Agricultural practices that favour the increase of soil organic matter. Philippe Ciais and Pete Smith

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1 Agricultural practices that favour the increase of soil organic matter Philippe Ciais and Pete Smith

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

3 Ecosystem carbon balance Chapin 2002

4 Soil and vegetation? Reservoir size [Pg] Turnover time [yr] Flux into atm. [Pg yr -1 ] Sediments & rocks 77,000,000 >>1,000,000 <1 Deep ocean 37,000 2, (?) Soils 2300 <1-5, (het.) frozen and wet total Vegetation Atmosphere Fossil carbon 1200 (- >6000) - ~5.6(-8.1) after Reeburgh et al. (1997), Sabine et al. (2004), Canadell (2007), pers. com.

5 Global carbon stocks in ecosystems after Gruber et al. 2004, Lal, 2005, Davidson & Janssens 2006 Most of soil C stock is at high latitudes Croplands hold small amount of soil C Grasslands hold variable amount of soil C

6 How soil C is sampled

7 Stock vs. sequestration: partitioning between vegetation and soil fraction Soil is a larger C stock but a smaller C sink than vegetation

8 Soil carbon stock is large & vulnerable PgC Stock Current gain per century Potential loss by century after Davidson & Janssens 2006

9 Climate factors affecting soil C balance Temperature Water balance CO 2 N- deposition Temperature Decomposition - + Primary production Soil carbon

10 Climate factors affecting soil C balance Temperature Water balance CO 2 N- deposition Temperature Decomposition - + Primary production tillage irrigation Soil carbon fertilization erosion Human factors affecting soil C balance Land use change Farmers practice

11 Global changes in biomass and Soil C affected by climate and land use change

12 Temperate regions USA Europe Former Soviet Union China Contrasted land use histories ; Large depletion of soil C pools in the first half of 20th Century in USA and FSU by agriculture

13 Soil C change associated to land use change

14 What are our emission reduction targets? European target To reach the 2 C goal, the EU needs to adopt a reduction target of at least 30% below 1990 levels in 2020 and additional action to reduce emissions in developing countries Corresponds to an emission reduction target of 0.3 Pg C y -1 Corresponds to an emission reduction effort higher than this

15 Emission reduction targets and current agricultural land C budgets? EU target means a cumulative emission reduction of 3 Billion tc between 2010 and 2020 If this sequestration had to be realized over European agricultural lands (250 Million ha), it would imply to increase 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 t C ha -1 y -1 Grassland is a sink of t C ha -1 y -1

16 How does soil C sequestration work? Increase C inputs Organic carbon source...or reduce C losses e.g. residue management, organic amendments, increased plant C input e.g. restore & rewet farmed organic soils Add to soil CO 2 Soil C in soil Some C is stabilised in the soil Soil C cycle

17 How does soil C sequestration work? reduced disturbance No-till Tillage C C C C C Tillage breaks open aggregates C C C Organic material (C) more exposed to microbial attack and weathering Key: = microbe C = C inside aggregate = weathering

18 Mechanisms for soil C sequestration in agriculture Activity Practice Specific management change Increase Decrease Reduce C inputs C losses disturbance Cropland management Agronomy Increased productivity X Rotations X Catch crops X Less fallow X More legumes X Deintensification X Improved cultivars X Nutrient management Fertilizer placement X Fertilizer timing X Tillage / residue management Reduced tillage X Zero tillage X Reduced residue removal X X Reduced residue burning X X Upland water management Irrigation X Drainage X Set-aside and land use change Set aside X X Wetlands X X Agroforestry Tree crops inc. Shelterbelts etc. X X Grazing land management Livestock grazing intensity Livestock grazing intensity X Fertilization Fertilization X Fire management Fire management X Species introduction Species introduction X More legumes More legumes X Increased productivity Increased productivity X Organic soils Restoration Rewetting / abandonment X X Degraded lands Restoration Restoration X X X Smith et al. (2008)

19 Manure large & long-lasting effects Organic C in Soil (t ha -1 ) Farmyard manure annually Farmyard manure nothing thereafter 20 Unmanured Year Rothamsted Hoosfield Jenkinson 1998

20 Carbon depleted soils have large potential to accumulate C in early recovery stage Example : the largest LUC change in the northern hemisphere! Former Soviet Union 20 Mha abandonned from 1990 to 2000 Steppe This process causes net C accumulation in soils Conclusions Farmland abandonment caused C sink ~0.5 tc ha-1 y-1 over past decade gc m-2 Cereals Year after abandonment Vuichard et al, Glob. Biogeoch. Cycl. (2008) 7

