Supplemental On-Line Material Soil Carbon Pool and the Global Carbon Cycle

Four global issues of the 21st century directly interacting with the soil organic carbon

(SOC) pool and its dynamics are: (i) the atmospheric concentration of CO2 at 379 ppm and

increasing at the rate of 1.8 ppmv or more, (ii) tropical deforestation estimated at 7.1 million

hectares (Mha)/yr (S1), (iii) soil degradation at moderate plus level affecting 1.2 billion hectares

(S2), (iv) global food insecurity affecting about a billion persons (S3, S4, S5), and (v) over-

reliance on fossil fuels for energy. Meeting the food demand in developing countries, which may

increase drastically by 2025 and 2050 (S3), necessitates restoring soil quality through

enhancement of its SOC pool.

Five principal global C pools are inter-connected (Fig. S1), and fluxes among them are

impacted by anthropogenic perturbations (e.g., deforestation, soil cultivation, draining wetlands,

fossil fuel combustion). The soil C pool is large (S6, S7) and an important component of the

terrestrial pool. The SOC pool has been steadily decreasing since the dawn of settled agriculture,

about 10,000 years ago. The magnitude and rate of SOC depletion are exacerbated by soil

degradation processes such as accelerated erosion by water and wind. Soil erosion is a 4-step

process: detachment, transport, redistribution and deposition of soil particles. Being located in

the vicinity of soil surface and of low density (1.3 Mg/m3 for SOC versus 2.6 Mg/m3 for

minerals), SOC is preferentially removed by surface runoff and blowing wind. However,

breakdown of aggregates, with exposure of organic matter hitherto encapsulated and physically

protected from microbial processes along with changes in soil moisture and temperature regimes

caused by soil erosion, increases mineralization of transported and redistributed soil organic

2

matter. As much as 20 to 30% of the displaced carbon may be mineralized prior to and after

deposition or burial in the depressional sites (S8, S9).

Carbon Sequestration

There are four strategies of C sequestration: transfer of atmospheric CO2 into the

terrestrial pool (trees and soils) called the terrestrial sequestration, that into geologic strata

(abandoned coal mines, low yielding oil wells or saline aquifers) or the geologic sequestration,

increasing productivity of oceanic biota through iron fertilization or injecting CO2 deep into the

ocean or the oceanic sequestration, and precipitation of industrial CO2 as carbonates of Ca+2 or

Mg+2 or the chemical sequestration. Soil C sequestration implies enhancing concentration/ stocks

of SOC and secondary carbonates through adoption of RMPs on soils of agricultural, grazing and

forestry ecosystems, and conversion of degraded soils and drastically disturbed lands to

restorative land uses. Soil C sequestration is a natural, and considering the technological know-

how during the first decade of the 21st century, also a very cost-effective, risk-avoidance and an

environmentally-friendly option (Table S1). Yet, its attainable capacity is finite (one-half to two-

thirds of the historic loss of 78+ 12 Gt) and can be filled over 2 to 5 decades. In some rare cases,

the soil C sink capacity in managed ecosystems may exceed those of native ecosystems through

adoption of innovative technology (Fig. S2). For example, the net primary productivity (NPP) of

soils of acid tropical savannas in south America is constrained under natural ecosystems by low

levels of P and toxic concentrations of Al+3 and Mn+3. Applications of P fertilizers and lime to

nullify the effects of Al+3 and Mn+3 by Ca+2 and Mg+2 can alleviate these soil-related constraints,

increase NPP, return more biomass to the soil and eventually enhance SOC pool in managed

ecosystems more than that under natural conditions (S10, S11, S12, S13). Observed rates of SOC

3

sequestration on agricultural ecosystems are more on medium than coarse-textured soils,

structurally active than inert soils, moderately drained than excessively drained soils, and in cool

and humid than warm and arid climates (S14-S25) (Table S2). The humification efficiency or the

efficiency of conversion of biomass C into humus C is about 5 to 15% in humid temperate

climates(S26, S27) and 2 to 5% in dry tropical regions.

Establishing trees in degraded ecosystems enhances SOC pool and improves soil quality.

