is likely to be burned, particularly in Canterbury

where most arable cropping occurs, C inputs may not be greatly different under irrigated versus

dryland cropping. Also, irrigation can increase decomposition and C turnover. The net effect may

be little change in soil C. Under sheep-grazed pasture, border dyke irrigation has been shown to

decrease soil C at Winchmore even though dry matter production and, by extension, C returns to

the soil, were substantially greater than under dryland pasture (Condron et al., 2006).

5.6 Effects of fertiliser

Using fertiliser to maximize crop yields should also maximize C inputs to the soil in post-harvest

residues. Overseas data confirm that, over the long-term, inadequate fertilisation does lead to

depletion of soil C under arable cropping (Campbell and Zentner, 1993). Effects of fertiliser

management on C levels in NZ arable soils have not been determined experimentally.

5.7 Effect of plant type and crop rotation

Crop type has a strong influence on amounts of C returned in plant residues, with perennial

grasses (pasture) returning the largest amounts, vegetable crops the least, and cereals being

intermediate. In a mixed cropping system where arable crops are rotated with short-term (3-5 yrs)

pasture, soil C may increase during the pasture phase and decline under the arable phase

(Francis et al., 1999). A comparison of soil C (0-15 cm depth) between long-term pasture and

long-term arable soils showed that arable soils were lower in C by between 5 and 14 t/ha,

depending on soil type (Figure 4.2). Because maize produces large quantities of residues, it can

maintain higher soil C levels than other arable crops. Data from the central North Island showed

that C inputs were similar under maize and ryegrass pasture (~9 t/ha per annum) when most of

the crop residue from maize crop was incorporated into soils (Parfitt et al., 2002). Soil C (0-60

cm) did not differ significantly between maize and pasture paddocks.

5.8 Effects of farm systems (organic and conventional) on soil carbon

Formal trials comparing soil C under organic and conventional arable cropping systems have not

been conducted in New Zealand. A number of studies are available in which paired comparisons

were made between commercial organic farms and nearby conventional farms (Reganold et al.,

1993: Nguyen et al., 1995; Murata and Goh, 1997). A problem that is common to these studies is

that only the top 10 or 15 cm of soil was sampled, making it impossible to make a full assessment

of the effect of the alternative system. Although some studies suggest that C levels may be higher

70


in organic than in conventional system, there does not appear to be a consistent difference.

Yields are generally substantially lower under organic production and C inputs in crop residues

are also likely to be reduced when a switch is made to an organic system. Further, organic

growers are more reliant on soil cultivation to control weeds and frequent tillage may promote

organic matter decomposition. Basic principles would suggest that soil C should decrease, rather

than increase, following a move to organic production. However, organic growers are more likely

to include grass-clover pasture in the rotation (partly to provide an input of fixed N) and frequent

inclusion of pasture may help maintain soil C levels.

Table 5.1 Yield range, harvest index, and estimated C inputs to soil from major arable

crops.

Crop Yield (t/ha Harvest Index Estimated C input from above-

ground residues (t/ha)

Wheat 7-12 0.50 2.9-5.0

Barley 7-11 0.53 2.8-4.3

Oats 5-7 0.43-0.47 2.3-3.2

Maize (grain) 8-14 0.5 3.4-5.9

Maize (silage) 16-25 0.48-0.52 (but 0.95 is

harvested)

0.7-1.1

Peas 6-7.5 0.49-0.63 (avg 0.5) 2.5-3.2

5.9 Vegetable production and soil carbon

Whereas horticultural production has sought out soils of low C, vegetable production in New

Zealand has been based on soils that were initially high in organic matter. These areas include

Pukekohe in South Auckland, Opiki in the Manawatu, and Marshlands near Christchurch.

Vegetable production often involves multiple crops per year, and this requires multiple cultivation

events per crop. Not surprisingly, given the high harvest index of vegetable crops, levels of soil

organic matter are declining in such regions.

Haynes and Tregurtha (1999) examined the soils of the Patamohoe clay loam near Pukekohe

that had been cropped for different periods of time and they were able to determine the impact of

the length of cropping on soil on soil C stocks (Figure 5.1).

71


Figure 5.1 The impact of the length of intensive vegetable production on soil C stocks 0-

15 cm in a Patamohoe clay loam near Pukekohe (Haynes and Tregurtha, 1999).

The drop off in soil C soon after the commencement of intensive vegetable production is

dramatic, and then it appears to slow down and stabilise at low values after 40 or so years.

Because of the loss of soil function at low values of soil C, vegetable growers are adopting less

intensive cultivation practices and building multiple-use beds. Also, they are importing composts

to raise levels of soil C.

Koerber et al. (2009) have determined geographical variation in the C balance of soils growing

vegetables. For a range of vegetable crops across a range of European and African regions they

calculated the net ecosystem production (NEP) and net biome production (NBP). NEP is net

primary production, namely aboveground C allocation plus belowground C allocation, less C

emitted by decomposition in the soil. NBP is NEP less the C removed at harvest. The NEP and

NBP were mainly negative (Figure 5.2 ), hence results like Figure 5.2 are to be expected. The

study stressed the importance of soil losses of C in the life cycle of vegetables.

To minimise the soil C loss from crop production, and also in the long term to improve soil

functioning, Koerber et al. (2009) recommended that there be greater returns of organic matter to

the soil.

72


Figure 5.2 The net ecosystem production (NEP) of C (t-C ha-1

yr-1

) during vegetable

production for various crops in various regions (top), and the net biome production

(NBP=NEP less C removed at harvest) (bottom) (Koerber et al. 2009).

73


C Stocks in Cropping Soils

RegionAuck/W

aikato

Hawke's Bay

Canterbury

Southland

Manawatu

Gisborne

To

tal C

(tC

/ha

, 0

-30cm

)

0

50

100

150

200

250

300

Figure 5.3 Box and whisker plots showing soil C stocks by region. Each box represents the middle 50% of the values measured for each region, the line across each box is the median value and the values plotted outside the boxes are the upper and lower quartiles of values (Unpublished data of M. Beare and E. Lawrence).

Land use

LT Pasture STP/STA LT Arable

Soil

Org

anic

carb

on (

t ha

-1)

40

45

50

55

60

65

Brown (n = 67)

Gley (n = 23)

Pallic (n = 114)

- 5.3 t C/ha

- 13.6 t C/ha

- 10.5 t C/ha

Figure 5.4 Average soil C stocks (t C ha

-1) in the top 15 cm of Brown, Gley and Pallic soils

under long-term sheep pasture (LT Pasture), short-term pasture of arable cropping

(STP/STA) and long-term continuous arable cropping (LT Arable). The difference in C

stocks between LT Pasture and LT Arable is also shown as the average C loss under

continuous cropping, n = the number of paddocks representing each soil order in the data

set. (Unpublished data of M. Beare and E. Lawrence)

74


5.10 Predicting effects of management practices on soil carbon

A computer-based tool called the Land Management Index (LMI) has been developed by

Plant and Food Research that can be used by farmers to predict how specific

management practices will affect soil C levels (Figure 4.5). The LMI is based on scientific

measurement of crop and tillage factors and soil quality indicators on 750 paddocks, covering a

wide range of soil types, land uses and management practices across New Zealand. The LMI

decision support tool is designed for use on arable, vegetable (process and fresh sectors) and

extensive sheep/beef pastoral farms on flat and rolling lands of New Zealand.

The four LMI Soil Quality Indicators are: - Soil structural stability - Soil structural condition - Soil organic matter (total C) - Biologically active C.

The LMI allows farmers to: 1. predict changes in soil quality (including C) and the likely effects of this on crop

performance (losses or gains) based on soil and crop management information,

2. explore the effects of applying different management practices (e.g. tillage type, crop

rotation) on soil quality.

3. identify best management practices and options to mitigate any adverse effects of land

use change and intensification on soil quality.

The LMI decision support tool is available on CD (free-of charge) through Plant and Food

Research, contact Erin Lawrence ([email protected]). It can also be obtained through

Regional Councils and the Foundation for Arable Research. The system is easy to run, and has a

help guide and e-mail support.

mailto:[email protected]

75


Figure 4.5 Land Management Index (LMI) decision support system (opening screen)

76


6. Opportunities for increasing carbon sequestration

Summary For most pastoral soils there are few production gains from lifting soil C levels beyond the target

range (see Section 6), which for most soils covers a wide range of C contents (3.5-9.0%). There

would be an interest in arresting any long-term decline, if the soil C content was below 3-4%.

Similarly, as mentioned frequently above, there is little interest, over and above maintaining

essential soil health, in increasing the fertility of horticultural soils. However, arable soils would

benefit considerably by enhancing soil C stocks in the plough zones both from production and

environmental viewpoints. Realising this, arable land that is rested under pasture rotation is

sought after by vegetable growers who are prepared to pay higher premium for leasing such land

from owners. Conservation of soil C is already covered in the arable section of this review. This

section will deal with other options that can aid in building soil C stocks such as the addition of

organic wastes (e.g. bio-solids, pulp and paper waste, green waste and manures inclusion of tree

options in pasture systems as well as agrichar and biochar in agricultural soils.

