Soil organic carbon source and values
Soil organic carbon (SOC) is derived from organic matter which ranges from living organisms to decaying plant material to charcoal. If the addition of organic matter to soil falls, then the SOC% will also fall.
Organic matter has beneficial physical, chemical and biological influences on soil condition and plant growth, and in some soils is the major source of plant available nutrients. SOC is in continuous flux or turnover, as new material is added and older material is broken down by microorganisms and CO2 released to the atmosphere.
SOM exists as 4 distinct fractions which vary widely in size, turnover time and composition in the soil:
- dissolved organic matter – soluble root exudates, simple sugars and decomposition by-products generally makes up less than 5% of total soil organic matter, and has rapid turnover (days).
- particulate organic matter – fresh or decomposing plant and animal matter with identifiable cell structure makes up 2–25% of total soil organic matter, and has moderate turnover (months to years).
- humus – older, decayed organic compounds that have a slower rate of turnover, can make up more than 50% of total soil organic matter in suitable soils and has slow turnover (10s to 100s of years).
- resistant organically derived matter – relatively inert material, such as chemically resistant materials or organic remnants (e.g. biochar/charcoal), can be up to 10% of soil organic matter and has very slow turnover (100s to 1000s of years).
The inorganic carbon present in soil minerals (for example, calcium carbonate, CaCO3) is not considered in this information.
For more information, see What is soil organic carbon?
What determines SOC in Western Australia?
The attainable SOC content is largely determined by:
- Climate – increasing rainfall increases the potential for SOC%; increasing temperature decreases the potential for SOC%.
- Soil clay content – increasing clay content increases the protection for SOC, and increases the potential for SOC%.
- Annual biomass production – increasing biomass production and retention increases the input of organic carbon, which increases SOC%. Adding biomass from outside the farm can have a similar result – animal wastes, composts, other organic wastes.
- Management and site factors – interact to influence the actual amount of SOC.
- Cultivation increases SOC mineralisation and release of CO2.
- Waterlogging decreases biomass production and also increases mineralisation of SOC.
- Perennial pastures usually increase SOC over annual pastures, and also add SOC deeper in the soil profile.
The interaction of these factors results in low SOC percentages in the low rainfall, sandy soil, Mediterranean climate of Western Australia. Building persistent SOC takes decades, and losses can be very sudden (for instance, following cultivation and a low biomass season).
Climate change over the last 20 years has been to lower annual rainfall and increasing temperatures for most of the agricultural area, resulting in a lower potential SOC%. The forecast climate change is for even less annual rainfall and higher average temperatures.
WA soils are generally low in SOC by global standards, and for the south-west of WA, SOC levels typically range from 0.7% to 4%. In some areas, changing land use from native vegetation to agriculture increased SOC (for example, where annual biomass production has increased under land uses such as pasture), and some areas experienced a decline in SOC associated with cultivation.
The benefits of higher SOC
Management that results in higher SOC is also likely to have improved:
- nutrient cycling
- water-holding capacity
- soil stability and biological activity.
Soil organic carbon condition and trends in WA
Condition in the agricultural areas
Much of the cropping areas have about 1% SOC in the top 10cm of soil: in a soil of bulk density 1.2g/cm3 (grams per cubic centimetre) this equals 12tC/ha (tonnes of carbon per hectare) in the top 10cm. Western Australian soil tests usually report SOC in the top 10cm, while the international SOC reporting is for the top 30cm. So, 12tC/ha in 0–10cm equals 15–25tC/ha in 0–30cm (variation depending on the soil and SOC profile).
The SOC levels (0–10cm) in particular sites in the south-west of WA vary a hundred fold (0.1% to >10%): there is a strong geographic (regional) and local variation in these levels.
- WA soils are low in SOC by global standards, but these low levels are within expected levels for the climatic conditions and soil types in most areas.
- SOC levels are positively related to rainfall and increased biomass production.
- Highest SOC levels (and variability) occur in areas of higher rainfall that support increased biomass production, and on soils that are unconstrained by water availability.
- Actual SOC stocks are only known for a small portion of the south-west of WA.
- Forecast increases in temperature and decreases in rainfall are likely to result in decreased SOC potential levels.
Condition in the rangeland areas
- SOC levels in the WA rangelands are low by global standards, even in higher rainfall areas.
- Rainfall has very little influence on SOC where average annual maximum temperature is above 24°C. All of the WA rangelands fit in this category.
- Forecast increases in temperature across the rangelands are likely to decrease potential SOC levels.
- Forecast increases in rainfall are unlikely to significantly increase SOC potential levels.
There are no measured trends in SOC levels at district, regional or state level in WA.
For more information, see the soil organic carbon chapter (opens a PDF) in the Report card on sustainable natural resource use in agriculture.
How management affects SOC
In the extensive cropping and livestock areas of WA, any increases in SOC are largely a result of sustained increases in biomass growth. There are opportunities to increase the input of organic matter and to reduce the loss of organic matter in many agricultural systems, with the upper limit of stable SOC set by the climate and soil clay content.
Intensive cropping and livestock areas can build SOC by adding high levels of organic matter on a regular basis: manures and mulches are common sources.
Regenerative agricultural systems have a goal of increasing SOC, as an indicator of soil health and productive capacity.
Noticeable changes in stable SOC will usually take several decades.
For more information, see Managing soil organic carbon on-farm.