Soil carbon

Carbon is held within the soil, primarily in association with its organic content. This discussion begins with a brief introduction to soil carbon, its function within the soil, influences on soil carbon, and finally the impacts that might be expected from increasing soil carbon.

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Overview

Between 1200 and 1800 Gt of carbon (C) is stored in soils worldwide, twice the amount that is stored in all terrestrial plants. Carbon is exchanged between the soil and atmosphere in a cycle that is overwhelmingly driven by photosynthesis (Smil, 1997).

Plants absorb atmospheric carbon dioxide (CO2), which is then reduced through photosynthesis so that the carbon component is retained and the oxygen is returned to the atmosphere. The carbon that is retained by plants may be transferred to the soil via roots or decomposing plant residues. Soil carbon may be returned directly to the atmosphere from the soil, when the organic material in which it is held is oxidised by decomposition or burning.

There are two discrete pools in which soil carbon is stored (Bardgett, 2005): the soil’s microbial biomass and easily-decomposed plant residues that are cycled rapidly and in which carbon may reside for as little as a few weeks, and; the pool in which carbon is more tightly held by physical encapsulation within soil aggregates (singular masses of coherent soil particles, or peds) or chemical complexing – here, carbon may reside for tens of thousands of years.

Soil organic carbon (SOC) refers to the amount of carbon stored in the soil – it is expressed as a percentage by weight (g C/kg soil). SOC is closely related to the amount of organic matter in the soil (soil organic matter (SOM)), according to the approximation: SOC x 1.72 = SOM (Young &Young, 1991). The SOM of Australian soils ranges from 50 per cent in alpine humus soils to less than one per cent in dessert loams. Across Victoria the SOM range of both agricultural and non-agricultural surface soils is from 1.3 to 10.5 per cent (Leeper and Uren, 1993).

Soil carbon and soil health

Soil carbon improves the physical properties of soil. It increases the cation exchange capacity (CEC) and water-holding capacity of sandy soil and it contributes to the structural stability of clay soils by helping to bind particles into aggregates (Leeper and Uren, 1993). Soil organic matter, of which carbon is a major part, holds a great proportion of nutrients, cations and trace elements that are of importance to plant growth. It prevents nutrient leaching and is integral to the organic acids that make minerals available to plants. It also buffers soil from strong changes in pH (Leu, 2007). It is widely accepted that the carbon content of soil is a major factor in its overall health.

Managing soil carbon

Natural variations in SOM occur as a result of climate, organisms, parent material, time and relief (Young and Young, 1991). The greatest contemporary influence has been that of humans; for example, historical SOM in Australian agricultural soils may have been twice the present range that is typically from 1.6 to 4.6 per cent (Charman & Murphy, 2000 in Young and Young, 1991).

It has long been encouraged that farmers adjust practices to maintain or increase the organic component in the soil – on one hand, practices that hasten oxidation of carbon, such as burning crop stubbles or over-cultivation are discouraged; on the other hand, incorporation or organic material, such as manuring has been encouraged. Increasing soil carbon is not a straightforward matter – it is made complex by the relative activity of soil biota, which can consume and release carbon and are made more active by the addition of nitrogen fertilisers (Young & Young 2001).

Managing for catchment health

Much of the contemporary literature on soil carbon relates to its role, or potential, in sequestering carbon from the atmosphere to offset climate change. Despite this emphasis, a much wider range of soil and catchment health aspects are improved as soil carbon is increased. These benefits are difficult to quantify due to the complexity of natural resource systems and the interpretation of what constitutes soil health; nonetheless, several benefits are proposed in the following points:

• Reduced erosion, sedimentation: increased soil aggregate stability means greater resistance to erosion; mass movement is less likely when soils are able to retain structural strength under greater moisture levels.
• Greater productivity: healthier and more productive soils can contribute to positive socio-economic circumstances.
• Cleaner waterways, nutrients and turbidity: nutrients and sediment tend to be retained by the soil rather than leach or wash off, and are so kept from waterways
• Water balance: greater soil water holding capacity reduces overland flow and recharge to groundwater; the water saved and held by the soil remains available for use by plants.
• Climate change: Soils have the ability to retain carbon that may otherwise exist as atmospheric CO2 and contribute to greenhouse warming.
• Greater biodiversity: soil organic matter contributes to the health of soil flora and accordingly, the natural links with biodiversity in the greater biosphere.

Conclusion

The exchange of carbon between soils and the atmosphere is a significant part of the world carbon cycle, which is extensive both spatially and temporally. Carbon, as it relates to the organic matter of soils, is a major component of soil and catchment health. Several factors affect the variation that exists in soil organic matter and soil carbon - the most significant has, in contemporary times, been the influence of humans and agricultural systems. There are clear benefits for catchment health by focusing on soil carbon – efforts would need to be extensive and economical for the collective benefit to be realised.

See also

• Soil health

References

• Bardgett, RD 2005, The biology of soil: a community and ecosystem approach, Oxford University Press Inc, New York.

• Charman, PEV & Murphy, BW 2000, 2th edn, Soils, their properties and management, Oxford University Press, Melbourne, in Young, A & Young R 2001, Soils in the Australian landscape, Oxford University Press, Melbourne.

• Leeper, GW & Uren, NC 1993, 5th edn, Soil science, an introduction, Melbourne University Press, Melbourne

• Leu, A 2007, Organics and soil carbon: increasing soil carbon, crop productivity and farm profitability, viewed May 2007, [1]

• Smil, V 1997, Cycles of life: civilization and the biosphere, Scientific American Library, New York.

• Young, A & Young R 2001, Soils in the Australian landscape, Oxford University Press, Melbourne.

The initial content of this article is reproduced from and modified with permission of the Catchment Knowledge Exchange Project. This Project comprises a trial to engage the soil practitioner community in the area of soil health in Victoria, Australia. The original published location of the material is located at SoilWiki.