6.1 Bio-solids, green wastes, manures

Organic waste materials are good source of C. Application of these materials on agricultural soils,

particularly to soils under arable agriculture will benefit not only to improve soil C but will also help

in improving porosity, water retention supplementing nutrients for plant growth etc. Given the

wide C:N ratios in these waste materials which ranges from 10:1 to 250:1, large amounts of

waste-derived C (typically between 4 to 12 tonnes) are applied to soils each year. A large

proportion of this C particularly from solid waste may become a part of the soil organic matter and

remain in soils for a longer periods of time. There are a number of sources of C rich waste

materials available in New Zealand that can be applied on land such as municipal biosolids, herd

home manure, pulp and paper biosolids, poultry and piggery waste. There are also a number of

farm and industrial effluents that are commonly applied onto land in New Zealand that contain

significant amounts of C including; farm dairy, dairy factory, meat processing, poultry and piggery

effluents etc (see section 2.7). Green waste, which is generally blended with other waste

materials to produced green manure or dumped into landfills, can be composted or converted into

more stable forms of C in biochar. Application of these wastes on land would have a positive

effect on soil C sequestration. Given that intensification of land use is negatively impacting on the

soil C levels in New Zealand (Ghani et al., 1996; Lambert et al., 2000; Schipper et al., 2007), it is

good stewardship to protect soil C levels by multiple management options for example, minimum

tillage, incorporation of manures into soils, and restricting intensification etc.

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6.1.1 Availability of organic solid and semi-solid waste materials in NZ

Potentially available C from some of the solid or semi-solid organic wastes that can be applied on

land to accumulate C is listed in Table 7.1. It is estimated that more than 234,000 dry tonnes of

biosolids are produced in New Zealand (MfE database, 2006). Based on the average C content in

biosolids being 30% (Ghani et al. 2003b), this would equate to nearly 89,000 tonnes of C in

biosolids that can be potentially applied on land to increase C in soils. A substantial amount of C

from green waste can be converted into biochar which is thought to be a reasonable stable form

of C (Lehmann, 2007). Collectively, approximately 2.4 million tonnes of woody and green waste

biomass is available for biochar conversion in New Zealand from municipal amenities,

households, horticulture and arable cropping. Manures from herd-homes and wintering-off pads

contain significant amounts of C and are generally applied on the soils for which no C adjustment

is currently made. At present only 20% of the farms use wintering pads. Therefore, the quantity of

manure produced is relatively small at the moment but it is expected to increase as more and

more environmental compliance is enforced on farmers encouraging them to house animals or

keep animals on pads during the winter period.

Table 7.1 Some of the organic waste available annually in New Zealand that can be

applied on land to enhance C sequestration in soils. Carbon contents in various

wastes have been calculated based on the average concentration measured in

various wastes. (from Ghani et al. 2008b)

Waste Dry weight

(tonnes)

% C

(w/w)

% N

(w/w)

C:N C content

(tonnes)

Biosolids1 234,000 30 1.0 30:1 70,250

Pulp and Paper

Waste

150,000 40 0.5 80:1 60,000

Dairy manure2 30,000 38 2.0 19:1 11,400

Green waste3 182,5000 40 1.8 22:1 730,000

Horticulture woody

residues4

46,500 40 ND ND 18560

Agricultural crop

residues4

508,000 35 ND ND 177800

1 MfE Database 2003-2006

2 20% of national herd using herd-homes, wintering barns or stand-off pads for 60 days during winter.

Calculation of net wt of manure have been based on the information provided Gonzales-Avalos and

Ruiz-Suarez, (2001). 3

Green Party NZ data on estimation of green waste going to landfills 4

Hall P and Gifford J (2008) Bioenergy Options for New Zealand, Scion 2008

ND means no data was available

There are a number of co-benefits to increasing C content in soils through application of organic

waste materials such as less reliance on imported nutrients, and improved biodiversity of micro-

and meso-fauna and also increased water holding capacity. To ensure land remains productive

78


and fit for growing plants that can be consumed by humans, organic waste materials that may

pollute soil with heavy metals, endocrine disruptors and harmful microorganisms should be

treated with care. However, some of the land mining areas that are already polluted with heavy

metals and not suitable for agricultural production, less desirable organic wastes can be used to

produce biomass that can be used for biofuel i.e. not entering food chain (Ghani et al. 2008b).

6.1.2 Carbon accumulation in soils through application of waste

There is very little New Zealand data showing impacts of application of solid or semi-solid waste

materials on soil C. There are a number of studies published from overseas showing increases in

soil C levels due to application of biosolids (see Fig. 1.11 Rothamstead Cropping trial, UK).

Biosolids contain a high proportion of their total C in lignin and cellulose, forms which decompose

very slowly, hence with annual or biannual application of biosolids even at 200 kg N/ha equivalent

application rate, levels of soil C increase over time. A relatively medium-term study (4 years)

being conducted at Lincoln university shows build up of C in the top 10 cm of the soil profile (Ron

McLaren, personal communication, 2008). He noted higher build-up of C when biosolids were

applied at higher rates of N (Table 4). A short-term (6 months) study conducted by Ghani et al.

(2003b) observed over 50% of the non-decomposed biosolids remained within the soils, a

qualitative evidence of increased C in soils. Su et al. (2007) reported a significant increase in soil

C in the top 100 mm soil when biosolids were repeatedly applied at 600 kg N/ha every three

years since 1997 (Table 6.2). Evidence from these studies further reinforces the point that C

sequestration in soils can be achieved through application of organic waste materials.

6.1.3 Co-benefits of carbon sequestration in soils

Besides offsetting the emissions from other production systems, there are a number of co-

benefits of increasing C in soils particularly in C deficient and lighter soils. These benefits include:

More nutrients, less reliance on imported nutrients

Enhanced biodiversity in soil ecosystems

Reducing nitrate leaching through increased immobilisation of N by C rich wastes

Better moisture retention due to increased organic matter

Better structural stability particularly in lighter soils

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Table 6.2. Some New Zealand studies showing various level of C sequestration in the top

0-100 mm soil depth. (from Ghani et al. 2008b)

Waste Soils Rates of N

application

(kg N/ha/yr)

Estimated

rates

of C

(kg C/ha/yr)

Control

% Soil C

(w/w)

Treated

% Soil C

(w/w)

Reference

Biosolids

Nelson 200a 12,000 1.08 2.12** Su et al. (2007)

Canterbury Templeton 200 4,000 3.33 3.67** McLaren (2008)

800 16,000 3.23 4.72*** McLaren (2008)

Waihou 200 4,000 8.66 10.37*** McLaren (2008)

800 16,000 9.81 10.31*** McLaren (2008)

Waikato Horotiu 200 4,000 5.8 6.2** Ghani et al.,

(2003b)

Effluent

Dairy farm Manawatu ND ND 3.1 3.7 Yeates (1976)

Dairy

Factory

Horotiu 1190b 14,700b 7.20 6.55 Degan et al.

(2000a)

Meat Fairton, 600 5.6 6.8** Speir et al, (1987)

Meat Horotiu 1000 5.6 5.9** Russell (1986)

Significantly different at ** P < 0.05, *** P <0.001 a Received 600 kg N/ha every three years b Received dairy factory effluent for 22 years.

6.1.4 Risks association with addition of organic carbon in soils

There are also risks associated if C levels exceed beyond the optimum manageable levels.

Excessive accumulation of C through waste application can create following risks:

Low bulk density hence poor support for growing plants, dangerous for heavy machinery

Excessive build up of nutrients that can be lost through run-off in particulates or movement of

soils through floods etc.

Undesirable leaching of dissolved organic nutrients

Accumulation of heavy metals derived from soil waste materials

6.2 Pastoral fallow

Fallowing is an ancient technique used in semi-arid regions to accumulate moisture for a crop, for

weed control and for lifting soil fertility. In a New Zealand study a traditional sabbatical fallow had

a positive impact on pasture species in a low fertility hill pasture, increasing legume content in the

years following the fallow (Nie et al. 1998), but had no measurable impact on soil C (Ross et al.

80


1995). The high soil organic C contents of the soil might been one reason for finding no change

in soil C content, despite the input of a large amount of C over the duration of the fallow, which

included the spring, summer autumn months. In a soil with little soil C a fallow might offer an

approach for adding large amounts of C in situ.

6.3 Tree pasture systems

There is some evidence to show that under mature agroforestry systems the organic C content of

the soil is higher than that of a pasture or cropped soil (Young 1997). Data in NZ on the effect of

poplar and willow, planted for increasing the stability and hence sustainability of hill soils, on soil

C stock is scarce. In one of the very few studies Guevara et al. (2002) found no difference in the

soil C stocks in the topsoil of a pasture-poplar system in 3 of the 4 sites he examined. While the

type and nature of the litter from the trees within a pasture-poplar system would tend to have a

long-half life than the litter from pasture plants, the soils under poplar have a higher pH than open

pasture, encouraging greater microbial activity and litter turnover.

Figure 6.1. Poplar pasture system in the Hawkes Bay of NZ.

Figure 6.2. Estimates of the static C pools in the poplar-pasture (PP) and open pasture (OP)

systems at the Pohangina 1 site. Numbers indicate C (t/ha) for the topsoil (0-75 mm) and

vegetative components of two systems. When Guevara et al., (2002) added the C stocks in the

standing poplar trees to the C stock in the topsoil of the pasture-poplar systems, there was 30%

more C than in the open alongside. This was for a system with 37-40 mature poplar stems/ha.

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Figure 6.2 Estimation of C pool in poplar-pasture system.