Regenerative fallow, tree plantations and some agroforestry systems can mimic important

characteristics of undisturbed ecosystems, increase the biomass returned to the soil and enhance

SOC pool (S28-S33). In addition to C sequestration in biomass and soil, tree plantations of site-

adapted species (e.g., Acacia, Albizia, Casuarina, Eucalyptus, Gmelina, Leucaena, Pinus,

Shorea) are needed for timber and more importantly as fuel wood for household cooking in

developing countries. Establishments of planted fallows and tropical plantations is a useful

strategy whenever natural regeneration is not effective in restoring degraded soils and

ecosystems. Growing cover crops in degraded agricultural soils is important to enhancing SOC

pool and improving soil quality (Fig. S3).

Crop Residue and Biofuel

The importance of crop residue as a biofuel re-emerged in the first decade of the 21st

century as a potential source of ethanol or hydrogen (S34). One t of corn residue is equivalent to

300 L of ethanol (S35), 3 x 106 Kcal, 16 x 106 BTU or 18.6 x 109 J of energy (S36). Assuming

that 30% of the crop residue can be removed from U.S. cropland for ethanol production, it has

been estimated that 42 Mt (million tons) of corn and 8 Mt of wheat straw may be available for

4

biofuel production in the U.S. to produce 5 GL (gigaliter or billion liter) of ethanol (S37, S38).

However, this is only 2.4% of the 617 GL of ethanol consumed in the U.S. in 2000 (S39).

Removing residue of corn or other crops for biofuel, primarily for household cooking as

in Asia and Africa or for production of ethanol or hydrogen or even co-firing with coal as in

developed countries, reduces the amount of residue returned for SOC sequestration, soil quality

enhancement and soil and water conservation. There are indeed numerous competing uses of the

crop residue. Although crop residue is a renewable resource and a large quantity is produced

annually (Table S3) (S36, S40, S41), its indiscriminate removal can have long-lasting adverse

effects on soils and the environment. Nonetheless, biofuel is an important strategy of reducing

fossil fuel combustion. Therefore, identifying lands for production of biomass as stock for

biofuel production by growing specifically adapted energy crops (e.g., switch grass, willows,

poplar and tree plantations) is an integral component of the holistic strategy of mitigating the

climate change. Most of the crop residue produced must be recycled for soil quality enhancement

and SOC sequestration.

Ancillary Benefits of Soil Carbon Sequestration

Over and above reducing the rate of enrichment of atmospheric CO2, there are at least 30

on-site and off-site ancillary benefits of SOC sequestration (Table S4). The most significant

benefit of enhancing SOC stock lies in improving soil quality and advancing food security for

resource-poor farmers of Africa, Asia and elsewhere in the tropics, and reducing the risks of

hidden hunger through improvement in nutritional value of the food grown on restored soils of

improved quality.

5

Sustainable Management of Agricultural Soils

Conversion of world croplands from plow till to no-till farming is a high priority.

Worldwide no-till farming is practiced only on 70 Mha, comprising 22 Mha in the U.S. (20% of

cropland), 17 Mha in Brazil (45%), 13 Mha in Argentina (50%), 9 Mha in Australia (17%), 4

Mha in Canada (9%), and 1 Mha in Paraguay (60%) (S42). Adoption of no-till farming in Asia

and Africa is seriously constrained by removal of crop residues for fodder and animal waste for

household fuel, lack of appropriate seeding equipment which can seed crops on unplowed and

mulched surface, and availability of herbicides only at prohibitively expensive prices. In addition

to being a health hazard, biomass burning for household cooking in South Asia has reportedly

influenced the climate and hydrological cycle (S43). Along with no-till based on mulch farming

and cover crops/forages in the rotation cycle, use of integrated nutrients and pest management

are equally important to reducing C-based input. For example, the introduction of Bt cotton

varieties with engineered insect resistance has reduced the use of insecticides in the U.S. by 1.2

million kg with the attendant reduction of about 6 million kg of C/yr (S44). Emission of N2O

from no-till soils is also an issue (S45-S50). However, there are management techniques which

reduce production of N2O and CH4 by agricultural practices. It is the realization of the need for

the stewardship of the land through adoption of good farming practices that will restore degraded

soils, enhance soil quality, achieve food security, and at the same time reduce the rate of

enrichment of atmospheric CO2. The strategy of soil C sequestration is an important solution to

all three global issues addressed by U.N. conventions on global climate change, desertification

control and biodiversity. It is a win-win option until non-carbon fuel sources become available.

