Soil carbon explained

Heather Field, Agriculture Victoria Climate Change Service Development Officer

Are you keen to understand more about soil carbon?

Agriculture Victoria has developed a soil carbon eLearning module that will provide a great introduction for those seeking a better understanding of soil carbon, its role and function in agriculture.

Over recent decades farmers have been actively working towards conserving and increasing soil carbon and reducing soil carbon losses by improved farm management practices. Soil carbon is critical for soil health, improving productivity, profitability and resilience. While soil carbon is easy to lose, and can be challenging to increase, it’s important to remember that maintaining your soil carbon levels whilst producing food and fibre is a great outcome.

The introduction to soil carbon eLearn delves into:

• soil health benefits of soil carbon
• difference between soil carbon and soil organic matter
• influence of soil type, climate and land management on soil carbon stocks
• impacts of agriculture practices on soil carbon
• where to go for further information and resources.

If you would like to learn about the importance of soil carbon, this is a great resource.

Ten key considerations for soil carbon changes

Accurate longer-term measurement and monitoring is essential to determine changes to soil carbon levels. Factors such as soil carbon testing methods and accuracy, the age of trials (particularly if less than 5 years old), plus rainfall and seasonal variability are all factors which must be carefully considered before conclusions are made.

Increasing carbon input rates or decreasing carbon loss rates can improve soil carbon levels and have other benefits including improved soil nutrient uptake, (where nutrients are available), water holding capacity and overall productivity.

While soil organic carbon (SOC) can function as a source of nutrients for farm production, it is also important to consider the reverse of this process, as increasing soil carbon levels will require nutrients to be locked away and bound up with the sequestered carbon.

Soil carbon occurs in a number of different fractions, each having different properties, vulnerabilities and rates of decomposition. The Particulate Organic Carbon or labile fraction can be easily lost and decomposed in the soil and subsequently released back into the atmosphere as carbon dioxide.

The capacity for soils to sequester carbon is finite and there are specific maximum achievable equilibrium levels of soil organic matter for most farming systems due to climatic and primary productivity limits to plant dry matter production and decomposition rates.

For carbon accounting purposes, genuine carbon sequestration must result in an additional net transfer of carbon from the atmosphere to land, not just movement of a carbon source from one site to another.

Changes in land management which lead to increased carbon in soil must be continued indefinitely if farmers wish to maintain the increased stock of SOC. For many farmers, committing to long term land use may be undesirable if it reduces their ability to adjust land management to meet changing market or profitability drivers over the longer term.

Some management practices may only be reducing losses of soil carbon and not actually sequestering additional atmospheric carbon into the soil. Many soils are still responding to initial cultivation of the native soil and experiencing soil carbon decline.

Increasing soil carbon may potentially lead to perverse impacts as a consequence of the links between soil carbon, nitrous oxide and methane cycles. For example, changing from annual crops to permanent pastures may increase soil carbon, but may also lead to an overall increase in total net emissions via increased ruminant livestock production. Soil carbon needs to be considered in a wider systems context.

Climate change and changing patterns of seasonal variability will affect the ability of soils to maintain or sequester carbon. For some regions this may make the task of maintaining or improving soil carbon levels even more challenging over coming decades.