Agricultural practices that favourthe increase of soil organic matter

Philippe Ciais and Pete Smith

• Current soil C stocks distribution• Vulnerability of soil C• Land use change• Agricultural practice• Future climate change• Uncertainties and outlook

Outline

Ecosystem carbon balance

Chapin 2002

Soil and vegetation ?

<1>>1,000,00077,000,000Sediments &rocks

~5.6(-8.1)-1200 (- >6000)Fossil carbon

-3-5750Atmosphere

601-1000800Vegetation

52 (het.)60-80 total

<1-5,0002300+1600 frozen and wet

Soils

18 (?)2,00037,000Deep ocean

Fluxinto atm. [Pg yr-1]

Turnovertime [yr]

Reservoir size[Pg]

after Reeburgh et al. (1997), Sabine et al. (2004), Canadell (2007), pers. com.

Global carbon stocks in ecosystems

after Gruber et al. 2004, Lal, 2005, Davidson & Janssens 2006

Most of soil C stock is at high latitudesCroplands hold small amount of soil CGrasslands hold variable amount of soil C

How soil C is sampled

Stock vs. sequestration: partitioningbetween vegetation and soil

frac

tion

Soil is a larger C stock but a smaller C sink than vegetation

Soil carbon stock is large & vulnerable

after Davidson & Janssens 2006

-400

-200

0

200

400

600

800

Stock Current gain percentury

Potential loss bycentury

2000Pg

C

Climate factors affecting soil C balance

Temperature Waterbalance CO2

N-deposition

Primaryproduction

Soil carbon

Decompo-sition - +

Tempe-rature

Climate factors affecting soil C balance

Temperature Waterbalance CO2

N-deposition

Primaryproduction

Soil carbon

Decompo-sition - +

Tempe-rature

Human factors affecting soil C balance

tillage

Land use changeFarmers practice

erosion

irrigation

fertilization

Global changes in biomass and Soil Caffected by climate and land use change

Former Soviet Union China

Europe

Temperate regions

USA

Contrasted land use histories ;Large depletion of soil C pools in the first half of

20th Century in USA and FSU by agriculture

Soil C change associated toland use change

What are our emission reduction targets?

• European target“To reach the 2°C goal, the EU needs to adopt areduction target of at least 30% below 1990 levelsin 2020 and additional action to reduce emissionsin developing countries”

• Corresponds to an emission reduction target of0.3 Pg C y-1

• Corresponds to an emission reduction efforthigher than this

• EU target means a cumulative emission reduction of 3Billion tC between 2010 and 2020

• If this sequestration had to be realized over Europeanagricultural lands (250 Million ha), it would imply toincrease ecosystem carbon by 12 tC ha-1 in one decade.

• This means an extra sink of 1.2 tC ha-1 y-1 in one decade• This sink need can be compared to the current

‘inadvertent’ C balance of agricultural land(Carboeurope numbers, Schulze et al. 2010)➩ Cropland is a source of 0 - 2.5 t C ha-1 y-1

➩ Grassland is a sink of 0.6 - 0.9 t C ha-1 y-1

Emission reduction targets and currentagricultural land C budgets ?

How does soil C sequestration work?

Organic carbon source

Add to soil

C in soil

CO2

Some C is stabilisedin the soil

Soil

Soil C cycle

e.g. residue management,organic amendments, increased plant C input…

Increase C inputs…e.g. restore & rewet farmedorganic soils

...or reduce C losses

= microbe C = C inside aggregate

C

No-till

C

C

CC

C

C

Tillage

Tillage breaksopen aggregates

= weatheringKey:

COrganic material (C)more exposed to microbial attack and weathering

How does soil C sequestration work? –reduced disturbance

Activity Practice Specific management change Increase C inputs

Decrease C losses

Reduce disturbance

Cropland management Agronomy Increased productivity XRotations XCatch crops XLess fallow XMore legumes XDeintensification XImproved cultivars X

Nutrient management Fertilizer placement XFertilizer timing X

Tillage / residue management Reduced tillage XZero tillage XReduced residue removal X XReduced residue burning X X