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6

S2 L.R. Oldeman, Soil Resilience and Sustainable Land Use (CAB International,

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S4 C. Somerville, J. Briscoe, Science 292, 2217 (2001).

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S9 P. Jacinthe, R. Lal, Land Degrad. & Dev. 12, 329-339 (2001).

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S12 C.C. Cerri et al., For. Ecol. Manage. 38, 247-257 (1991).

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S18 R.C. Dalal, Soil Sci. Soc. Am. J. 53, 1511-1515 (1989).

S19 G. Dersch, K. Bohm, Soil & Tillage Res. 52, 73-81 (2001).

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S21 R. Lal, Prog. Env. Sci. 1, 307-326 (1999).

7

S22 R. Lal, Adv. Agron. 71, 145-191 (2000).

S23 C.A. Campbell et al., Can J. Soil Sci. 80, 193-202 (2000).

S24 M.S. Pal, Aust. J. Expl. Agric. 43, 603-615 (2003).

S25 D.E. Buschiazzo et al., Soil & Tillage Res. 49, 105-116 (1998).

S26 S.W. Duiker, R. Lal, Soil & Tillage Res. 52, 73-81 (1999).

S27 P.A. Jacinthe et al., Soil & Tillage Res. 67, 750-758 (2002).

S28 J. Beer et al., Agroforestry Systems 12, 229-249 (1990).

S29 F. Montagnini, C. Porras, Env. Management 22, 459-470 (1998).

S30 J.A. Parrotta, Agric. Ecosyst. Environ. 41, 115-133 (1992).

S31 C.K. Smith et al., For. Ecol. Manage. 164, 257-263 (2002).

S32 N. Sanginga, M.J. Swift, Agric. Ecosyst. Environ. 41, 55-65.

S33 E. Cuevas et al., Plant Soil 135, 257-268 (1991).

S34 USDOE (Office of Energy Efficiency and Renewable Energy, Washington, D.C.,

www.ott.gov/biofuels/energysecurity.html#reliance, posted 15 May 2002, Verified 8

October 2003).

S35 D.E. Buschiazzo et al., Soil & Tillage Res. 49, 105-116 (1998).

S36 R. Lal, J. Sust. Agric. 5, 51-78 (1995).

S37 R.G. Nelson, Biomass Energy 22, 349-363 (2002).

S38 W.W. Wilhelm et al., Agron. J. 96, 1-17 (2004).

S39 Energy Information Administration, Alternative to Traditional Transport Fuels 2000

(www.eia.doe.gov.cneaf/alternate/page/datatables/table10.html).

S40 FAO, Production Yearbook, FAO, Rome, Italy (1991).

S41 FAO, Production Yearbook, FAO, Rome, Italy (2001).

8

S42 R. Derpsch, J.R. Benites, Second World Congress on Conservation Agriculture, 11-15

August 2003, Ignassu Falls, Brazil (2003).

S43 V. Ramanathan et al., Science 294, 2119-2124 (2001).

S44 J. Carpenter, Science 292, 637 (2001).

S45 N. Chareonsilp, C. Buddhaboon, P. Promnart, R. Wassmann, R.S. Lantin, Nutr. Cycl.

Agroecosyst. 58, 121-130 (2000).

S46 E. Ishibashi et al., Jap. J. Soil Sci. Plant Nutr. 72, 535-541 (2001).

S47 M. Hanaki, T. Ito, M. Saigusa, Japanese J. Soil Sci & Plant Nutr. 73, 135-143 (2002).

S48 G.P. Robertson et al., Science 289, 1922-1925 (2000).

S49 C.C. Passianoto et al., Biol. & Fert. Soils 38, 200-208 (2003).

