Soil carbon

What are the sources, factors and forms of soil organic carbon, why is it important and how is it measured?

Sources of organic carbon

Swamp banksia (Banksia robur) seedlingsLand-based organic carbon is derived from plant production. Plants use sunlight and atmospheric carbon dioxide (along with water and soil nutrients) to grow and generate sugars and organic plant tissue through the process of photosynthesis. This organic material is then consumed by other organisms through the food chain and is excreted or dies and decomposes.

The organic material that is produced along the way either decays to produce water and carbon dioxide. The carbon component of this soil organic matter then becomes soil organic carbon. Soil organic carbon is one part of soil organic matter, together with hydrogen, oxygen, nitrogen, phosphorus and sulfur.

Soil organic carbon is an important soil property.

Why soil organic carbon is important

The nature and quantity of organic carbon in the soil affects a wide range of physical, chemical and biological soil properties.

Soil nutrients. Decomposition of organic materials in the soil releases soil nutrients such as nitrogen, phosphorus etc.

Soil structure. Soil organic carbon promotes good soil structure by binding soil particles together in stable aggregates. Improved structure aids aeration, water holding capacity, etc.

Soil biology. Organic matter and organic carbon in the soil are a food source for a range of soil organisms and so enhance soil biodiversity and biological health. A wide range of organisms in the soil also helps release nutrients and create pores and can help protect against crop diseases.

Soil protection.  Adequate soil carbon reduces the severity and costs of natural phenomena (eg drought, flood, and disease) and can increase farm production. By contributing to, or protecting, the soil, soil organic carbon contributes to farm production and increasing soil organic carbon is valuable for a range of soil health, sustainability and production benefits.

Factors influencing soil carbon levels

Climate. Rainfall and temperature have by far the strongest influence on soil organic matter levels. Soil organic matter content is usually higher where rainfall is higher and temperatures are cooler. Soil organic matter tends to decompose more rapidly in warmer soils. Therefore, across the Border Rivers Gwydir catchment, soil organic matter levels tend to decrease from east to west as average rainfall decreases and average temperatures increase.

Soil type. Soils that are naturally more fertile will tend to have higher organic matter contents due to the greater amount of both living and dead organic matter (biomass) that can be produced. Soils high in clay will tend to retain more soil organic matter than sandy soils. For these reasons, basalt and clay soils in the Border Rivers Gwydir catchment will tend to have higher organic matter contents than sandy or granite soils. Soils in a given location will have a limit on how much carbon can be captured before they become carbon saturated, which is defined by their physical environment.

Soil moisture. Moist soils tend to have more carbon than their drier counterparts. Waterlogged soils or soils in wet depressions will tend to have more carbon.

Soil aeration. Soils that are better aerated tend to lose carbon more rapidly. 

Topography. Soils at the bottom of slopes will usually have higher organic matter levels because these areas are generally wetter and have higher clay contents. Poorly drained areas also have much slower rates of organic matter decomposition and higher organic matter contents. 

Plant productivity. The greater the plant growth, the more organic matter is made available to the soil (assuming that this is not all removed). The hardier the plant is, the slower the decomposition rate and hence the higher the soil organic matter levels. 

Management effects.  Land-use and land management practices can also influence the amount of organic matter in the soil. Land management affects soil carbon due to the balance of carbon inputs against outputs (ie. how much organic matter is produced, how much is removed from the site and how much remains to be added to the soil). Land uses and management systems that generate more organic matter and retain it on-site tend to have higher soil organic carbon levels.

Forms of soil organic carbon

Soil organic carbon is a complex mixture of organic compounds at different stages of decomposition.

Because different forms of carbon behave differently, they are often grouped into three distinct pools:

  1. labile pool
  2. slow pool 
  3. inert pool.

Carbon pools are important when we consider soil management. We can increase soil carbon by implementing a number of management practices. The nature of the carbon pool that is increased will determine how long the carbon stored will remain in the soil (ie. how fragile the additional soil carbon is).

Labile carbon includes fresh plant and animal material and micro-organisms which are easily decomposed.

The slow pool includes well-decomposed organic materials called humus.

The inert pool is the soil carbon fraction that is old, resistant to further breakdown and in the last stage of decomposition. Soils differ not only in total soil organic carbon but also in the composition of the different organic carbon pools.

It is our aim to maximise the amount of slow pool and inert pool carbon as this will maximise soil condition, health and stability.

Measuring soil organic carbon

A number of techniques exist by which differences in soil organic matter and carbon can be assessed between sites, soils, land-uses, land-management types and through time.

The currently accepted standard for measuring soil carbon is the LECO method. This method uses a total combustion approach where a soil sample is heated to more than 1000 degrees Celsius. The carbon dioxide liberated from the sample during this process is measured directly to give a measure of total soil carbon. This is now the standard technique for soil carbon estimation for carbon accounting.