A more detailed examination of the effect of conservation plant on soil C levels is warranted, as is

a fresh analysis of the scope that poplar-pasture and willow-pasture systems might have as a

systems option for increasing C stocks above 1990 baseline values. With heavier and/or more

frequent extreme rainfalls expected, especially where mean rainfall increases are predicted under

New Zealand climate projections for the 2030‘s and 2080‘s (MfE 2004, updated in NIWA 2007)

there may be up to a four-fold reduction in storm return period by 2080‘s, although there is little

quantitative information available. The frequency of extra tropical cyclones (which bring large

storm events to the north and east of New Zealand) is predicted to decrease but their intensity is

expected to increase. If rainfall increases as predicted for many parts of the country, then

erosion rates can be expected to increase substantially. Soil C losses via erosion processes

already represent a significant part of the national C budget. Those losses are being used at

present to explain why soil C levels in hill soils are still increasing. Increasing the stability of

landscapes has the potential to increase current accumulation rates.

Exploring how the tree pasture systems might best be assembled to capture a future C market

warrants future study, as it has the potential to offer multiple benefits to both the land owner and

wider community.

6.4 Managing soil carbon sequestering through biochar

there is little interest, over and above maintaining essential soil health, in increasing the fertility of

horticultural soils, for this would only result in negative consequences through creating excessive

vegetative vigour. So increasing the ‗active‘ stocks of soil C is not a favoured option for

integrated orchard systems. Nonetheless, there could be value and benefit in using horticultural

soils for storage of ‗inactive‘ forms of C, such as biochar.

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Biochar is a charcoal produced from biomass. The context of biochar as a soil C sequestration

strategy relies on biomass of charcoal produced by pyrolysis. Biochar is the residue of pyrolysis.

Under complete or partial exclusion of oxygen, ‗waste‘ biomass is heated to moderate

temperatures, usually between 400 and 500°C (namely low temperature pyrolysis), yielding fuel

energy, and biochar as a C-rich and more stable by-product.

Biochar seems especially well suited for a use in orchard/vineyard systems. For example, it

should not increase vegetative vigour. As opposed to other biomass-derived C materials (e.g.,

compost), biochar is not easily decomposed. As a consequence, the application of biochar does

not lead to large amounts of plant-available nutrients such as nitrogen in soil. Also biochar could

improve the efficiency of fertilizers, and possibly reduce the leaching of nitrogen and phosphorus,

thus improving the overall eco-efficiency of nutrient management in orchards/vineyards (see

below).

Recently biochar has been discussed nationally, and internationally, as a potential strategy for

soil-C sequestration. Biochar needs to fulfil at least four criteria to be a successful strategy for soil

C sequestration in orchards/vineyard systems: The half-life of biochar that is incorporated into soil

needs to be at least 100 years. This is the criterion for any strategy to be considered as a soil C

sequestration under the IPCC and other regulatory frameworks, such as the proposed PAS 2050

(British Standards Institute, 2008)

The use of biochar results in a net reduction of equivalent CO2 emissions for a horticultural

enterprise. A full life cycle analysis, including the energy needed for its production, transport and

incorporation into the soil thus needs to be considered

Biochar could become locally available at a cost-effective price, and large amounts of biochar

could be incorporated into soils without compromising the product yield and quality in

orchards/vineyards

There would, at present, seem to be no short or long-term negative consequences of biochar

applications for product yield and quality or for the environment.

6.4.1 Stability of biochar in soils

Large accumulations of charred material with residence times in excess of 1000 years have been

found in soil profiles (Saldarriaga & West 1986; Glaser et al. 2001; Forbes et al. 2006). Most

authors (Glaser et al. 2003) attribute the presence of large stocks of pyrogenic black C, as can be

found in Amazonian dark earths or terra preta several hundred years after the cessation of

activities that added it to the soil, to its chemical recalcitrance.

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However, very little is known about the half-life of specific types of ‗industrial‘ biochar. The

recalcitrance of biochar in soils depends on a multitude of factors, including the type of biomass

used for pyrolysis, the pyrolysis conditions, soil properties, and local climate. Typically, the half-

life of biochar from low-temperature pyrolysis is longer than 100 years (Lehmann et al. 2006;

Singh & Cowie 2008).

The adaptation of microorganisms in the soil to biochar, as a C source, might lead to a shorter

half-life of biochar, given that certain microorganisms exist that are able to live with biochar as the

sole C source (Hamer et al. 2004).

6.4.2 Net reduction of equivalent CO2 emissions due to the use of biochar

Biochar leads to a reduction of equivalent CO2 emissions from soils because of its long half-life

compared with other biomass-derived C (e.g., compost). However, the slow pyrolysis-based

bioenergy systems produce not only biochar for soil C sequestration, but also energy. The

combined use of energy production and soil C sequestration avoids about 2–19 t CO2e ha-1

year-

1. Some 41–64% of these avoided emissions are related to the retention of C in biochar; the rest

to the offsetting fossil fuel use for energy, fertilizer savings, and avoidance of soil emissions other

than CO2, such as nitrous oxide (Gaunt & Lehmann 2008).

The proportion of C retained in biochar during pyrolysis varies with pyrolysis temperature and the

type of biomass (Lehmann et al. 2006). A typical level of C recovery is 50% of the initial C

content. This C has a typical half-life of more than 100 years (Lehmann et al. 2003, 2006).

6.4.3 Practicality and cost-effectiveness of biochar use

While no large-scale facility for low-temperature pyrolysis is currently available in New Zealand,

this might change. Another question relates to how much biochar can be effectively and

practically applied to soils.

From the data available for highly weathered tropical soils, it appears that crops respond

positively to biochar additions up to 50 t C/ha, and may only show growth reductions at very high

application rates (Lehmann et al. 2006). For most plant species and soil conditions, this maximum

was not reached even with applications/additions of 140 t C/ha (Lehmann et al. 2006). We note

that most knowledge is derived from experiments with highly weathered tropical soils and very

low natural soil organic C contents. Little is known about the effect of biochar additions to

relatively fertile soils in a temperate climate.

The cost of incorporating biochar in soil, instead of using biomass solely for electricity generation,

was estimated as US $47/t of CO2 contained in biochar (Gaunt & Lehmann 2008). This does not

84


incorporate the additional costs associated with the transport of biochar from the pyrolysis plant to

the site of application, and the ‗costs‘ of incorporation of biochar into soil in existing enterprises.

Currently, the market price for one tonne of CO2 is US $9–16/t, and, therefore, the incorporation

of biochar in soil is not yet a cost-effective option (Gaunt & Lehmann 2008). However, future C

prices and emissions-trading costs could be much higher, for example, in the European Union

Emission Trading Scheme the price is US $20, and would lie around US $25–85 if the social

costs of climate change were used as the basis for the calculation (Stern 2007).

6.4.4 Possible short- or long-term consequences of biochar applications

No published data are available on the possible negative consequences of biochar applications in

soil, at least those that are based on field-scale studies. Field-scale studies of biochar

incorporated into soils have only recently started. Below we give an assessment of the potential

risks that have not yet been evaluated thoroughly, especially under the horticultural conditions of

New Zealand‘s soils and climate.

The type of biomass and pyrolysis conditions can modify the amount and composition of

phytotoxic and potentially carcinogenic organic materials that are byproducts of pyrolysis (Lima et

al. 2005).

Biochar contains aromatic and aliphatic organic compounds that may cause or enhance the

occurrence of soil water repellency. Many New Zealand soils have been found to be water

repellent after dry summers, which causes a decrease in pasture growth (Deurer et al. 2008a).

The run-off of water and nutrients into surface waters is also another deleterious consequence of

repellency (Doerr et al. 2000). Many topsoils in New Zealand already have very high C contents,

and the C content is generally positively correlated with the occurrence of water repellency (Doerr

et al. 2000). Although no studies have yet been undertaken to investigate if biochar could cause

or enhance soil water repellency, water repellency was reported to occur in reclaimed mine soils

that contained sandy sediment mixtures with significant proportions of lignite (brown coal) (Gerke

et al. 2001). Another indication of the potential risk of using biochar and causing soil water

repellency is that hydrophobicity often occurs in topsoils after forest fires, which in a way ‗mimics‘

pyrolysis (Doerr et al. 2000).

A study in boreal forests conclude that the potential of biochar for soil-C sequestration might be

overstated (Wardle et al. 2008). In this study, charcoal was prepared and mixed with the forest

soils, then left in the soils of different contrasting forest stands in northern Sweden for 10 years.

Because of the incorporation of biochar microorganisms significantly increased. As a

consequence, the loss of ‗native‘ soil organic matter increased, and the net soil C sequestration

was small. How this might relate to productive enterprises, such as orchards/vineyards, is

unknown.

85


6.4.5 Impact of biochar on physical and chemical soil properties

It has been found that in highly weathered, coarse-textured soils, biochar improves the soil‘s

filtering and buffering capacity for nutrients.

Biochar adsorbs more cations per unit C than most other soil organic matter, because of its

greater surface area, greater negative surface charge, and greater charge density (Liang et al.

2006). However, the magnitude of the cation-exchange capacity depends on the type of biomass,

and pyrolysis conditions. Also, the biochar‘s properties can change considerably with time during

the exposure to the soil environment (Lehmann 2007).

Biochar retains nutrients, especially nitrogen and phosphorus (Glaser et al. 2002; Lehmann et al.

2003), and also increases the nitrogen fertilizer-use efficiency for plants (Chan et al. 2007).

Biochar was found to have reduced the leaching of nitrate, ammonium, phosphorus and other

ionic compounds (Beaton et al. 1960; Radovic et al. 2001; Lehmann et al. 2003; Mizuata et al.

2004), and has also been observed to absorb hydrophobic organic contaminants (Gustaffson et

al. 1997; Accardi-Dey & Gschwend 2002).

