Soil carbon and climate change

What is soil carbon?

Soils used for growing crops and pastures are also very important for storing carbon:

• Soils can store large amounts of carbon, up to 50-300 tonnes per hectare, which is equivalent to 180-1100 tonnes of carbon dioxide.¹
• The above-ground component (or above-ground biomass) of pastures and crops store between 2 and 20 tonnes of carbon per hectare.
• The above-ground component of plantation forests can store 250 tonnes of carbon per hectare.

To appreciate how valuable the soil is in storing carbon, we need to understand the carbon cycle. Find out more about the carbon cycle.

Soil organic carbon

Soil organic carbon, the major component of soil organic matter, is extremely important in all soil processes. Soil organic carbon refers to all the different carbon compounds found in soils that are, or were previously, a living organism.

Soil organic matter

Soil organic matter is generally divided into three components:

1. Particulate material refers to the bits and pieces of plant material, or material that is available to soil organisms for decomposition. Soil organisms break down particulate matter to create humus, which is the final product of the decaying process; it will break down no further.
2. Humus is important for binding soil particles together. It improves the water and nutrient holding capacity of soils, and these are essential for plant growth. Humus stores or sequesters carbon for decades, or even centuries.
3. Charcoal is the result of incomplete burning of plant material or fossil fuels. It is believed to be biologically and chemically unreactive compared with other soil organic matter components.² This means that the carbon stays locked in the charcoal in the soil and isn't readily released or taken up by soil organisms.

Healthy soil characteristics

Healthy soils with a high organic carbon content contribute to the following soil characteristics, which are important for soil fertility:

• Nutrient availability. Decomposition of soil organic matter releases nitrogen, phosphorus and a range of other nutrients for plant growth.
• Soil structure and soil physical properties. Soil organic carbon promotes a healthy soil structure by holding the soil particles together, thereby improving soil physical properties such as water-holding capacity, water infiltration, gaseous exchange, root growth and ease of cultivation.
• Biological soil health. Soils contain microscopic plants and animals which live between, and feed on, the many soil particles. Soil organic matter plays an important role in the soil food web by controlling the number and types of soil inhabitants. These inhabitants serve important functions such as cycling nutrients, making nutrients available for other organisms, assisting root growth, assisting plant nutrient uptake, creating burrows, and even suppressing crop diseases.
• Buffer for toxic and harmful substances. Soil organic matter can lessen the effect of harmful substances, for example toxins such as heavy metals, by acting as a buffer. Heavy metal toxins can bond very tightly to soil particles, preventing their release into waterways where contamination affecting the food chain may occur. Soil organic matter also increases the adsorption of pesticides, thus reducing the amount of chemicals that may enter groundwater.

Soil carbon sequestration

Plants sequester (remove) carbon from the atmosphere through growth. Carbon dioxide is converted into plant tissue through photosynthesis. After a plant dies, the plant material is decomposed primarily by soil microorganisms, and carbon is released back into the atmosphere through respiration or it is left behind as humus.

So plants and the microorganisms in the soil provide the link between the carbon in the atmosphere and how it can be stored or fixed to biological matter in the soils - this process is called soil carbon sequestration.

Problems caused by traditional practices

Where farmers have used the land for many years for growing crops, the soils typically have low levels of organic carbon content, due to disturbance, erosion and regular periods when very few organic nutrients have been added to the soil, for example during fallow (leaving the soil bare between crops) and during the early stages of crop growth.

Harvesting a crop leaves little organic material for decomposition, and this, often coupled with traditional farming methods, results in what little soil carbon is present in the soils being released.

The tillage method of farming uses machinery to turn over the soil to reduce weeds and loosen and aerate the soil, allowing the deeper penetration of roots. In doing this, however, it is now known that more soil carbon may actually be released through disturbance of the soil structure and soil microorganisms.

Improving practices to increase soil fertility and soil carbon levels

It has been estimated that Australia's cropping soils have lost a substantial amount of carbon, estimated to be 1050 Mt (megatonnes), following the introduction of intensive cropping.³ This signifies that there is significant potential to increase carbon stocks in these carbon-poor soils by improving land management practices.⁴ It is believed that over time, a change in land use from forest or grassland to cropping generally leads to a loss of 50% or more in soil carbon.⁵

Some of the management practices farmers are implementing which can increase soil organic carbon stocks while improving production output include the following:

