Soil carbon

Template:Globalize/Australia Soil carbon is the generic name for carbon held within the soil, primarily in association with its organic content. Soil carbon is the largest terrestrial pool of carbon. Humans have, and will likely continue to have, significant impacts on the size of this pool. Soil carbon plays a key role in the carbon cycle and thus is important in global climate models.

Overview

Although the figure is frequently being revised upwards with new discoveries, over 2700 Gt of carbon is stored in soils worldwide, which is well above the combined total of atmosphere (780 Gt) or biomass (575 Gt), most of which is wood. Carbon is taken out of the atmosphere by plant photosynthesis; about 60 Gt annually becomes various types of soil organic matter including surface litter; about 60 Gt annually is respired or oxidized from soil.^[1]

Soil carbon is the last major pool of the carbon cycle. The carbon that is fixed by plants is transferred to the soil via dead plant matter including dead roots, leaves and fruiting bodies. This dead organic matter creates a substrate which decomposer respire back to the atmosphere as carbon dioxide or methane depending on the availability of oxygen in the soil. Soil carbon can also be oxidized by combustion and returned to the atmosphere as carbon dioxide.

Soil carbon is primarily composed of biomass and non-biomass carbon sources. Biomass carbon primarily includes various bacteria and fungi. Non-biomass carbon sources or substrates reflect the chemical composition of plant biomass and primarily include cellulose, starch, lignin and other diverse organic carbon compounds. Some of the substrate carbon will bind to the mineral soil becoming encapsulated in soil aggregates (singular masses of coherent soil particles, or peds) or chemical complexing.

The biomass feeds off of the substrate carbon compounds at different rates. Some of the carbon compounds are easily digested and respired by the microbes resulting in a relatively short residence time. Others, like lignin, humic acid or substrate encapsulated in soil aggregates, are very difficult for the biomass to digest and have very long residence times.

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.^[2] 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.^[3] It is widely accepted that the carbon content of soil is a major factor in its overall health.

Losses of soil carbon

Although exact quantities cannot be documented, human activities have caused massive losses of soil organic carbon.^[4] First was the use of fire, which removes soil cover and leads to immediate and continuing losses of soil organic carbon. Tillage and drainage both expose soil organic matter to oxygen and oxidation. In the Netherlands, East Anglia, Florida, and the California delta, subsidence of peat lands from oxidation has been severe as a result of tillage and drainage.

Grazing management that exposes soil (either excessive or insufficient recovery periods) can also cause losses of soil organic carbon.

Managing soil carbon

Natural variations in SOM occur as a result of climate, organisms, parent material, time and relief.^[5] 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.^[6]

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 of 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 fertilizers.^[5]

Managing for catchment health

Much of the contemporary literature on soil carbon relates to its role, or potential, as an atmospheric carbon sink 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.

Australian Soil Carbon Accreditation Scheme (ASCAS)

ASCAS^[7] pays farmers for cultivating in such a way (pasture cropping, also known as no till farming) that carbon is captured and retained in the soil. The payments are determined by validated soil carbon increases above initial baseline levels determined for each Defined Sequestration Area, so vary according to actual carbon captured rather than a fixed sum.

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 realized.

References

^ Lal, Rattan (2008). "Sequestration of atmospheric CO₂ in global carbon pools". Energy and Environmental Science. 1 (1): 86–100. doi:10.1039/b809492f.
^ Leeper, G.W. (1993). Soil science, an introduction (5th edn ed.). Melbourne: Melbourne University Press. ISBN 0-522-84464-2. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Leu, A (2007). "Organics and soil carbon: increasing soil carbon, crop productivity and farm profitability" (PDF). {{cite journal}}: Cite journal requires |journal= (help)
^ Ruddiman, William (2007). Plows, Plagues, and Petroleum: How Humans Took Control of Climate. Princeton, NJ: Princeton University Press. ISBN 9780691146348.
^ ^a ^b Young, A. (2001). Soils in the Australian landscape. Melbourne: Oxford University Press. ISBN 9780195515503. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Charman, P.E.V. (2000). Soils, their properties and management (2nd edn ed.). Melbourne: Oxford University Press. ISBN 9780195517620. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Australian Soil Carbon Accreditation Scheme

[Lal-2008-1] Lal, Rattan (2008). "Sequestration of atmospheric CO₂ in global carbon pools". Energy and Environmental Science. 1 (1): 86–100. doi:10.1039/b809492f.

[leeper-uren-1993-2] Leeper, G.W. (1993). Soil science, an introduction (5th edn ed.). Melbourne: Melbourne University Press. ISBN 0-522-84464-2. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[leu-2007-3] Leu, A (2007). "Organics and soil carbon: increasing soil carbon, crop productivity and farm profitability" (PDF). {{cite journal}}: Cite journal requires |journal= (help)

[ruddiman-2007-4] Ruddiman, William (2007). Plows, Plagues, and Petroleum: How Humans Took Control of Climate. Princeton, NJ: Princeton University Press. ISBN 9780691146348.

[young-young-2001-5] Young, A. (2001). Soils in the Australian landscape. Melbourne: Oxford University Press. ISBN 9780195515503. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[charman-murphy-2000-6] Charman, P.E.V. (2000). Soils, their properties and management (2nd edn ed.). Melbourne: Oxford University Press. ISBN 9780195517620. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[7] Australian Soil Carbon Accreditation Scheme

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