S50 E.M. Baggs et al., Plant Soil 254, 361-380 (2003).

9

Figure captions for SOM figures Fig. S1 Estimates of five principal global C pools and fluxes between them. The mean residence

time (MRT) is computed at a global scale as a ratio of the pool:average annual flux. The MRT for biota and soil computed on a global scale may be misleading because at a species or pedon scale it depends on site-specific conditions. For example, the MRT for some recalcitrant humic fractions may be thousands of years, and also depends on land use and management. The MRT in biota depends on species, and may also be thousand of years for trees and tree products.

Fig. S2 A Schematic of the soil C dynamics upon conversion from a natural to agricultural

ecosystem, and subsequent adoption of recommended management practices (RMPs). In most cases, the maximum potential equals the magnitude of historic C loss. Only in some soil-specific situations, the adoption of RMPs can increase SOC pool above that of the natural system. An example of this is acid savanna soils of South America (Llanos, Cerrados) where alleviation of soil-related constraints can drastically enhance the SOC pool.

Fig. S3 Incorporating cover crops in the rotation cycle is important to enhancing SOC pool and

improving soil quality. The cover crop shown in this photo is Mucuna utilis being grown on a degraded Alfisol in western Nigeria. This fast growing legume produces a large quantity of biomass, smothers weeds, fixes nitrogen, and dies naturally at the end of the long dry season so that corn and other corps can be grown through the dead sod with no-till farming with a minimal input of herbicides.

10

Table S1. Comparative analyses of soil versus geologic sequestration. C sequestration strategy Parameter Soil/terrestrial Geologic 1. The process

2. C sink capacity

3. Rate of C sequestration

4. Technology application

5. Duration

6. Cost/Mg of C sequestered

7. Risks

- Crop/biomass yield reduction

- Human health

- Environmental

- Leakage/permanence

8. Monitoring and verification

9. Regulatory measures

Natural

Finite (50-100 Gt)

~1 Gt C/yr

Immediate

25 to 50 years

Negative, none or low

Minor to low

Minor to low (pesticides)

Positive

Only with a change in land use

Verification possible from

continuation of the

recommended practices,

simple and routine methods

available

Monetary incentive may be

required

Engineering

Very large

High

10 to 20 years (by 2020)

Very long time

High

--

Can be high

Unknown

High risks

Complex and expensive

procedures

Legislative and policy

measures needed

11

Table S2. Range of soil carbon sequestration rates for commonly adopted management practices in different climates (data compiled from S10, S12-S24, S26, S29-S33 and others cited in Fig. 2). Temperate climates Tropical climates Technological options Humid Semi-arid Humid Semi-arid ---------------------------t C/ha/yr----------------------- A. Cropland

(i) No-till/conservation tillage

(ii) Cover crops and elimination of bare

fallow

(iii) Manuring (10-20 Mg/ha/yr)