21 Grass biofuel production vs. C sequestration How many years until biofuel cultivation overpasses passive C seuestration by return to natural steppe? Future climate change impacts the results Gain carboné cumulé (gc m-2) Avec évolution climatique simulée par IPSL-CM4 Current Sans évolution climate climatique % Nombre d'années de production Vuichard et al,

22 Global mitigation potential in agriculture 1600 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 Global biophysical mitigation potential (Mt CO 2-eq. yr -1 ) Cropland management Water management Rice management Setaside, LUC & agroforestry Grazing land management Restore cultivated organic soils Mitigation measure Restore degraded lands N2O CH4 CO2 Bioenergy (soils component) Livestock Manure management Smith et al. (2008)

23 High and low estimates of the mitigation potential in each region A potential European sink 200 Tg C y -1 Current sink in European agricultural land 20 Tg C y Mt CO2-eq. yr Southeast Asia South America East Asia South Asia Eastern Africa Russian Federation North America Western Europe Western Africa Central Asia Northern Europe Middle Africa Eastern Europe Region Oceania Southern Europe Central America Northern Africa Western Asia Southern Africa Carribean Japan Polynesia Smith et al. (2007)

24 Effect of C price on implementation up to 20 USD t CO2-eq.-1 up to 50 USD t CO2-eq.-1 up to 100 USD t CO2-eq.-1 Mt CO 2-eq. yr Restore cultivated organic soils Cropland management Grazing land management Restore degraded lands Rice management Livestock Setaside, LUC & agroforestry Manure management Measure Smith et al. (2007)

25 Global mitigation potential in agriculture (Mt CO 2 -eq. yr -1 ) Price range (USD t CO 2 -eq. -1 ) Scenario >>100 (technical potential) B A1b B A Smith et al. (2007)

26 Global economic mitigation potential for different sectors at different carbon prices 7 GtCO 2 -eq <20 <50 <100 Energy supply <20 <50 <100 Transport Buildings Industry Agriculture Forestry Waste Non-OECD/EIT EIT OECD World total US$/tCO2-eq IPCC WGIII (2007)

27 How do we cut GHG emissions and how much will it cost? From: McKinsey (2009) - Pathways to a low-carbon economy Version 2 of the Global Greenhouse Gas Abatement Cost Curve

28 How do we cut GHG emissions and how much will it cost? From: McKinsey (2009) - Pathways to a low-carbon economy Version 2 of the Global Greenhouse Gas Abatement Cost Curve

29 Smith (2008) International Journal of Agricultural Sustainability 6(3), There are a number of well rehearsed arguments against reliance on carbon sequestration for tackling climate change, involving saturation of the carbon sink (the carbon is only removed from the atmosphere while the tree is growing or until the soil reaches a new equilibrium soil carbon level; Smith, 2005), permanence (carbon sinks can be reversed at any stage by deforestation or poor soil management; Smith, 2005), leakage/displacement (e.g. planting trees in one area leads to deforestation in another; Intergovernmental Panel on Climate Change (IPCC), 2000), verification issues (can the sinks be measured; Smith, 2004), and total effectiveness relative to emission reduction targets (only a fraction of the reduction can be achieved through sinks; IPCC, 2007).

30 Saturation the time course of C sequestration C stock Soil C Vegetation C Management change Time since management change Sink saturation ~ years Sink strength declines towards new equilibrium Smith (2004a)

31 Permanence Total SOC to 23 cm (t C ha -1 ) Conversion to low-input cropland Management change Manure treatment in red, Woodland in blue Year Smith (2005)

32 Leakage / displacement: are we actually sequestering carbon or just moving it about? More manure here.but..less manure here Manure Manure Mineral N Farm with more manure Farm with less manure Effect over the whole cropland area = zero

33 Verification issues Minimum accuracy required for verification Cost Zero return Cost Zero return Progress in measurement technology Value of C sequestered Value of C sequestered No. of samples required to demonstrate increase in soil C Accuracy of estimate (or precision if only a change is verified) Smith (2004b)

34 Trying to sequester the geosphere in the 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 add up! It is easier to leave the marbles in the jar than to tip them out and try to pick them all up again W.H. (Bill) Schlesinger Soil C sequestration is time limited, non-permanent, difficult to verify and is no substitute for GHG emission reduction Soil C sequestration may have a role in reducing the short term atmospheric CO 2 concentration, and buying us time to develop longer term solutions, largely in the energy sector

35 Conclusions Soil C sequestration globally has a large, costcompetitive 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 term sequestered C ; verification is not in place Don t forget the limitations: time limited, not permanent, doesn t replace genuine emission reduction