In highly weathered, coarse-textured soils, biochar improves the soil‘s water retention properties.

In Amazonian charcoal-rich anthrosols, the field water-retention capacity was 18% higher than for

surrounding soil without charcoal (Glaser et al. 2002). However, in another study (Tryon 1948)

with three different textures (sandy, clayey and loamy), charcoal increased the plant-available

water contents in the sandy soil, but had no effect in the loamy soil, and decreased it in the

clayey soil.

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7. Soil carbon as a soil quality indicator and soil natural capital

and ecosystems services

The C in SOM represents a significant reservoir of C within the global C cycle. Sequestrating C in

soils has the potential to be a GHG mitigation option for some soils in New Zealand, and would

increase their versatility by adding to their natural capital (e.g. hill soils). On other soils, sustaining

current levels might be a challenge, as is the current situation under pasture agriculture on

lowland soils and under a number of horticultural and arable practices.

At present, the amount of organic C stored in soil is not part of the C budget considered under the

Kyoto protocol. This seems almost unbelievable as it is known that soil is a huge store of C and to

be influenced by land management practices and natural events (e.g., erosion). The omission of

soil C from the C budget and from trading seems surprising (ignoring the elephant in the room?),

given the potential magnitude that changes in soil C could have on national and global C

balances There have been suggestions that C depleted soils could be used to sequester (store)

extra C by appropriate management (Lal 2001; Paustian et al.1997), and that farmers could earn

credits simply by increasing the C content of their soils. However, to date no such trading in soil

organic C has been established.

Sparling et al. (2006) modelled the benefits of restoring organic matter on C depleted soils (after

prolonged cropping) for both increased production and for the hypothetical value if NZ

landowners were recompensed by trading on international markets for the increased C storage in

soil. They estimated that over the recovery period (which took from 36-125 years) the benefit of

increasing soil C content to pasture production was NZ$546–1237 (per ha basis). This amount

was small compared to the NZ$1619–NZ$16 001 if the stored C had been able to be traded for C

credits (typically NZ$6–17 per tonne of C). The environmental ―value‖ in conserving and

increasing soil C is much greater than the benefits to production under typical pasture

management in New Zealand. That environmental value was only estimated for C storage and

does not include additional benefits such as nitrogen storage, soil stability, resistance to erosion,

regulation of water flows and chemical buffering. Inclusion of those environmental services would

increase further the value of soil organic C.

In this section current knowledge on soil C as a soil quality indicator is reviewed. The

implications of using sequestered C in soils as a potential C-offset for GHG emissions, on the

interpretation and setting of the optimum C content and target range for this soil quality indicator

are also discussed. The concept of natural capital stocks of our soils is introduced in this section,

as is the use of this approach in measuring and valuing the soils ecosystems services.

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7.1 Soil carbon and soil quality indicators

Organic C/ organic matter (SOM) is at the living heart of soil. SOM affects a range of soil

attributes and processes. For example soil C influences soil microbial diversity and health,

nutrient and water cycling, soil aggregate building and stability, filtering and buffering of the soil‘s

water through to colour. These attributes and processes underpin the soils natural capital and

ecosystem services which includes, supporting, provisioning, regulating and socio-cultural

services (See section 8.2). Because of the pivotal role soil C plays in soil function, it is a very

useful soil quality indicator for a wider range of soil services. With soil C sequestration emerging

as a potential C-off-set for GHG emissions, measuring soil C for quantifying C stocks in soil, has

added another use for this soil quality indicator. In this section of the report current knowledge on

soil C as a soil quality indicator (―fitness for purpose‖) is reviewed and summarised. The

implications of using C sequestered in soils as a potential C-offset for GHG emissions, on the

setting and interpretation of the optimum and target range for this soil quality indicator is also

discussed

7.1.1 Soil quality indicators

Soil quality indicators inform use about the characteristics or condition of the soil. A large number

of soil properties have been proposed as indicators of soil quality, but to be an effective and

quantitative indicator, the property needs to have an interpretive framework. Ideally, a response

curve is available to show the relationship between a quantitative soil characteristic and the

production, environmental or social goal of interest and how that is influenced by land use or land

management practices. We need to know whether a particular value is desirable, and what to do

about it if it is not. The interpretation of indicators is usually based on one of the following

descriptions: `More is better‘, `Less is better‘ or an `Optimum Range‘. The setting of targets for

soil quality characteristics has proved contentious, with a divergence of opinion about what

constitutes good soil quality. Much of the contention arises because of the question ―Soil quality

for what purpose?‖ Fitness for purpose is also raised as part of that debate. A very good

example is the optimum soil C content for pasture or crop management, versus the optimum soil

C content for viticulture. For pasture and cropping more is better and for viticulture production

less is better. This example also raises the potential conflict between production and

environmental goals within a land use. Under a pastoral use increasing soil C aligns with both

improved production and environmental outcomes. Under viticulture this is not the case with

declining soil C an undesirable environmental goal. Due to differing pedogenesis, soils have

different characteristics, and hence the target and optimum range a target that is attainable and

suitable for one soil may not be appropriate for another (Fig. 1.2).

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7.1.2 Soil quality indicators for NZ soils

Provisional targets for state-of-the-environment indicators have been published in Provisional

targets for soil quality indicators in New Zealand (Sparling et al. 2003). The indicators include a

measure of soil acidity (soil pH); two measures of compaction (bulk density and macroporosity); a

measure of soil P fertility (Olsen P); and three measures of soil organic matter (total C, total N

and mineralisable N). Response curves have also been derived for some additional soil quality

measures, including earthworm numbers, aggregate stability, depth of topsoil, potential rooting

depth, and the C and N balance. The soil quality response curves for each of the above

indicators, along with the optimum and target range were derived by averaging the views of a

wide range of experts. This is a useful approach where knowledge is incomplete, because it

allows experimental data, statistical metrics, and simulation modelling where available to be

synthesized with personal experience, anecdotal evidence, and best guesses based on an

understanding of soil processes and relationships (Sparling et al. 2003a). Three of the soil quality

indicators, including soil C were re-examined with some revision made in 2007 (Beare et al.

2007).

7.1.3 Soil carbon as a soil quality indicator

Total C (TC) measures the amount of C (C) in soil. In New Zealand soils, TC is largely organic C

in soil organic matter (a complex mixture of organic compounds originating from living and

partially decomposed plant, microbial and animal remains), but can also include inert forms such

as charcoal and inorganic forms such as carbonates. Total C is generally measured in the

surface soil horizon where C content is greatest, often from 0−7.5 or 0−10 cm depth. With soil C

becoming a commodity the depth of sampling has been extended to 30 cm in line with

international standards. Total C contents are normally reported as percentage C (%C = weight of

C/weight of dry soil x 100), but can also be expressed as weight per volume (preferred for valid

comparison between different soils where bulk density may differ). Soils differ in the amounts of

organic matter they contain depending on their mineralogy, climate and land use.

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Figure 7.1 Expert soil quality response curves for total C.

The Semi-Arid Soils, Pumice and Recent formed one distinct group, and Allophanic soils another

distinct group, sufficiently different to warrant their own specific response curves. As the organic

soil order, by definition, contains more than 16% C, C content is not a useful measure of soil

quality for that order. The response curves fitted the ―more is better‖ model. Total C targets for

soil quality for environmental protection were higher than those for production (Fig. 7.1)

The target for the Semi-Arid soil order was lower than other orders (Table 5.1) recognising that

organic matter content in that soil type and environment rarely attain the levels of other soil

orders. Conversely, total C contents for the Allophanic soils are higher than the others because

the high contents of allophanic clays and hydroxy-aluminium compounds tend to stabilise larger

amounts of organic C in those soils. Relatively little information is available on the relationship

between total soil C and pasture productivity across a range of different soil orders. The

consensus at the present time is that pasture production is insensitive to a change in soil C

across the target range. A similar conclusion is reached for forest productivity. Our

understanding of the relationship between total soil C and productivity is probably most advanced

for cropping soils (Section 4). There appears to be no information showing the direct linkage, if

any, between soil organic C and performance of horticultural crops.

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Table 7.1. Provisional quality classes and target ranges for total C.

There were no upper limits defined (more is better), but there will be a point where the organic

matter content reduces bulk density. The desirable lowest level for Allophanic Soils was 3%, for

Semi-Arid, Pumice and Recent Soils the lower limit was 2%, and 2.5% for all other orders except

the Organic Soils (Table 7.1).

The setting of a lower limit is contentious. Sparling et al. (2003a) suggested a justifiable lower

limit could be based on the concept of intergenerational equity (e.g. current generations do not

have the right to deplete non-renewable resources to the disadvantage of the next generation). In

terms of soil organic C, this suggests that total C should not be depleted to a point where it

cannot be restored within a 25-year timeframe (Table 1.4).

For each land use there will be an optimum amount of soil organic matter for achieving production

and environmental goals. There will also be a range around that optimum value for which there

appears to be little measurable change in production or environmental outcomes, but will have a

significant impact on the C budget. Inclusion of soil C as a potential C-offset for GHG emissions,

will require a rethink of the current target range or fit for purpose definition for this soil quality

indicator and a revision of the provisional targets for state-of-the-environment indicators published

in Provisional targets for soil quality indicators in New Zealand (Sparling et al. 2003a) and

undated by (Beare et al. 2007).

