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Very large quantities of carbon are currently stored in soils in the form of soil organic carbon (SOC). SOC provides a vital ecosystem function, contributing to inherent soil fertility, water holding capacity, structure, biological activity and carbon sequestration. Carbon stored as SOC is however highly sensitive to changes in land management and SOC levels almost always fall following the conversion of native ecosystems to agricultural use. The picture shows  degraded land after conversion from forest to pasture in Brazil. This is well documented in a range of environments, and is primarily driven by (i) reduced organic matter inputs; and (ii) increased oxidation of SOC due to aggregate disruption and increased aeration. A range of factors determine the stabilisation of carbon in soils, but key variables are clay content and type, pH, hydrology, climate and organic matter inputs. For a given climate, three principle mechanisms account for SOC decline: reduced organic matter input, increased erosion, and increased oxidation as a result of tillage. In the tropics, both intensification of existing cropping land and land clearance for new cropping land are responsible for substantial net losses of soil carbon. In addition, anticipated increases in global mean temperatures will also tend to deplete SOC stocks through enhanced oxidation. The net result of both processes is an increase in carbon flux to the atmosphere. Conversely, application of innovative agricultural management practices, promoting the recovery of soil C stocks, and reduced rates of land clearance can either reduce soil CO2 emissions or actively sequester carbon in soil.

From a biodiversity perspective, SOC, a principle constituent of soil organic matter, is known to significantly influence biodiversity both in the soil and above-ground.Soil organic matter has a direct correlation with soil organism density and hence soil biodiversity. Critical to any model for sustainable soil management is the role that soil organisms play. Soil organic matter affects a wide range of processes such as the movement of nutrients through plants and the movement of soil and water in catchments, thus resulting in different levels of vegetation stress and in turn, ecosystem functioning as a whole. This correlation points to the importance of SOC for vital ecosystem function, contributing to inherent soil fertility, water holding capacity, structure, biological activity in addition to carbon sequestration.

Regional, spatially-explicit estimates of SOC stocks and changes are needed to quantify CO2 fluxes under various climate/soil/land-use conditions. Suitable tools for estimating these changes and further carbon sequestration possibilities as a consequence of land management at national and sub-national scale are however lacking as mapping methods are not dynamic and therefore cannot be used in a predictive capacity. Further, such tools need to be developed and systematically evaluated for the wide range of soils found in the tropics, especially for vertisols and oxisols (given the high P fixation of this highly weathered soil type). The need to have reliable data on present SOC stocks, and an estimate of how these might change under different environmental and management scenarios is of high priority for many nations.


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Last updated: 20 March 2007.