Soil: carbon sinks

Extra facts

• Soils in agro-ecosystems lose 25 to 75 percent of their organic carbon during the initial conversion of these ecosystems from natural to agricultural and due to such soil degradation processes as erosion, salinization and nutrient depletion. This is equivalent to a global loss of 78 ± 12 billion tonnes of carbon [286,000 ± 44,000 MtCO₂e per year] through historic land use and soil degradation (Lal 2011). With the adoption of recommended soil management practices, rates of soil organic carbon sequestration (higher for cool and humid climates) range from 50 to 1,500 kilograms of carbon per hectare per year [0.2 to 5.5 metric tonnes of carbon dioxide equivalent per hectare per year (tCO₂e/ha/yr)].
• The sequestration rates of organic carbon in grazing land soils are generally (but not always) lower than those of cropland soils. The rates range from 200 to 500 kilograms of carbon per hectare per year (kgC/ha/yr) [0.75-1.85 tCO₂e/ha/yr] (Lal 2011).
• Fertilizers (particularly nitrogen) increase crop yields and, in 29 of 34 long-term experiments, increased soil organic carbon. Soil organic carbon increased by an average of 50 kgC/ha/yr [0.2 tCO₂e/ha/yr] when the soil received nitrogen fertilizer at a rate of 144 kilograms of nitrogen per hectare per year. However, annual emissions associated with nitrogen fertilizer manufacture and use are around 200 kilograms of carbon per hectare [0.73 tCO₂e/ha]—four times greater than the annual increase in soil organic carbon (Powlson et al. 2011).
• The conversion of arable land to woodland leads to an increase in soil organic carbon of 44 to 64 metric tonnes of carbon per hectare [161-234 tCO₂e/ha] over a 120 year period. (Powlson et al. 2011). The conversion of arable land to grassland leads to an increase of 18 metric tonnes of carbon per hectare (Johnson et al. 2009).
• Recommended land use and management practices enhance the amount of carbon in soils. In turn, this improves soil quality, increases agronomic productivity, advances global food security, enhances soil’s resilience to extreme climatic events and offsets fossil fuel emissions—mitigating climate change.
• It is estimated that, increasing the soil carbon pool by 1 tonne of carbon hectare per year [3.7 tCO₂e/ha/yr] in developing countries can enhance agronomic production by 32 ± 11 million metric tonnes per year (Mt/yr) of cereals and pulses and 9 ± 2 Mt/yr of roots and tubers (Lal 2011).

Methods, caveats and issues

Methods

• Conversion from carbon to carbon dioxide is done by multiplying the amount of carbon by 44/12.

Definitions

• Carbon sequestration is defined as any increase in soil organic carbon content caused by changes in land management, with the implication that increased soil carbon storage mitigates climate change (Powlson et al. 2011).

Issues

• Recommended sustainable management practices to increase soil carbon stocks in croplands include increasing yield, reducing soil disturbance and residue management, planting trees alongside crops (agroforestry) and avoiding bare soil that is prone to erosion and nutrient leaching (Bellarby et al. 2008).
• Although soil carbon sequestration is generally thought to be the highest potential source of mitigation, the exact mitigation potential is dependent on many factors and can be difficult to measure (FAO 2011).
• With increasing demand for food, the removal of land from agriculture in one place will lead to the expansion of agricultural land in another. Hence, changing land use away from agriculture for the purposes of carbon sequestration is an option only on surplus agricultural land or on cropland with limited or marginal productivity (Smith et al. 2007).
• Constraints in measuring carbon sequestration include high natural variability in soil carbon, difficulties in establishing reliable baselines, problems detecting changes over short time periods or across small spatial extents and a lack of consistency in units and measurement techniques. However, much progress is currently being made in establishing global norms for measurement (Smith et al. 2012).

Sources

• Bellarby J, Foereid B, Hastings A, Smith P. 2008. Cool Farming: Climate Impacts of Agriculture and Mitigation Potential. Amsterdam: Greenpeace International.
• [FAO] Food and Agriculture Organization of the United Nations. 2011. Climate Change Mitigation Finance for Smallholder Agriculture: A guide book to harvesting soil carbon sequestration benefits. (Available from http://www.fao.org/docrep/015/i2485e/i2485e00.pdf)
• Lal R. 2011. Sequestering carbon in soils of agro-ecosystems. Food Policy 36:S33–S39.
• Powlson DS, Whitmore AP, Goulding KWT. 2011. Soil carbon sequestration to mitigate climate change: a critical re-examination to identify the true and the false. European Journal of Soil Science 62:42-55.
• Smith P, Martino D, Cai Z, Gwary D, Janzen H, Kumar P, McCarl B, Ogle S, O’Mara F, Rice C, Scholes B, Sirotenko O. 2007. Agriculture. In: Metz B, Davidson OR, Bosch PR, Dave R, Meyer LA, eds. Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge: Cambridge University Press.
• Smith P, Davies CA, Ogle S, Zanchi G, Bellarby J, Bird N, Boddey RM, McNamara NP, Powlson D, Cowie A, van Noordwijk M, Davis SC, Richter DD, Kryzanowski L, van Wijk MT, Stuart J, Kirton A, Eggar D, Newton-Cross G, Adhya TK, Braimoh AK. 2012. Towards an integrated global framework to assess the impacts of land use and management change on soil carbon, current capability and future vision. Global Change Biology 18:2089-2101.