7.2 Natural capital and ecosystems services

7.2.1 Soil Natural Capital

Natural capital is the extension of the economic idea of capital (manufactured means of

production: buildings) to environmental goods and services. Natural capital is thus the stock of

natural ecosystems that yields a flow of valuable ecosystem goods or services into the future

(Costanza & Daly 1992). Soils across the globe are significant and perhaps an unheralded

91


category of natural capital, even in industrial and post-industrial economies less dependent on

agriculture. Soils are considered to be ―critical‖ natural capital (Ekins et al. 2003), as food

production and other ecosystem services would not be available without soils and human survival

would not be possible. Soils cannot be replaced or simply substituted by any manufactured

capital, in any absolute sense, although at the margins some substitutions are possible through

for example the use of hydroponics.

Only a fraction of the goods and services the soil-plant-atmosphere systems which cloak our

earth provide are valued within the world‘s economy. In a landmark paper in Nature, Costanza et

al. (1997) estimated the annual value of 17 terrestrial ecosystem services, all involving the soil-

plant-atmosphere system, to be US$5.74 trillion. In ―Growing for Good‖, Morgan Williams (PCE,

2004) noted that New Zealanders are highly dependent on our natural capital stocks of our

waters, soils and biodiversity to sustain our wealth-generating capacities.

It is often thought that New Zealand is blessed with large areas of versatile and elite soils rich in

organic matter (i.e. high in natural capital) that supply beneficent productive and ecosystem

services. However, the reality is that over 65% of New Zealand soils have a physical limitation to

use for pastoral agriculture (i.e. limited natural capital) and the hectares suitable for sustained

horticultural and arable use are even smaller (Mackay 2008). This lack of natural capital in some

soils can be linked directly to their low SOM contents. Practices that actively built SOM would

add natural capital value to these soils. Addressing the physical limitations in others, e.g. poor

drainage, has the potential to reduce SOM levels. Given that much of the present agricultural

expansion is on to our marginal and fragile landscapes, developing practices to lift SOM would be

an attractive proposition to land owners, offering not only potentially production gains, but also

increasing the efficiency of current resource use through better services of nutrient and water

regulation and by reducing nutrient leakage. In this section the concept of natural capital stocks of

our soils is introduced, as is the use of this approach in measuring and valuing the soils

ecosystems services.

Soils with high natural capital value for pastoral agriculture include those with deep silt loam-

texture and free-draining top soils (e.g. Allophanic and Brown soils). Soils with less natural capital

for pastoral agriculture include those with poorly developed structures (e.g. podzols), shallow soil

horizons and plant-rooting depth (e.g. stony soils), weak cation and anion storage and supply

capacities (e.g. coastal sands), and low water-holding capacity (e.g. gravels and pumice). For

New Zealand, natural capital of soil has been defined as the capacity of the soil to sustain a

legume-based pasture sown with improved germplasm, supplied with essential plant nutrients

other than N, kept at an optimum pH and under optimum grazing practice (Clothier et al. 2008). A

legume-based pasture is a self-regulating biological system with an upper limit on the amount of

N that can be fixed and made available for plant growth. This definition of the natural capital

92


reflects the underlying capacity of soil to retain and supply nutrients and water and sustain plant

growth under the pressure of grazing animals.

The introduction of a range of technologies, including irrigation, drainage, N fertiliser, wintering

pads, off-farm grazing and imported feeds, have lifted potential livestock production levels

significantly beyond the inherent productive capacity of the original legume-based pasture system

(Fig. 7.2). These production technologies have most benefitted those soils with the least natural

capital, but often at the expense of a greater impact on the environment. For example, drainage

is a production technology that can lift pasture growth rates, pasture utilisation and stocking rates

significantly, by removing excess water from poorly structured soils. However, drainage also

increases leaching volumes and nutrient losses to the wider environment, since many of our

poorly drained soils have low nutrient-retention capacities. To gain sustained benefits from

investment in drainage, and given the increasing threat to groundwater and surface water quality

from pastoral agriculture (Clothier 1997), an investment to compensate for the lack of soil

properties and processes that regulate nutrient emissions needs to be added.

Figure 7.2 Contribution of the natural capital of a soil and production technologies (Tech1

and 2) to production and emissions from a soil with either a low or high natural capital

(Mackay 2009).

Soil quality indicators have been used to assess the suitability a soil for a particular use, mainly in

regard to productive capacity but they haven‘t been linked to the notion of Natural Capital and

ecosystem services (Mackay 2008). Soil quality indicators inform us about the characteristics or

condition of the soil. There are two broad categories of indicators, those that describe properties

of soils (e.g. texture, mineralogy) that, along with extrinsic factors (e.g. climate), define their

suitability for particular land uses (e.g. arable, pastoral) and those indicators that change in

response to human use and management (e.g. soil organic matter, bulk density, biodiversity)

normally over relatively short time frames. Linked to soil processes they can inform the provision

Low High Low High

Pro

du

ctio

n/E

mis

sio

ns

Natural capital of soil

Tech2

Tech1

Production

Emissions

93


of ecosystem services from soils. Palm et al. (2007) discuss how best to determine the natural

capital of soils, proposing that the natural capital of soils underlying ecosystem services is

primarily determined by three core soil properties – texture, mineralogy and soil organic matter,

the first two inherent and the third that changes in response to human use and management.

7.3 Classifying and measuring soil natural capital and ecosystem services

Soil classifications and associated properties cannot be used for compiling an inventory of the

natural capital stocks of soils. To soil classifications must be added a human use (land-use) or

purpose before a ―value‖ can be assigned to the natural capital stocks in terms of the ecosystem

services they provide. Dominati et al. (2009) has drawn on our understanding of soil forming

processes, soil taxonomy and classification and soil processes and built on current thinking on

ecosystem services to develop a framework for classifying and measuring soil natural capital and

ecosystem services (Fig. 7.3). In soils some properties are inherent, a product of soil forming

processes and cannot be modified on a short-time period, and some are more manageable (Lynn

et al. 2009). Knowing what type of properties is involved in the processes and therefore in the

services provision is important when it comes to management and land use decisions.

Supporting processes are the processes that maintain the integrity of ecosystems. They insure

the formation and maintenance of soil natural capital. Regulating and provisioning processes

relate to human welfare and how natural capital stocks provide goods and services to humans.

Identifying how processes are organised is critically important to avoid double counting when

valuing ecosystem services. The value of a soil‘s Natural Capital and ecosystem services is very

dependent of the land use applied to it. Soils with the same Natural Capital and properties will

have very different values for human welfare according to what use the land is put. For example,

a deep stony soil will be very well suited for grape growing, average for sunflower cropping and

completely unsuitable for corn cropping, because these different crops require very different

optimal water and drainage conditions.

94


Regulating soil processes

Water quantity regulation: Provision of

flood control

Water quality regulation: filtering of

contaminants and Nutrients pools

regulation

Ecological interactions: Biological

control of pests

Degradation and recycling of wastes:

Destruction of harmful compounds

Erosion regulation Physical integrity

Air quality regulation: Limitation of

green house gases emissions

Climate regulation

Natural Hazard regulation

Supporting processes

Soil formation: weathering

and pedological processes

Nutrients cycling

Water cycling

Biological activity

Primary production:

Photosynthesis

Secondary production

Climate activity and

regulation

Provisioning soil processes

Provision of nutrients for plants

Provision of water for plants

Provision of water to humans

Provision of habitat and physical support

Capital

degradationDegradation processes

•Physical

–Erosion

–Crusting

–Compaction

•Chemical

–Salinisation

–Nutrients depletion

–Acidification,

eutrophication

–Toxification

•Biological

–Loss of OM

–Decline in biodiversity

–Loss of structure

Socio-cultural processes

•Values

•Perception

•Behaviour

•Beliefs

Capital formation

and maintenance

AnthropogenicTechnology

Farming practises

Land uses

NaturalClimate

Natural hazards

Geology

Biodiversity

External drivers

Inherent properties•Slope

•Orientation

•Subsoil pans or

•other rooting impediments

•Depth

•Clay types

•Texture

•Stoniness

•Strength in subsoil

•Size of aggregates in subsoil

•Subsoil Wetness class

Manageable properties•Soluble Phosphate

•Mineral nitrogen

•Soil Organic Matter

•Total carbon

•Temperature

•pH

•Land cover

•Macropores

•Bulk density

•Strength in topsoil

•Size of aggregates in topsoil

•Field capacity

Soil

Natural Capital

SOIL HUMANS

Ecosystem

Goods and

Services

•Spiritual and

Cultural

•Aesthetics,

inspiration

•Knowledge and

education

•Recreation

•…

•Human habitat

•Human health

•Food

•Fibre

•Fuel

•Clean water

Human

need

Hierarchy

S

E

R

V

I

C

E

S

Figure 7.3 Draft Framework for Ecosystem services provision from soils Natural Capital.

Prepared for May 2009 Title 92

7.4 Application of soil Natural capital in land management

A natural capital based approaches to land-use planning and managing nutrient and

land valuation is an emerging methodology that will in future be at the forefront of

sustainable developments, as the efficiency of agricultural production systems will be

measured increasingly on the basis of their sustainable exploitation of natural capital,

whilst minimising external costs to the environment. In this section of the report four

examples of the use of a natural capital approach to resource manage are briefly

described. The first and last example looks at how a change in soil C influences the

soils natural capital and how that affects the cost of production.