Upland water management Irrigation XDrainage X

Set-aside and land use change Set aside X XWetlands X X

Agroforestry Tree crops inc. Shelterbelts etc. X XGrazing land management Livestock grazing intensity Livestock grazing intensity X

Fertilization Fertilization XFire management Fire management XSpecies introduction Species introduction XMore legumes More legumes XIncreased productivity Increased productivity X

Organic soils Restoration Rewetting / abandonment X XDegraded lands Restoration Restoration X X X

Mechanisms for soil C sequestration in agriculture

Smith et al. (2008)

Manure – large & long-lasting effects

Rothamsted Hoosfield – Jenkinson 1998

0

20

40

60

80

100

1850 1890 1930 1970

Year

Organic C in Soil(t ha-1)

Farmyard manure annually

Farmyard manure 1852-1871 nothing thereafter

Unmanured

Carbon depleted soils have large potential toaccumulate C in early recovery stage

Conclusions• Farmland abandonment

caused C sink• ~0.5 tC ha-1 y-1 over past

decade

Cereals Steppe

Former Soviet Union

20 Mha abandonnedfrom 1990 to 2000

7

Example : the largest LUC change in the northern hemisphere !

This process causes net Caccumulation in soils

Vuichard et al, Glob. Biogeoch. Cycl. (2008)

Year after abandonment

gC m

-2

Grass biofuel production vs. C sequestration

Future climate changeimpacts the results

8

How many years until biofuel cultivation overpasses passive Cseuestration by return to natural steppe ?

-200

0

200

400

600

800

1000

0 10 20 30 40 50

Nombre d'années de production

Gai

n ca

rbon

é cu

mul

é (g

C m

-2)

Avec évolution climatique simuléepar IPSL-CM4

Sans évolution climatique

Vuichard et al, 2010

+50%Current climate

Global mitigation potential in agriculture

-200

0

200

400

600

800

1000

1200

1400

1600

Crop

land

man

agem

ent

Wat

er m

anag

emen

t

Rice

man

agem

ent

Setas

ide,

LUC

&ag

rofo

restr

y

Graz

ing

land

man

agem

ent

Resto

re c

ultiv

ated

orga

nic s

oils

Resto

re d

egra

ded

lands

Bioe

nerg

y (so

ilsco

mpo

nent

)

Live

stock

Man

ure

man

agem

ent

Mitigation measure

Glob

al b

ioph

ysic

al m

itiga

tion

pote

ntia

l (M

t CO2-e

q. y

r-1)

N2OCH4CO2

Smith et al. (2008)

A potential sink of 0.9 Pg C y-1

Compared to :• Global land sink today 4.7 ± 1.2 Pg C y-1

• Global land use source 1.2 ± 0.7 Pg C y-1

-200

0

200

400

600

800

1000

1200

1400

1600

1800

Sout

heas

t Asi

a

Sout

h A

mer

ica

East

Asi

a

Sout

h A

sia

East

ern

Afr

ica

Rus

sian

Fed

erat

ion

Nor

th A

mer

ica

Wes

tern

Eur

ope

Wes

tern

Afr

ica

Cen

tral A

sia

Nor

ther

n Eu

rope

Mid

dle

Afr

ica

East

ern

Euro

pe

Oce

ania

Sout

hern

Eur

ope

Cen

tral A

mer

ica

Nor

ther

n A

fric

a

Wes

tern

Asi

a

Sout

hern

Afr

ica

Car

ribe

an

Japa

n

Poly

nesi

a

Region

Mt C

O2-

eq. y

r-1

Smith et al. (2007)

High and low estimates of the mitigation potentialin each region

A potential European sink 200 Tg C y-1

Current sink in European agricultural land 20 Tg C y-1

Effect of C price on implementation

Smith et al. (2007)

0

200

400

600

800

1000

1200

1400

Res

tore

cul

tivat

edor

gani

c so

ils

Cro

plan

dm

anag

emen

t

Gra

zing

land

man

agem

ent

Res

tore

deg

rade

dla

nds

Ric

e m

anag

emen

t

Live

stoc

k

Seta

side

, LU

C &

agro

fore

stry

Man

ure

man

agem

ent

Measure

Mt C

O2-

eq. y

r-1

up to 20 USD t CO2-eq.-1up to 50 USD t CO2-eq.-1up to 100 USD t CO2-eq.-1

Global mitigation potential in agriculture(Mt CO2-eq. yr-1)

