Soil organic carbon (SOC) is essential for soil health. It assists in soil structure and provides food for soil microbes that in turn benefits plants and helps regulate nutrient cycling. SOC is in a constant state of flux as microbes and other soil fauna decompose and convert carbon into carbon dioxide. SOC can be maintained or increased by increasing organic carbon inputs, or by reducing organic carbon losses. Management strategies aimed at increasing soil carbon may also have negative impacts. For example, changing from annual crops to permanent pastures may increase soil carbon, but it may also lead to an overall increase in total emissions when the additional ruminant livestock production (methane emissions) is taken into account.
The amount of soil carbon varies across the Corangamite region with peat soils in the region’s southwest at levels greater than 10%, through to areas with high cultivation histories where the carbon level is typically less than 1%. This variation is a result of many factors, namely:
Soil and vegetation type, which determines the carbon-holding capacity
Climate, especially rainfall and temperature which determine the rate of decomposition
Land management practices, both current and historic.
In general terms, on agricultural land, higher soil organic carbon is likely to be found in areas of higher rainfall where traditionally perennial pastures have been used (i.e. dairy areas), whereas lower rainfall areas with annual cropping rotations have lower soil organic levels.
A recent study by the Woady Yaloak Catchment Group - on 950 soil samples collected over 20 years - indicated the majority of results fell in a band of between 2.0% and 3.5%. They determined that cropping tended to decrease total soil organic carbon while perennial pastures increased the rate. (Woady Yaloak Catchment Group, 2012).
The amount of carbon in soil can be maintained or increased with the rate of loss influenced by the:
The amount and type of organic matter, both plant and animal, entering the soil
Management practices which reduce carbon inputs, increase losses and/or increase decomposition rates. Examples include cultivation, stubble removal and overgrazing
Climate conditions such as rainfall, temperature and sunlight
Soil properties including the clay, silt or sand content.
The natural perennial vegetation cover that existed prior to European settlement declined as a result of traditional farming practices along with the region’s soil organic carbon.
Since the 1990s, the region has made progress in adopting more perennial based grazing systems and encouraged minimal cultivation cropping. Industry groups such as Sustainable Grazing Systems, Evergraze and programs delivered by the state government have led to more land management practices that have minimised carbon losses. An interest in soil carbon sequestration has also led to this shift in land management.
Regional soil organic carbon potential
There are three main practices to improve soil carbon in the region’s soils. These are:
Grow pastures, crops and trees to maximise above ground biomass and root production whilst appropriately ‘fitting’ the farming system.
Retain as much existing soil carbon as possible.
Use carbon rich sources or practices that increase soil function (biochar, humates, biological products, biosolids, green manure, high carbon using crops).
More information on soil organic carbon potential can be found in the "Brown Book", a sustainable agricultural information resource for the Corangamite region. It addresses the needs of farmers to develop simple solutions in effective management of soils to enhance productivity, including soil organic carbon. More information on the Brown Book can be found at www.ccmaknowledgebase.vic.gov.au/soilhealth/brown_book/home