Valuing the carbon in a pastoral soil - Sparling et al. (2006) assessed the relative

value of organic matter for pasture production and for C and N sequestration, by

calculating the value in restoring organic matter in a depleted pastoral soil following

cropping. The value of soil organic matter to production was estimated from the value

of dairy milk solids based on a computer simulation of pasture dry matter yield and

organic matter accumulation. The hypothetical financial gain associated with organic

matter recovery if C credits were issued for organic C sequestration, and N credits for

N sequestration were also estimated. The simulations and estimates were completed

for three real-life contrasting soils and climate regimes in New Zealand. The analysis

provided only a partial estimate of the economic value of soil organic matter. It did not

include the contribution soil organic matter made to the other soil provisioning and

regulating services.

Natural Capital and Land-Use Planning - Productive-sector environments are

undergoing rapid land-use change in many regions of New Zealand, including the

northern region of the Kapiti Coast District. The northern region is currently

characterised by dairying, other pastoral and horticultural activities. But competition

from urban sub-division, and an increase in lifestyle properties are rapidly encroaching

on the viability of the land-based primary production sector. Consideration of the

natural capital value of a district‘s biophysical resources, along with consideration of its

ecosystem services, provided a means for policy analysts and the community to

assess and appraise future land-use options, their potential value and the

environmental impacts to assist the local community in long-term planning for the

District Council, through their Long Term Council Community Plan (Clothier et al.,

2008). Taking into account the value of a region‘s natural capital stocks and

ecosystem goods and services provided a more complete picture of the future options

for land-use and trade-offs.

93


Natural Capital and Nutrient Policy - It is imperative for our productive and

ecological futures that we sustainably manage our lands to protect the natural capital

of our ground and surface waters. We need to understand better how land-

management practices control groundwater quantity and quality (Clothier, 1997). How

could environmental policy for nutrient management in relation to nitrogen, across a

diverse landscape, be developed to protect the quality of our receiving waters better

through limiting leaching losses, without the need to be prescriptive about current or

future land uses, by linking directly to the soils natural capital. Horizons Regional

Council One Plan, which has been notified and is currently in front of Commissioner,

has adopted an approach that is independent of current land use and links a N loss

limit directly to the underlying natural biophysical resources (Clothier et al. 2008).

Allocating a N loss limit based on the natural capital of the soil in the catchment offers

an approach for developing policy that is linked directly to the underlying resources in

the catchment. This is not too dissimilar to the concept of a water-use take limit. It is

independent of current land use. The approach also provides a direct link between

soil properties (soil organic matter) and land practices and nutrient loss.

Change in natural capital through a change in soil carbon - The soil and climate

(―terroir‖) of viticultural land provides a ―natural capital‖ which growers exploit by

appropriate management to obtain desirable economic and environmental outcomes.

Using wine grapes as an example, we discuss here an approach to valuing this natural

capital, as a function of soil C, by using simple models to predict outcomes and

incorporating economic and environmental values of inputs and outputs.

Given a vine growth model, a soil, seasonal weather, and a management system, it is

possible with the aid of simple models to estimate both the economic return and the

environmental impact of operating a vineyard for one season. If environmental and

economic impacts are quantified in the same units, namely dollars, then these

seasonal outcomes can be summarised into a single monetary value. A comparative

value for the natural capital of the terroir is then defined as the expected value of this

total, which can be obtained by running the model (Hall et al. 2009) for a large number

of simulated, or historical, seasonal weather series, and averaging the results. We will

first run the model for a soil with a total C content of 1% v/v, and then re-run for the

same conditions except with nearly double the amount of C.

For this economic evaluation of the impact of C on natural capital value of viticultural

soil we use the terroir calculator of Hall et al. (2009) whose model includes the

following:

94


(a) Economic Model: Hall et al. (2009) place a value on the net income per hectare

which could be obtained from a vineyard under conditions where water is ―naturally‖

available in exactly the right quantity throughout the season, then deduct from this

costs associated with water management (―irrigation cost‖) and any loss of crop value

due to inability to control the water perfectly (―loss of quality cost‖). The same applies

to fertiliser management, with their being costs of purchase and applications, and as

well we have now included environmental costs as they relate to nitrous oxide in IPCC

calculations for C dioxide equivalents,, which enable us to assign a cost through the

price of C.

(b) Plant model: Three time series define the plant response to water and nitrogen.

The first details the time course of canopy cover during the growing season, used as a

crop factor in calculating evapotranspiration. The second describes the ―ideal‖ soil

water curve which would lead to optimum crop value. The soil should be near field

capacity early in the season but lower following veraison (berry softening). Deviations

from this ideal curve (either above or below) reduce the net value of the crop (see

Figure 6.4). Next there is the supply of N, and the ideal curve for this is shown in

Figure 6.5. Nitrogen needs to be available through canopy development up to

flowering, and then it should decline to limit vegetative vigour through veraison to

harvest.

(c) Environmental impacts: Here we have chosen to include the cost of irrigation

water as an economic cost, assuming that if it is a scarce resource it will need to be

paid for. Water which drains through the soil is assumed to have a negative

environmental impact due to leachates being transported into the groundwater. We

also include IPCC impacts for leached nitrogen and nitrous oxide emissions.

(d) Soil water model: A simple water balance is applied to a single layer of soil. At the

end of each time step, after taking into account evapotranspiration, rainfall, and

irrigation, any water in excess of field capacity is assumed to drain to groundwater.

(e) Water and fertiliser management: Irrigation is applied whenever soil water drops

more than some threshold amount below the ―ideal‖ for the plant at that time. For

such a simple system, it is possible to ―optimise‖ management by taking into account

the cost of irrigation relative to the costs of deviations from the ideal soil water content

at that time in the season (c). A dressing of 20 kg-N is applied in spring, and then the

only N available to the plant comes from that mineralised in the soil.

(f) Weather: Only rainfall and reference evapotranspiration in each time period are

used, but of course to estimate the natural capital value as defined above many years

of actual or simulated weather data are necessary, in order to obtain a good estimate

of the expected response.

95


Figures 7.4 and 7.5 The ideal seasonal pattern of soil-water and nitrogen that

we consider maximises terroir value.

Beyond veraison (berry softening) it is advantageous to have low levels of water and N

to limit leaf growth, and thereby encourage higher Brix in the berries. However it is

necessary to have a sufficient level of N to avoid low levels of yeast available N (YAN)

in the berries which can cause stuck ferments during wine making.

The terroir model was run over 30 years using weather data for Marlborough.

Penalties on terroir are assigned for any deviation from the ideal patterns of soil water

and nitrogen shown in Figures 7.4 and 7.5. The penalty for deviation is not taken to

be linear, but is set at the square of the deviation from the ideal line. The penalty cost

for water above is taken to be $2 (%v/v day-1

)2 and for nitrogen it is $0.40 (kg-N ha

-1)2.

Because of the relative detriment from vegetative vigour, relative to low YAN, the

penalty for being above the nitrogen line is weighted two-fold greater than that for

being below the line.

Figures 7.6 and 7.7. The value of terroir ($/ha) in a low C (1%) soil as a function

of depth. On the right is the same result but for soil with a C content of 1.75%.

0.0

0.1

0.2

0.3

0.4

0 4 8 12 16 20 24

Week

Op

tim

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so

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ate

r fr

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(L/L

)

FloweringO

ptim

um s

oil w

ater

con

tent

(m3

m-3

)

Veraison

Weeks after bud-burst

Soil Water Store


0

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10

15

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9-Sep 29-Oct 18-Dec 6-Feb 28-Mar

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nitr

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(kg-

N h

a-1 )

Nitrogen

0.0

0.1

0.2

0.3

0.4

0 4 8 12 16 20 24

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oil w

ater

con

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(m3

m-3

)

Veraison


0.0

0.1

0.2

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0.4

0 4 8 12 16 20 24

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ptim

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oil w

ater

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(m3

m-3

)

Veraison


Soil Water Store


0

5

10

15

20

25


Date

Targ

et

Nit

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en

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nte

nt

(kg

/Ha)

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0 4 8 12 16 20 24

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(kg-

N h

a-1 )

Nitrogen


0

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Date

Targ

et

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rog

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(kg

/Ha)

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0 4 8 12 16 20 24

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)

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Veraison

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nitr

ogen

stor

age

(kg-

N h

a-1 )


0

5

10

15

20

25


Date

Targ

et

Nit

rog

en

co

nte

nt

(kg

/Ha)

0.0

0.1

0.2

0.3

0.4

0 4 8 12 16 20 24

Week

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tim

al

so

il w

ate

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acti

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(L/L

)

0

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Date

Targ

et

Nit

rog

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co

nte

nt

(kg

/Ha)

0.0

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0.2

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0.4

0 4 8 12 16 20 24

Week

Op

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(L/L

)

Flowering

Veraison

Tar

get

nitr

ogen

stor

age

(kg-

N h

a-1 )

Nitrogen

Using 30 years' historical Blenheim

data

-$5,000

$0

$5,000

$10,000

$15,000

$20,000

0 500 1000 1500 2000

Soil depth

Terro

ir v

alu

e

Comm value

Env value

Net Value

Using 30 years' historical Blenheim

data

-$5,000

$0

$5,000

$10,000

$15,000

$20,000

0 500 1000 1500 2000

Soil depth

Terro

ir v

alu

e

Comm value

Env value

Net Value

96


The environmental costs are subtracted from the commercial value to provide a net

value. On the right is the same, but for a soil higher in C (1.75%). In Figure 6.6 is

shown the value ($ ha-1

) of terroir for a soil in Blenheim, as a function of depth. If the

soil is too shallow there is neither enough water nor nitrogen, so the natural capital

value is lower. Conversely, if it is too deep, there too much water and nitrogen for

good berry development, and so again the terroir value declines. The optimum depth

to maximise terroir value is around 1 m (Fig. 7.7)

Now, if we consider the same soil but with an increase in C of 0.75% v/v, up from a

total C content of 1% v/v then there would be more water storage and more N

mineralisation,. Now the optimum depth to maximise terroir value would be very

shallow and closer to 0.5 m (Fig. 7.6). The soil here is considered to be 80% sand,

and 5% clay. So now only shallow ‗fertile, water holding‘ soils would be valuable, for

as depth increase, as does water supply and provision of N. As a result, the pruning

‗penalty costs‘ go up from $1400 ha-1

for a 400 mm deep soil, through to $13,000 ha-1

that is needed to control the vigour on a 2m deep soil that can supply more water and

nitrogen. C here has negative values here for viticulture. If there is a worry about soil

quality and functioning as a result of low C, maybe the application of just a surface

mulch of composted prunings and marc (crushed grape skins) is an option so as to

avoid vegetative vigour, yet retain soil structure and soil functioning. This mulching

value can be seen in Figs 7.6 and 7.7 for it is akin to just raising the C content of a

shallow soil. Indeed, there is not much difference in the terroir value as a function of

soil C between soil having a depth of 400mm. An alternative could be to sustain soil

functioning and C sequestration in viticulture soils through the use of biochar.

