Fractions of carbon from rapidly decomposable raw pieces of plants and micro-organisms scanned using electronic microscopy.

Soil carbon: the basics

Soil organic carbon is a complex and varied mixture of materials and makes up a small but vital part of all soils.

• 21 November 2008 | Updated 14 October 2011

• What is soil carbon?
• Different types of soil carbon
• Key functions of the different types of soil carbon 
• Movement between soil carbon fractions 

What is soil carbon?

Soil carbon, or soil organic carbon (SOC) as it is more accurately known, is the carbon stored within soil.

It is part of the soil organic matter (SOM), which includes other important elements such as calcium, hydrogen, oxygen, and nitrogen.

Soil organic matter is made up of plant and animal materials in various stages of decay.

Un-decomposed materials on the surface of the soil, such as leaf litter, are not part of the organic matter until they start to decompose.

Different types of soil carbon

Soil organic matter is often reported in soil tests as the percentage of soil organic carbon present in the soil sample.

However, although determining the amount of soil organic carbon in soil is important for understanding soil health, knowing the type of organic carbon present is also important as this can greatly impact soil productivity. 

CSIRO scientist, Dr Jeff Baldock and his team have identified four biologically significant types or fractions of soil organic carbon:

• crop residues – shoot and root residues less than 2 mm found in the soil and on the soil surface
• particulate organic carbon – individual pieces of plant debris that are smaller than 2 mm but larger than 0.053 mm
• humus – decomposed materials less than 0.053 mm that are dominated by molecules stuck to soil minerals
• recalcitrant organic carbon – this is biologically stable; typically in the form of charcoal.

The different types of soil organic carbon not only differ in size but are also composed of different materials with different chemical and physical properties and different decomposition times.

Key functions of the different types of soil carbon

Each fraction of soil carbon has different functions, most of these are due to the relative stability and biological availability of each fraction:

• crop residues
  • readily broken down and provide energy to soil biological processes
• particulate organic carbon
  • broken down relatively quickly but more slowly than crop residues
  • important for soil structure, energy for biological processes and provision of nutrients
• humus
  • plays a role in all key soil functions
  • particularly important in the provision of nutrients - for example the majority of available soil nitrogen derived from soil organic matter comes from the humus fraction
• recalcitrant organic carbon
  • is usually charcoal - a product of burning carbon-rich materials. As 'biochar', it is attracting interest as both a carbon sink and, possibly, a source of soil benefits.
  • decomposes very slowly and is therefore unavailable for use by micro-organisms
  • many Australian soils have high levels of charcoal from millennia of burning.

The amount of each type of organic carbon in Australian agricultural soils varies significantly.

In rainforests or good soils organic carbon can be >10 per cent, while in many poorer soils or soils which are heavily exploited, levels are typically <1 per cent.

The proportion of some fractions can also vary due to management practices. This is important as different fractions decompose at different rates and contain different quantities of nutrients, which will have an impact on the health and productivity of the soil.

Balancing carbon inputs and outputs

The amount of organic carbon in soil is a balance between the build-up which comes from inputs of new plant and animal material and the constant losses where the carbon is decomposed and the constituents separate to mineral nutrients and gases, or are washed or leached away.

Carbon levels build up where water, nutrients and sunlight are plentiful.

Carbon is lost where:

• microbial activity is high (such as in warm, moist environments)
• where there are fallow periods with no plant inputs.

Read more about Factors which influence soil carbon levels.