Price range (USD t CO2-eq. -1)

Scenario 0-20 0-50 0-1000->>100 (technical

potential)

B1 1925 2384 3149 5480

A1b 1982 2439 3254 5670

B2 2047 2495 3330 5844

A2 2119 2549 3330 5957

Smith et al. (2007)

Energy supply

0

1

2

3

4

5

6

7

<20<50

<100

<20<50

<100

GtCO2-eq

Transport Buildings Industry Agriculture Forestry Waste

Non-OECD/EITEITOECDWorld total

US$/tCO2-eq

Global economic mitigation potential fordifferent sectors at different carbon prices

IPCC WGIII (2007)

How do we cut GHG emissions andhow much will it cost?

From: McKinsey (2009) - Pathways to a low-carbon economy Version 2 of the GlobalGreenhouse Gas Abatement Cost Curve

How do we cut GHG emissions andhow much will it cost?

From: McKinsey (2009) - Pathways to a low-carbon economy Version 2 of the GlobalGreenhouse Gas Abatement Cost Curve

Smith (2008) International Journal ofAgricultural Sustainability 6(3),169–170

• “There are a number of well rehearsed arguments against reliance oncarbon sequestration for tackling climate change, involving saturationof the carbon sink (the carbon is only removed from the atmospherewhile the tree is growing or until the soil reaches a new equilibriumsoil carbon level; Smith, 2005), permanence (carbon sinks can bereversed at any stage by deforestation or poor soil management;Smith, 2005), leakage/displacement (e.g. planting trees in one arealeads to deforestation in another; Intergovernmental Panel on ClimateChange (IPCC), 2000), verification issues (can the sinks be measured;Smith, 2004), and total effectiveness relative to emission reductiontargets (only a fraction of the reduction can be achieved throughsinks; IPCC, 2007)”.

Soil CVegetation C

Time since management change

C s

tock

Management change

Saturation – the time course of Csequestration

• Sink saturation ~ 20-100 years• Sink strength declines towards new equilibrium

Smith (2004a)

Permanence

25

35

45

55

65

75

85

1844 1894 1944 1994 2044 2094

Year

Tota

l SO

C to

23

cm (t

C h

a-1)

Management change

Conversion to low-input cropland

Manure treatment in red, Woodland in blue Smith (2005)

Leakage / displacement: are we actuallysequestering carbon or just moving it about?

Farm with more manure Farm with less manure

Manure Manure Mineral N

More manure here….but……..less manure here

Effect over the whole cropland area = zero

Value of C sequestered

No. of samples required to demonstrateincrease in soil C

Cost

Zero return

Verification issues

Smith (2004b)

Value of C sequestered

Accuracy of estimate(or precision if only a change is verified)

Cost

Zero return

Minimum accuracyrequired for verification

Progress in measurement

technology

“Trying to sequester the geosphere inthe biosphere”

• The C we release through fossil fuel burning has been locked up for~300 Million years and was accumulated over many millions of years –we are trying to lock that up over years / decades – it does not addup!

• “It is easier to leave the marbles in the jar than to tip them out andtry to pick them all up again” W.H. (Bill) Schlesinger

• Soil C sequestration is time limited, non-permanent, difficult to verifyand is no substitute for GHG emission reduction

• Soil C sequestration may have a role in reducing the short termatmospheric CO2 concentration, and buying us time to developlonger term solutions, largely in the energy sector

Conclusions

• Soil C sequestration globally has a large, cost-competitive mitigation potential

• Useful to meet short / medium term targets –especially if these are high (e.g. in UK)

• Many co-benefits – soil fertility, workability,water-holding capacity etc. (see other talk)

• Uncertainties are very large on long termsequestered C ; verification is not in place

• Don’t forget the limitations: time limited, notpermanent, doesn’t replace genuine emissionreduction