97


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9. Glossary

acid soil–Soil with a pH value <7.0.

adsorption–The process by which atoms, molecules, or ions are taken up from the soil solution

or soil atmosphere and retained on the surfaces of solids by chemical or physical binding.

aeration, soil–The process by which air in the soil is replaced by air from the atmosphere. In a

well-aerated soil, the soil air is very similar in composition to the atmosphere above the

soil. Poorly aerated soils usually contain a much higher content of CO2 and a lower content

of O2 than the atmosphere above the soil. The rate of aeration depends largely on the

volume and continuity of air-filled pores within the soil.

aerobic–(i) Having molecular oxygen as a part of the environment. (ii) Growing only in the

presence of molecular oxygen, such as aerobic organisms. (iii) Occurring only in the

presence of molecular oxygen (said of chemical or biochemical processes such as aerobic

decomposition).

aggregate–A group of primary soil particles that cohere to each other more strongly than to

other surrounding particles.

aggregation–The process whereby primary soil particles (sand, silt, clay) are bound together,

usually by natural forces and substances derived from root exudates and microbial activity.

air dry–(i) The state of dryness at equilibrium with the water content in the surrounding

atmosphere. The actual water content will depend upon the relative humidity and

temperature of the surrounding atmosphere. (ii) To allow to reach equilibrium in water

content with the surrounding atmosphere.

allophane–An aluminosilicate with primarily short-range structural order. Occurs as

exceedingly small spherical particles especially in soils formed from volcanic ash.

Alluvial soil–(i) A soil developing from recently deposited alluvium and exhibiting essentially

no horizon development or modification of the recently deposited materials. (ii) When

capitalized the term refers to a great soil group of the azonal order consisting of soils with

little or no modification of the recent sediment in which they are forming. (Not used in

current U.S. system of soil taxonomy.)

anaerobic–(i) The absence of molecular oxygen. (ii) Growing in the absence of molecular

oxygen (such as anaerobic bacteria). (iii) Occurring in the absence of molecular oxygen (as

a biochemical process).

anaerobic respiration–The metabolic process whereby electrons are transferred from a

reduced compound (usually organic) to an inorganic acceptor molecule other than oxygen.

The most common acceptors are carbonate, sulfate, and nitrate. See also denitrification.

anion exchange capacity–The sum of exchangeable anions that a soil can adsorb. Usually

expressed as centimoles, or millimoles, of charge per kilogram of soil (or of other

adsorbing material such as clay).

arable land–Land so located that production of cultivated crops is economical and practical.

ash (volcanic)–Unconsolidated, pyroclastic material less than 2 mm in all dimensions.

Commonly called "volcanic ash". Compare cinders, lapilli, tephra.

aspect–The direction toward which a slope faces with respect to the compass or to the rays of

the sun.

available nutrients–(i) The amount of soil nutrient in chemical forms accessible to plant roots

or compounds likely to be convertible to such forms during the growing season. and (ii)

The contents of legally designated “available” nutrients in fertilizers determined by

specified laboratory procedures which in most states constitute the legal basis for

guarantees.

available water (capacity)–The amount of water released between in situ field capacity and

the permanent wilting point (usually estimated by water content at soil matric potential of -

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1.5 MPa). It is not the portion of water that can be absorbed by plant roots, which is plant

specific. See also nonlimiting water range.

biodegradable– A substance able to be decomposed by biological processes.

biological availability–That portion of a chemical compound or element that can be taken up

readily by living organisms.

bulk density, soil ( b)–The mass of dry soil per unit bulk volume. The value is expressed as

Mg per cubic meter, Mg m-3

.

carbon cycle–The sequence of transformations whereby carbon dioxide is converted to organic

forms by photosynthesis or chemosynthesis, recycled through the biosphere (with partial

incorporation into sediments), and ultimately returned to its original state through

respiration or combustion.

carbon-organic nitrogen ratio–The ratio of the mass of organic carbon to the mass of organic

nitrogen in soil, organic material, plants, or microbial cells.

cation exchange capacity (CEC)– The sum of exchangeable bases plus total soil acidity at a

specific pH, values, usually 7.0 or 8.0. When acidity is expressed as salt extractable

acidity, the cation exchange capacity is called the effective cation exchange capacity

(ECEC) because this is considered to be the CEC of the exchanger at the native pH value.

It is usually expressed in centimoles of charge per kilogram of exchanger (cmolc kg-1

) or

millimoles of charge per kilogram of exchanger. See also acidity, total.

carbon sequestration – Storage of CO2 through biological or physical process.

chronosequence–A group of related soils that differ, one from the other, primarily as a result

of differences in time as a soil-forming factor.

clay–(i) A soil separate consisting of particles <0.002 mm in equivalent diameter. See also soil

separates. (ii) A textural class. See also soil texture. (iii) (In reference to clay mineralogy)

A naturally occurring material composed primarily of fine-grained minerals, which is

generally plastic at appropriate water contents and will harden when dried or fired.

Although clay usually contains phyllosilicates, it may contain other materials that impart

plasticity and harden when dried or fired. Associated phases in clay may include materials

that do not impart plasticity and organic matter.

compost–Organic residues, or a mixture of organic residues and soil, that have been mixed,

piled, and moistened, with or without addition of fertilizer and lime, and generally allowed

to undergo thermophilic decomposition until the original organic materials have been

substantially altered or decomposed. Sometimes called “artificial manure” or “synthetic

manure.” In Europe, the term may refer to a potting mix for container-grown plants.

composting–A controlled biological process which converts organic constituents, usually

wastes, into humus-like material suitable for use as a soil amendment or organic fertilizer.

cover crop–Close-growing crop, that provides soil protection, seeding protection, and soil

improvement between periods of normal crop production, or between trees in orchards and

vines in vineyards. When ploughed under and incorporated into the soil, cover crops may

be referred to as green manure crops.

crop rotation–A planned sequence of crops growing in a regularly recurring succession on the

same area of land, as contrasted to continuous culture of one crop or growing a variable

sequence of crops.

degradation–(i) The process whereby a compound is transformed into simpler compounds. (ii)

(no longer used in SSSA publications) The changing of a soil to a more highly leached and

a more highly weathered condition; usually accompanied by morphological changes such

as development of an A2 horizon.

evapotranspiration–The combined loss of water from a given area, and during a specified period of time, by evaporation from the soil surface and by transpiration from plants.

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field capacity, in situ (field water capacity)–The content of water, on a mass or volume basis,

remaining in a soil 2 or 3 days after having been wetted with water and after free drainage

is negligible. See also available water.

fulvic acid–The pigmented organic material that remains in solution after removal of humic

acid by acidification. It is separated from the fulvic acid fraction by adsorption on a

hydrophobic resin at low pH values. See also soil organic

GHG- Green house gases e.g. N2O, CH4, CO2, vapour etc.

gravitational water–Water which moves into, through, or out of the soil under the influence of

gravity. See also soil water, soil water potential.

green manure–Plant material incorporated into soil while green or at maturity, for soil

improvement.

green manure crop–Any crop grown for the purpose of being turned under while green or

soon after maturity for soil improvement.

greenhouse effect–The absorption of solar radiant energy by the earth's surface and its release

as heat into the atmosphere; longer infrared heat waves are absorbed by the air, principally

by carbon dioxide and water vapor, thus, the atmosphere traps heat much as does the glass

in a greenhouse.

heavy metals–Those metals which have densities >5.0 Mg m-3

. In soils these include the

elements Cd, Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, Pb, and Zn.

humus–Total of the organic compounds in soil exclusive of undecayed plant and animal

tissues, their “partial decomposition” products, and the soil biomass. The term is often used

synonymously with soil organic matter.

immobilization–The conversion of an element from the inorganic to the organic

IPCC- Intergovernmental panel on climate change

labile–Readily transformed by microorganisms or readily available to plants.

labile pool–The sum of an element in the soil solution and the amount of that element readily

solubilized or exchanged when the soil is equilibrated with a salt solution.

landscape–A collection of related landforms; usually the land surface which the eye can

comprehend in a single view.

leachate–Liquids that have percolated through a soil and that contain substances in solution or

suspension.

leaching–The removal of soluble materials from one zone in soil to another via water

movement in the profile. See also eluviation.

litter–The surface layer of the forest floor which is not in an advanced stage of decomposition,

usually consisting of freshly fallen leaves, needles, twigs, stems, bark, and fruits.

mesofauna–Nematodes, oligochaete worms, smaller insect larvae, and microarthropods.

microbial biomass–(i) The total mass of living microorganisms in a given volume or mass of

soil. (ii) The total weight of all microorganisms in a particular environment.

microfauna–Protozoa, nematodes, and arthropods of microscopic size.

microflora–Bacteria (including actinomycetes), fungi, algae, and viruses.

mineralization–The conversion of an element from an organic form to an inorganic state as a

result of microbial activity.

net primary productivity (NPP)–Net carbon assimilation by plants. NPP = GPP - respiration

losses. NPP can be estimated for a given time period as B + L + H, where B = biomass

accumulation for the period, L = biomass of material produced in the period and shed (i.e. foliage, flowers, branches), and H = biomass produced in the period and consumed by

animals and insects.

