Loss of Soil Organic Carbon
Soil organic carbon (SOC) is derived from organic matter which ranges from living organisms to decaying plant material to charcoal. 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.
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%. Historically, drier regions of the south-west of WA were inherently low in SOC because of the strong influence of climate on the amount of organic inputs and the prevalence of sandy textured soils. While in some areas changing land use to agriculture actually increased SOC (e.g. where biomass production has increased under land uses such as pasture), large areas of land converted from native vegetation to agricultural systems experienced a decline in SOC associated with cultivation. While the attainable SOC content is defined by climate (rainfall and temperate) and soil clay content, management and site factors interact to influence the actual amount of SOC.
SOC underpins a number of critical soil functions, such as nutrient turnover. However, no “desirable” level can currently be set for any soil-location-management situation as there might be for soil pH in relation to crop production. In general, the loss of SOC is considered to have adverse effects on nutrient supply, soil water storage and carbon storage. Gains in SOC will generally have the reverse effect.
Soil Organic Carbon and Climate Change
While there are no measured trends in SOC levels over time as yet, current knowledge allows us to predict possible changes in SOC associated with a changing climate. The possible influence of changing climate on driving factors and impact on SOC trends is presented in the Table below (Table 2.4.4 in DAFWA Report Card 2013), with many scenarios associated with a decline in SOC. While some hypothetical climate change scenarios have been presented, there are no expectations that these will occur or that they will occur in isolation
Increasing SOC requires increased plant growth, increased organic matter inputs or decreased decomposition of organic matter, or a combination of these. In principle, better water use efficiency and agronomic management to improve crop yields provides more plant growth and should support either maintenance or growth of SOC stocks. Decreasing plant residue losses also increases the potential for organic matter accumulation in soil.
Many practices have been advocated for increasing soil health, including the management of farming systems for increased SOC. The possible impact of common practices on SOC have been generalised n the table below (Table 2.4.5 from DAFWA Report Card 2013).
Key messages across south-west Western Australia (DAFWA Report Card 2013)
Condition and trend
- WA soils are low in soil organic carbon (SOC) by global standards.
- 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.
- It is likely to take a decade or more of monitoring to detect significant changes in SOC because of its high spatial and temporal variability.
- SOC levels are likely to decline in response to predicted declining rainfall where there is a corresponding decrease in biomass production.
- Actual SOC stocks are only known from a small portion of the south-wets of WA.
- Detecting differences in SOC levels due to existing land use and management in the south-west of WA is difficult because of:
- large influences of rainfall and soil moisture on SOC levels
- land use and management practices are applied selectively to soil types and across regions.
- For SOC levels to be maintained or increased, management should focus on:
- increasing plant biomass production by addressing soil constraints, and improved agronomic management
- minimising organic matter and soil loss from water and wind erosion
- increasing the frequency and amount of organic matter returned to soil
- applying organic inputs where profitable.
- Management systems that support a greater proportion of the year under an actively growing crop or pasture are more likely to result in potential gains in SOC.
- Increasing SOC is widely regarded as beneficial to soil function and fertility and has been associated with increased agricultural productivity.
- Promote management actions which boost biomass production as a strategy for increasing SOC.
- Promote erosion mitigating actions as a strategy for increasing SOC and soil health.
- Determine how soil and water conditions and land use and management types influence the levels of SOC in the medium and high rainfall areas as a way of identifying areas of highest potential for increase in SOC.
- A method be developed to predict the climate change related biomass production and consequential trends in SOC in the south-west of WA.
Projects addressing the Loss of Soil Carbon
SWCC supports a number of landholders and partners to undertake projects addressing agricultural practices including soil organic carbon. One project addressing the loss of soil carbon is:
- Lebeckia: A perennial legume for non-wetting deep sandy soils.
Further information on this project can be found on the Agtrialsites website here.
The Agtrialsites website www.agtrialsites.com is a cross regional NRM WA initiative that aims to provide information on sustainable agriculture projects across Western Australia.
- Griffin E, Hoyle F and Murphy D (2013). ‘Soil organic carbon’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.