• Retaining forest slash and crop residues, rather than burning them, to increase organic matter input and protect against erosion of the carbon-rich surface soil.⁶
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• Applying fertiliser to overcome nutrient deficiencies, thereby enhancing plant growth and, consequently, litter inputs.⁷ Fertiliser rates and timing should be matched to the requirements of the crop, to maximise efficiency of fertiliser use and limit leaching and runoff. However, greenhouse gases are emitted during the manufacture of nitrogen fertilisers, and their application can also cause nitrous oxide emissions. Therefore a balance must be struck between the soil carbon gain and greenhouse gas emission.
• Applying organic amendments - recycled organics such as manures, biosolids, composts and char.
• Selecting cropping, forest or pasture systems that will maximise plant growth. Each species has a different carbon allocation strategy that results in a different pattern, rate, quality and quantity of organic carbon input to the soil. Mixed-species planting (i.e. two species) can maximise biomass production where the two species allow one another to grow better in each other's presence. Examples are: nitrogen-fixing species such as acacia planted with eucalypts;⁸ lupin with pine;⁹ and clover with pasture grasses.¹⁰ These combinations commonly produce greater growth outputs when planted together than if a single species is planted on its own.
• Minimising cultivation disturbance, to reduce mineralisation and erosion losses. This minimising of soil disturbance will conserve soil carbon, particularly in erodible soils.
• Modifying grazing management to maintain pasture cover, thereby minimising erosion losses and maximising organic input to soil.

While these land management actions are recommended for increasing soil carbon stocks, researchers are still looking for cost-effective ways to estimate soil carbon changes under changed land management practices.

Carbon farm gas calculator

Industry & Investment NSW - Primary Industries has developed an online calculator that can assist farmers to calculate greenhouse gas contributions from their farm operations. Learn more about the Carbon farm gas calculator

¹Watson, RT, Noble, IR, Bolin, B, Ravindranath, NH, Verado, DJ & Dokken, DJ (eds) 2000, 'Land use, land-use change, and forestry: a special report of the Intergovernmental Panel on Climate Change', Cambridge, Cambridge University Press, pp. 23–51>

²Smernik RJ, Skyemstad JO & Oades JM, 'Virtual fractionation of charcoal from soil organic matter using solid state 13C NMR spectral editing', Australian Journal of Soil Research, 38: 665-83, 2000.

³Swift & Skyemsted 1999, in Fairweather H & Cowie A 2007, 'Climate change research priorities for NSW primary industries', Discussion paper, NSW Department of Primary Industries. Orange NSW, 2007.

⁴Fairweather H & Cowie A, 'Climate change research priorities for NSW primary industries' discussion paper, NSW Department of Primary Industries, Orange NSW, 2007.

⁵Guo LB & Gifford RM, 'Soil carbon stocks and land-use change: A meta analysis', Global Change Biology, 8, pp. 345360, 2002, in Fairweather H & Cowie A 'Climate change research priorities for NSW primary industries' discussion paper, NSW Department of Primary Industries, Orange NSW, 2007.

⁶Rasmussen, PE & Parton, WJ 1994, ‘Long-term effects of residue management in wheat/fallow: I. inputs, yield, and soil organic matter’, Soil Science Society of America Journal, vol. 58, pp. 523–530 AND Ayanaba, A, Tuckwell, SB & Jenkinson, DS 1976, ‘The effects of clearing and cropping on the organic reserves and biomass of tropical forest soils’, Soil Biology and Biochemistry, vol. 8, pp. 519–525 IN: Fairweather, H. and Cowie, A., 2007. Climate change research priorities for NSW primary industries. NSW Department of Primary Industries, Orange, NSW.

⁷Johnson, DW 1992, ‘Effects of forest management on soil carbon storage’, Water, Air and Soil Pollution, vol. 64, pp. 83–120;  Schroeder, P 1991, ‘Can intensive forest management increase carbon storage in forests?’, Environmental Management, vol. 15, pp. 474–481; Turner, J & Lambert, M 1986, ‘Nutrition and nutritional relationships of Pinus radiata’, Annual Review of Ecology and Systematics, vol. 17, pp. 325–350; Dalal, RC & Chan, KY 2001, ‘Soil organic matter in rainfed cropping systems of the Australian cereal belt’, Australian Journal of Soil Research, vol. 39, pp. 435–464 IN: Fairweather, H. and Cowie, A., 2007. Climate change research priorities for NSW primary industries. NSW Department of Primary Industries, Orange, NSW.

⁸Bauhus, J, Khanna, PK & Menden, N 2000, ‘Aboveground and belowground interactions in mixed plantations of Eucalyptus globulus and Acacia mearnsii’, Canadian Journal of Forest Research, vol. 30, pp. 1886–1894 IN: Fairweather, H. and Cowie, A., 2007. Climate change research priorities for NSW primary industries. NSW Department of Primary Industries, Orange, NSW.

⁹Beets, PN & Madgwick, HAI 1988, ‘Above-ground dry matter and nutrient content of Pinus radiata as affected by lupin, fertilizer, thinning, and stand age’, New Zealand Journal of Forestry Science, vol.18, pp. 43–64 IN: Fairweather, H. and Cowie, A., 2007. Climate change research priorities for NSW primary industries. NSW Department of Primary Industries, Orange, NSW. 

¹⁰Ledgard, SF 1991, ‘Transfer of fixed nitrogen from white clover to associated grasses in swards grazed by dairy-cows, estimated using N-15 methods’, Plant and Soil, vol. 131, pp. 215–223 IN: Fairweather, H. and Cowie, A., 2007. Climate change research priorities for NSW primary industries. NSW Department of Primary Industries, Orange, NSW.