What is soil organic carbon?

The carbon cycle

Tamara Harris, Regional Agriculture Landcare Facilitator

The carbon cycle put simply is this: plants photosynthesise energy from the sun to take CO2 from the air, split it into carbon and oxygen, release the oxygen back into the air and use carbon to grow.

Humans and animals eat the plant (or animal in the food chain that originally ate the plant), metabolise the carbon, use it for their own growth and repair, and breathe out the CO2 or excrete the rest.

Carbon is continually cycled (and recycled) in many ways. It can be present as a gas, liquid or solid - and it moves easily and constantly between each of these forms as it is cycled by plants and animals through respiration, rumination, reproduction and all other processes of life.

Carbon sequestration

Our soil has a large capacity to absorb (or sequester) carbon. Sequestration of carbon in agricultural soils, through appropriate management actions, has been recognised as an important tool to mitigate climate change. Our soil is full of microorganisms and when plants take carbon from the air they use it in a symbiotic relationship to feed soil microbes in exchange for nutrients and other ecological functions.

Plants do this by releasing exudates via their roots in the form of many different carbon compounds - exuding liquid carbon.

These exudates can drive numerous interactions within the soil, including reproduction and life-cycles of many different soil microbes. Organic matter from roots, plant biomass and grazing animals all add to the carbon (as well as other nutrients). The microbes then convert this into complex and stable carbon compounds called Humus.


Organic matter is a diverse group of organic materials of differing composition and at different stages of decomposition. It comprises of partially decomposed organic residues, microscopic organisms, well - decomposed humus, and burnt residues such as charcoal. The transformation of organic residues into humus by soil organisms requires nitrogen, phosphorus and sulphur (and other elements in smaller quantities). These elements are constituents of organic matter and must be present in organic residues or added to the soil for humus to form.

Humus is the building block of soil and building new soil, by storing carbon in this stable form, is called carbon sequestration. Under healthy aerobic conditions, up to 40% of the carbon captured from the air by plants can be sequestered in the soil in this way.

Increasing carbon

Soil carbon increases through increased biomass production and retention and application of carbon rich amendments. The main losses of carbon from the soil are through organic matter decomposition by microorganisms, soil erosion, biomass burning, and product removal in food and fibre.

For more information on soil carbon read the NSW DPI Primefact 1185: Key soil carbon messages.

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