How important is soil organic carbon?
"Of the 2700 Gt of C [gigatonnes of carbon] stored in soils worldwide, 1550 GtC is organic and 950 GtC is inorganic carbon, which is approximately three times greater than the current atmospheric C and 240 times higher compared with the current annual fossil fuel emission. The balance of soil carbon is held in peat and wetlands (150 GtC), and in plant litter at the soil surface (50 GtC). This compares to 780 GtC in the atmosphere, and 600 GtC in all living organisms. The oceanic pool holds 38 200 GtC." Wikipedia viewed 17 October 2018.
Every tonne of organic carbon is the equivalent of about 3.67 tonnes of atmospheric carbon dioxide.
The potential for increasing organic carbon storage in soil depends on soil type, climate and management factors, many of which are outside our control.
Practicalities of increasing soil organic carbon
Most Western Australian soils have relatively low capacity to sequester more carbon as stable organic forms, and this is strongly dependent on soil type, climate and land use. However, small increases in soil organic carbon over the very large area of Western Australia make this option for carbon sequestration seem attractive.
In the agricultural areas
- 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 forecast declining rainfall with a corresponding decrease in biomass production.
- SOC levels are likely to decline in response to forecast rising temperatures, as biological turnover increases.
- Actual SOC stocks are only known for a small portion of the south-west of WA.
- Detecting differences in SOC levels due to existing land use and management in the south-west of WA is difficult because:
- rainfall and soil moisutre have a large influence 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 reducing soil constraints, and improving agronomic management
- minimising organic matter and soil loss from water and wind erosion
- increasing the frequency and amount of organic matter returned to soil (green and brown manuring, stubbles)
- applying organic inputs where profitable (mulches, manures).
- 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.
- Removing organic material from one place to add to another may have no net gain in SOC.
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.
- Predicted increases in temperature across the rangelands are likely to decrease potential SOC levels.
- Predicted increases in rainfall are unlikely to significantly increase SOC potential levels.
- SOC loss could be minimised by reducing the impact of wind and water erosion, fire damage and loss of vegetation.
- SOC levels could be increased by increasing plant biomass and groundcover, especially on potentially productive degraded land.
See the attached document which describes the status and trend of SOC in the agricultural areas of south-west WA, the 'See also' links, and the external links. You can also contact one of our staff mentioned below.