Carbon storage for healthy WA soils


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Types of soil carbon

Soil organic carbon (SOC) contributes to a range of soil functions that interact and influence crop production.

Plants take carbon dioxide (CO2) from the air for growth and add it to the soil through plant debris.

Microorganisms break down plant debris and convert a portion of it into soil organic carbon.

Soil organic carbon comprises living and non-living fractions and the main types are:

  1. Microbial biomass carbon – the living component of soil organic matter, which is mostly micro-organisms (such as fungi and bacteria). Microbial biomass carbon decomposes plant residues to release CO2 and nutrients into forms available for plants.
  2. Plant and animal residues– organic residues on and in soil (e.g. shoot, root, manure) greater than 2mm in size.
  3.  Particulate organic carbon – individual pieces of plant debris, smaller than 2mm but larger than 0.05mm. Plant residues and particulate organic carbon improve soil structure and can supply or reduce the availability of nutrients to growing plants. For example, as residues with high nutrient contents (such as legume residues) are decomposed, nutrients will be released. But the decomposition of low nutrient-content residues (such as stubble) will initially reduce the availability of nutrients.
  4. Humus - decomposed materials, smaller than 0.05mm, which are dominated by molecules stuck to soil minerals. Humus supplies nutrients and improves soil structure, water-holding capacity and cation exchange capacity.
  5. Resistant organic carbon - dominated by pieces of charcoal. Resistant organic carbon lifts the soil’s cation exchange capacity in sandy soils and its ability to hold water.

Roles of soil carbon

Each type of organic carbon has a different role in the soil and contributes to a range of physical, chemical and biological processes, including:

  • Chemical – cation exchange, pH buffering, reduces effects of sodicity.
  • Physical – water retention, soil structural stability, soil temperature.
  • Biological – energy for microbes, provision of nutrients and resiliency.

A decline in soil organic carbon often has a negative impact on soil structure and fertility, increases run-off and reduces crop yields.

Increasing levels of soil carbon can maintain or improve soil structure, soil water balance and soil productivity over time.

Amount of carbon in the soil

Typically, soils across the world contain about twice as much carbon as the atmosphere.

Most Australian agricultural soils contain between 10 and 250 tonnes of total organic carbon per hectare in the top 30cm. This wide range reflects differences between soil types, environment and use (crop versus pasture). 

Typically, cropping systems in WA have between 10 and 40 tonnes SOC per hectare (0-30cm).

The amount of organic carbon found in a soil can be calculated using values for the depth (cm) of the soil layer of interest, the soil bulk density (g/cm3) and the soil carbon content (%). 

For example, using  this equation, a 20cm layer of soil having a bulk density of 1.2g/cm3 and a carbon content of 1.2% contains approximately 29 tonnes of SOC/ha.

The amount of organic carbon a soil could potentially hold (or the carbon ‘bucket’) varies with factors such as soil clay content, soil depth and bulk density and is not influenced by management. 

The soil carbon ‘bucket’ is generally smaller for a sand than a clay soil.

The amount of carbon a soil actually holds results from the balance between inputs (plant residues) and losses (microbial decomposition and associated mineralisation) across many years. 

Inputs are controlled by the type and amount of plant residue added to the soil. Any practice that enhances productivity and the return of plant residues (shoots and roots) to the soil opens the input tap.

Losses of carbon from soil can result from erosion as well as the decomposition and conversion of carbon in plant residues and soil organic materials into CO2. Processes that accelerate decomposition open the losses tap further. 

How carbon is stored in the soil

Organic carbon exists in the soil in fractions, or ‘pools’, some of which are readily available and turnover rapidly (more labile/bio-available) and others that are more stable and break down slowly.

The higher the labile content of the total soil organic carbon pool, the more biologically fertile the soil will be and the greater the potential for nutrient turnover.

The labile pool of soil carbon can turn over in less than five years and results from addition of fresh residues, such as plant roots and living organisms.

Research has demonstrated that retention of crop stubbles can lift N supply by up to 4kg/ha/day from labile carbon pools.

Monitoring changes in labile carbon can provide some indication of changes in soil carbon content due to changes in management practices (but this still can take some years).

Simulation models, such as the soil carbon model RothC used by researchers, can also be used to predict the likely outcomes of management practices on soil carbon content.

How to lift soil carbon levels

Build-up of the soil organic carbon pool has many benefits, including improved soil structure, a more regulated soil water-holding capacity and nutrient cycling. 

In cropping systems, the potential to build soil organic carbon depends on the capacity to produce big quantities of crop biomass – through high yielding crops and/or high frequency crop rotations – and return and retain carbon in the soil.

Management practices such as minimum or no tillage, retaining stubble, managing soil erosion, increasing soil moisture, using well managed pasture phases and potentially using organic amendments will help to boost soil carbon.

But, even where all crop residues are retained, only a fraction of retained carbon will ultimately accumulate in soil. 

It takes about 15 to 20t of carbon to increase the organic carbon content of the top 10-15cm of the soil by one per cent. It can, therefore, take decades to significantly influence the carbon levels of many soils.

There is ongoing research into the value of adding Biochar (a fine grained charcoal high in organic carbon) to the soil, as it may have potential to contribute to the resistant organic matter pool. This is important for carbon sequestration.

The ideal soil organic carbon level depends on soil type, the kind of organic matter available and the role a farmer might want the soil carbon to provide.

For example, a sandy soil will need added organic matter to hold onto cations (such as potassium, calcium and magnesium).

But in soils with more clay, organic matter is not as necessary for retention of cations because the clay particles also contribute.

On high clay content soils, the effect of organic matter on cation retention may be insignificant compared with that of sand.

An active population of micro-organisms may also help suppress diseases and improve the flow of N within the soil for plants to access.

More research is needed to quantify the likely return on investment from lifting the organic carbon content for specific soils and different crops. 

Carbon and nutrients

A high organic carbon level (as indicated in a standard soil test) may mean there are more organic nutrients, such as N, phosphorus (P) and sulphur (S), potentially available in the soil bank.

The level of nutrients in the soil may also vary with the quality of the organic material in, and added to, the soil.

For instance, the residues from a good legume pasture, which has a relatively low carbon to N ratio (i.e. a higher N concentration) will release more N than residues from cereals.

The organic carbon result shown in soil test reports has historically been used to improve nutrient recommendations, particularly for N, because of the presence of organic N.

It also helps fine-tune S and zinc recommendations and, to a lesser extent, the advice about potassium (K) inputs.

Carbon sequestration

It is understood that when it comes to carbon sequestration, trading schemes are likely to require carbon to be stored in stable forms for decades.

Humus, the product of the breakdown of plant residues and soil microbes (typically representing 40 to 60 per cent of the total organic carbon pool), is the main stable form that can be influenced by growers.

But storing soil carbon has associated costs for growers.

Nutrients such as N, P and S are tied up along with stored humus - and soil carbon will not build up without adequate levels of these.

To sequester one tonne of carbon as humus, about 800kg of N, 200kg of P and 150kg of S must be available in the soil.

There is no doubt that soils could potentially hold more carbon. The challenge is to be able to do this while still maintaining an economically viable farm enterprise. 

The GRDC publication Managing Soil Organic Matter contains further advice: Visit the download page for the Managing Soil Organic Matter Guide.

Some potential options include:

  • Enhancing the proportion of perennial vegetation in pastures or conversion of paddocks that continually give negative returns to perennial vegetation.
  • Increasing retention of crop residues, reducing stocking rates and increasing use of green manure crops to return more plant material to the soil.
  • Optimising farm management inputs to maximise water use efficiency and thus maximise the return of crop residues to soil.