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nutrient–Elements or compounds essential as raw materials for organism growth and

development.

organic farming–Crop production system that reduces, avoids or largely excludes the use of

synthetically compound fertilizers, pesticides, growth regulators, and livestock feed

additives.

organic fertilizer–By product from the processing of animals or vegetable substances that

contain sufficient plant nutrients to be of value as fertilizers.

Pg- Petagram (1015 quadrillion metric tons)

physical properties (of soils)–Those characteristics, processes, or reactions of a soil which are

caused by physical forces and which can be described by, or expressed in, physical terms

or equations. Examples of physical properties are bulk density, hydraulic conductivity,

porosity, pore-size distribution, etc.

plant nutrient–An element which is absorbed by plants and is necessary for completion of the

normal life cycle. These include C, H, O, N, P, K, Ca, Mg, S, Cu, Fe, Zn, Mn, B, Cl, Ni,

and Mo.

porosity–The volume of pores in a soil sample (nonsolid volume) divided by the bulk volume

of the sample.

rhizosphere–The zone of soil immediately adjacent to plant roots in which the kinds, numbers,

or activities of microorganisms differ from that of the bulk soil.

runoff–That portion of precipitation or irrigation on an area which does not infiltrate, but

instead is discharged from the area. That which is lost without entering the soil is called

surface runoff. That which enters the soil before reaching a stream channel is called

ground water runoff or seepage flow from ground water. (In soil science runoff usually

refers to the water lost by surface flow; in geology and hydraulics runoff usually includes

both surface and subsurface flow.)

sedimentary rock–A rock formed from materials deposited from suspension or precipitated

from solution and usually being more or less consolidated. The principal sedimentary rocks

are sandstones, shales, limestones, and conglomerates.

soil biochemistry–The branch of soil science concerned with enzymes and the reactions,

activities, and products of soil microorganisms.

soil compaction–Increasing the soil bulk density, and concomitantly decreasing the soil

porosity, by the application of mechanical forces to the soil.

soil conservation–(i) Protection of the soil against physical loss by erosion or against chemical

deterioration; that is, excessive loss of fertility by either natural or artificial means. (ii) A

combination of all management and land use methods that safeguard the soil against

depletion or deterioration by natural or by human-induced factors. (iii) The branch of soil

science that deals with soil conservation (i) and (ii).

soil consociation–A kind of map unit comprised of delineations, each of which shows the size,

shape, and location of a landscape unit composed of one kind of component soil, or one

kind of miscellaneous area, plus allowable inclusions in either case. See also component

soil, soil complex, soil association, undifferentiated group, miscellaneous areas.

soil microbiology–The branch of soil science concerned with soil-inhabiting microorganisms,

their functions, and activities.

soil organic matter–The organic fraction of the soil exclusive of undecayed plant and animal

residues. See also humus.

soil organic residue–Animal and vegetative materials added to the soil of recognizable origin.

soil qualities–Inherent attributes of soils which are inferred from soil characteristics or indirect observations (e.g., compactibility, erodibility, and fertility).

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soil quality–The capacity of a soil to function within ecosystem boundaries to sustain

biological productivity, maintain environmental quality, and promote plant and animal

health.

soil structure–The combination or arrangement of primary soil particles into secondary units

or peds. The secondary units are characterized on the basis of size, shape, and grade

(degree of distinctness).

surface soil–The uppermost part of the soil, ordinarily moved in tillage, or its equivalent in

uncultivated soils and ranging in depth from 7 to 25 cm. Frequently designated as the

plough layer, the surface layer, the Ap layer, or the Ap horizon. See also topsoil.

sustainability–Managing soil and crop cultural practices so as not to degrade or impair

environmental quality on or off site, and without eventually reducing yield potential as a

result of the chosen practice through exhaustion of either on-site resources or non-

renewable inputs.

Tg – I million metric tons

tillage–The mechanical manipulation of the soil profile for any purpose; but in agriculture it is

usually restricted to modifying soil conditions and/or managing crop residues and/or weeds

and/or incorporating chemicals for crop production.

soil management–The combination of all tillage operations, cropping practices, fertilizer,

lime, and other treatments conducted on or applied to the soil for the production of

plants.

strip cropping (field strip cropping, contour strip cropping)–The practice of growing two

or more crops in alternating strips along contours, often perpendicular to the prevailing

direction of wind or surface water flow.

strip planting (strip till planting)–A method of simultaneous tillage and planting in isolated

bands of varying width, separated by bands of erect residues essentially undisturbed by

tillage.

topsoil–(i) The layer of soil moved in cultivation. Frequently designated as the Ap layer or Ap horizon. See also surface soil. (ii) Presumably fertile soil material

water drop penetration time (WDPT)–A measure of soil water repellency which uses the

imbibition time of drops of prescribed aqueous solutions as a discriminator.

water-stable aggregate–A soil aggregate which is stable to the action of water such as falling

drops, or agitation as in wet-sieving analysis.

weathering–The breakdown and changes in rocks and sediments at or near the

xenobiotic–A compound foreign to biological systems. Often refers to human-made

compounds that are resistant or recalcitrant to biodegradation and/or decomposition.

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10. Appendix 1 PAS 2050 protocol

10.1 The PAS 2050 protocol and the Pesticide Paradox

From a climate-change point of view, a constant, or even increasing, level of soil C

content in apple orchards is desirable, indicating that no net greenhouse gas

emissions from the soils are associated with the orchard operations. The greenhouse

gases resulting from soil-C losses caused by apple orchard management that fails to

balance the continuous loss of soil C by an equivalent input of organic matter might, in

the future, be added to the life cycle greenhouse gas emissions of apples in the PAS

2050 (BSI 2008). External inputs of long-lived C, such as biochar, or even some parts

of compost, or manure may lead to a long-term increase of the soil organic-C

contents.

The current version of the PAS 2050 does not consider any greenhouse-gas

emissions, or reductions, that arise from a change of the soil-C contents. The following

paragraph in the PAS2050 refers to the issue of the change of soil C contents:

PAS2050:2008, 5.6 Treatment of soil C change in existing agricultural

systems: Changes in the C content of the soils, either emissions or

sequestration, other than those arising from direct land use change shall be

excluded from the assessment of GHG emissions under this PAS.

…Note 2: While it is recognized that soils play an important part in the C cycle

both as a source and sink for C, there is considerable uncertainty regarding

the impacts of different techniques in agricultural systems. For this reason,

emissions and sequestration arising from changes in soil C are outside the

scope of this PAS. Inclusion of C storage in soils will be considered further in

future revisions of this PAS.

We highlight the anomaly that this ignorance of soil C creates by considering whether

a grassed tree-row would have a C footprint smaller than a herbicided tree rows. The

functional unit for an apple orchard would be a kg of apples, and the C footprint, or

component of the footprint, is then expressed as being kg-CO2 equivalent, CO2-e, as it

needs to take into account all greenhouse gases with their differing global warming

potentials (GWP). For example, the GWP of nitrous oxide gas, N2O, is 298 because

of its persistence in the atmosphere.

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A life cycle assessment (LCA) using the PAS 2050 of the use of herbicides to create a

bare strip along the tree results in a contribution of 1.3 g-CO2-e kg-1

of apples

produced. This includes the fuel use and embodied C in the herbicide. If the tree-row

were grassed, and then mowed and the clippings returned to the soil, the IPCC rules

require the 1% of the applied N in the clippings must be accounted for as emitted N2O.

This means that the footprint resulting from having a grass strip and mowing it is 4.2 g-

CO2-e kg-1

of apples produced. So to obtain a reduction in the C footprint, it would

seem sensible to adopt the use of a herbicided tree-row.

Changes in soil C are ignored in the PAS 2050. Yet if they were, then the changes in

soil C listed in Figure 3.22 for an integrated orchard would result in a C footprint

calculated at 730 g-CO2-e ha-1

yr-1

. This footprint size is nearly three times greater

than the footprint that results for all in-orchard operations and emissions!

So although there may be difficulties and uncertainties in bringing in soil C to

footprinting protocols, it is assuredly a large part of the overall orchard footprint.

Ignoring it could send the wrong signals. For the use of a herbicide strip, whatever its

other benefits might be, is definitely not a good option for reducing the C footprint of

production from the atmosphere‘s perspective. We can only hope that soil C will soon

be taken into account in footprinting protocols, for then their use to find reduction

options will benefit efforts for climate change mitigation.

a literature review of soil carbon under pasture, horticulture and

Documents