Recent interest in carbon sequestration has raised questions about how much organic carbon (OC) can be stored in soil. Total OC is the amount of carbon in the materials related to living organisms or derived from them. In Australian soils, total OC is usually less than 8 % of total soil weight (Spain et al., 1983) and under rainfed farming it is typically 0.7 – 4 %. Increasing the amount of OC stored in soil may be one option for decreasing the atmospheric concentration of carbon dioxide, a greenhouse gas.
Increasing the amount of OC stored in soil may also improve soil quality as OC contributes to many beneficial physical, chemical and biological processes in the soil ecosystem (figure 1) (see Total Organic Carbon fact sheet). When OC in soil is below 1 %, soil health may be constrained and yield potential (based on rainfall) may not be achieved (Kay and Angers, 1999).
Figure 1: Some of the beneficial physical, chemical and biological processes in soil that total OC contributes to.
The amount of OC stored in soil is the difference between all OC inputs and losses from a soil. The main inputs of OC to soil in rainfed farming systems are from plant material, such as crop residues, plant roots, root exudates and animal manure. Inputs of plant material are generally higher when plant growth is greater.
Losses of OC from soil are from decomposition by microorganisms, erosion of surface soil and offtake in plant and animal production. Decomposition occurs when microorganisms use OC in soil to obtain the carbon, nutrients and energy they need to live. During decomposition, OC is lost from soil because microorganisms convert about half of the OC to carbon dioxide gas (CO2). Without continual inputs of OC, the amount stored in soil will decrease over time because OC is always being decomposed by microorganisms.
Losses of OC from erosion of surface soil can have a large impact on the amount of OC stored in soil. This is because OC is concentrated in the surface soil layer as small particles that are easily eroded. In Australian agriculture, erosion can cause the annual loss of 0.2 t/ha of soil from a pasture, 8 t/ha from a crop and up to 80 t/ha from bare fallow.
Offtake of OC in plant and animal production is also an important loss of OC from soil. Harvested materials such as grain, hay, feed and animal grazing all represent loss of OC (and nutrients) from soil.
Figure 2: The influence of soil type, climate and management factors on the storage of organic carbon (OC) that can be achieved in a given soil. Based on Ingram and Fernandes (2001).
The potential storage of OC in soil depends on the soil type (figure 2). Clay particles and aggregates can reduce losses of OC by physically protecting organic matter from decomposition. Particles of organic matter can become adsorbed to clay surfaces, coated with clay particles or buried inside small pores or aggregates. All of these processes make it difficult for microorganisms to come in contact with organic matter. Therefore, the amount of OC stored in soil tends to increase with increasing clay content (figure 3). In contrast, in sand soil microorganisms are able to more easily access OC. This causes greater loss of OC by decomposition.
The potential storage of OC in soil is rarely achieved because climate reduces inputs of OC to soil.
Figure 3: The relationship between clay content and the organic carbon content of 220 soils in a 10 hectare area of a paddock under cereal-legume rotation in the central agricultural region of Western Australia.
Climate determines the attainable storage of OC in soil by regulating plant production (figure 2). Under dryland agriculture, rainfall is the climate factor that has most influence on plant productivity and therefore inputs of OC to soil. In regions with high rainfall, soils tend to have greater attainable storage of OC than the same soil type in a lower rainfall region.
Although it is not possible to increase the attainable storage of OC in soil, management practices determine whether or not the attainable storage of OC in soil is achieved.
Management practices determine the actual storage of OC in soil by increasing inputs and decreasing losses (figure 2). Practices that can increase the amount of total OC stored in soil include:
Hoyle F, Murphy D and Baldock G (2009) Soil organic carbon: Role in Australian farming systems. In: Rainfed Farming Systems. Springer, Chapter 14.
Ingram JSI and Fernandes ECM (2001) Managing carbon sequestration in soils: Concepts and terminology. Agriculture, Ecosystems & Environment. 87: 111-117.
Kay BD and Angers DA (1999) Soil structure. In: Handbook of Soil Science. (Ed ME Sumner), pp A-229-276. CRC Press, Boca
Spain AV, Isbell RF and Probert ME (1983) Soil organic matter. In: Soils – An Australian viewpoint, pp 551-563. CSIRO,
Melbourne Australia. Academic Press, London UK.
Author: Jennifer Carson (The University of Western Australia).
This soilquality.org.au fact-sheet has been funded by the Healthy Soils for Sustainable Farms programme, an initiative of the Australian Government’s Natural Heritage Trust in partnership with the GRDC, and the WA NRM regions of Avon Catchment Council and South Coast NRM, through National Action Plan for Salinity and Water Quality and National Landcare Programme investments of the WA and Australian Governments.
The Chief Executive Officer of the Department of Agriculture and Food, The State of Western Australia and The University of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.