(iv) Complex rotations with deep rooted

plants

(v) Integrated pest management

(vi) Irrigation and water management

(vii) Agroforestry

(viii) Rice paddies

0.5-1.0

0.2-0.4

0.5-1.0

0.2-0.4

0.1-0.2

--

0.4-0.6

--

0.1-0.5

0.1-0.2

0.2-0.5

0.05-0.1

0.05-0.1

0.2-0.5

0.2-0.4

--

0.2-0.5

0.1-0.2

0.2-0.4

0.1-0.2

0.02-0.05

--

0.2-0.5

0.2-0.5

0.1-0.2

0.05-0.1

0.1-0.2

0.02-0.05

0.01-0.2

0.4-0.2

0.1-0.2

0.05-0.1

B. Grazing land

(i) Improved pastures

(ii) Fertility management

(iii) Grazing management

0.4-0.6

0.1-0.2

0.2-0.4

0.1-0.2

0.05-0.1

0.1-0.2

0.5-1.0

0.2-0.4

0.4-0.6

0.1-0.2

0.05-0.1

0.1-0.2

C. Forest lands

(i) Timber harvest

(ii) Site preparation

(iii) Improved species

(iv) Stand management

0.1-0.2

0.1-0.2

0.2-0.4

0.1-0.2

0.05-0.1

0.05-0.1

0.1-0.2

0.05-0.1

0.2-0.4

0.1-0.2

0.4-0.8

0.1-0.2

0.1-0.2

0.05-0.1

0.1-0.2

0.05-0.1

D. Degraded soils

(i) Soil erosion by water

(ii) Soil erosion by wind

(iii) Salt affected soils

(iv) Mine soils

(v) Wetland restoration

0.5-1.0

--

--

0.5-1.5

0.5-1.0

0.2-0.4

0.1-0.2

0.5-1.0

0.2-0.5

0.2-0.4

0.2-0.4

--

--

0.2-0.4

0.2-0.4

0.1-0.2

0.05-0.1

0.2-0.5

0.1-0.2

0.1-.0.2

12

Table S3. Estimates of the world grains, tubers and crop residue production in 1991 and 2001 [land area and grain production are calculated from FAO (S36, S40, S41), and residue production was estimated using the harvest index for each crop]. Area (Mha) Residue production (106 Mt) Crop 1991 2001 1991 2001 Cereals Barley Corn Millet Oats Rice Rye Sorghum Wheat Others Total

76 129 37 21 148 14 45 224 10 704

54 138 37 13 152 10 43 214 10 671

254 479 44 34 780 41 87 826 18 2563

212 609 44 27 890 35 87 875 23 2802

Legumes Beans Broad beans Chick peas Groundnut Lentils Peas Pulses Soybeans Total

26 3 8 20 3 9 70 55 194

23 2 9 26 4 6 66 76 212

18 5 11 23 2 16 60 103 238

17 4 6 35 3 11 52 177 305

Oil Crops Linseed Rapeseed Safflower Seed cotton Sesame Sunflower Total

4 20 1 38 7 17 87

3 24 1 16 8 18 70

3 41 1 90 4 23 162

2 54 1 24 6 21 108

Sugar Crop Sugarbeet Sugarcane Total

9 17 26

6 19 25

76 264 340

59 314 373

Tubers Potato Sweet Potato Total Grand total

279 177 9 465 1476

322 193 9 524 1502

47 67 31 145 3448

59 77 34 170 3758

13

Table S4. Ancillary benefits of enhancing soil organic carbon pool. On-site benefits Off-site benefits

1. Improvement of water quality (i) Decrease in transport of pollutants (ii) Biodegradation and denaturing of pollutants and contaminants (iii) Reduction in sediment load and siltation of water bodies (iv) Decrease in non-point source pollution (v) Reduction in risks of hypoxia in water bodies (vi) Less damage to coastal ecosystems (vii) Low risks by floods and sedimentation to aquacultures (shrimp, fisheries etc.) (viii) Decrease in transport of pollutants out of the ecosystem 2. Improvement in air quality (i) Reduction in rate of enrichment of greenhouse gases (ii) Decrease in wind-borne sediments 3. Improvement in biodiversity (i) Increase in soil biodiversity (ii) Improvement in wildlife habitat and species diversity on restored ecosystems (iii) Improvement in aesthetic and cultural value

1. Improvement in soil quality (i) Increase in available water capacity (ii) Increase in nutrient retention (iii) Improvement in soil structure and tilth (iv) Buffering against changes in pH (v) Enhancement of soil biotic activity (vi) Improvements in soil moisture and temperature regimes 2. Increase in agronomic/forest productivity (i) Increase in crop yield (ii) Increase in use efficiency of input (e.g., fertilizers, water) (iii) Decrease in losses of soil amendments by runoff, erosion and leaching (iv) Improvements in edaphic conditions 3. Sustainability and food security (i) Increase in sustainable use of soil and water resources which have been drastically perturbed (ii) An important step in achieving food security (iii) Additional income from trading C credits (iv) Improvement in nutritional value of food (especially micronutrients) and avoidance of hidden hunger

4. Desertification control (i) Restoration of desertified lands (ii) Reversal of degradation trends (iii) Strengthening elemental recycling mechanisms