Ten ways to build soil carbon

Soil organic carbon (SOC) is the main component of soil organic matter (SOM) (approximately 58 percent) and is what actually gets measured in the laboratory. Soil organic matter is made up of organic materials in varying states of decay, including small pieces of roots, stems and leaves (less than 2 mm), partially decomposed organic matter, microbes and charcoal. Australian soils also contain char (fine charcoal) in varying quantities.

SOC is often divided into ‘active’ (typically 20-40 percent of soil carbon) and ‘stable’ (typically 60-80 percent of soil carbon) components. SOC is in a constant state of flux and can be decomposed quickly (days, months) or slowly (years). Stable SOC is often referred to as ‘humus’ and is more resistant to decomposition. Active SOC is often referred to as ‘labile’, turns over relatively quickly and relates to the biological fertility and nutrient availability of a soil.

 

Why is soil organic carbon important?

SOC plays an important role in stabilising soil structure, holding and releasing plant nutrients and contributing to soil water holding capacity. SOC is also a large sink of carbon with potential to mitigate climate change by storing carbon in the soil that would otherwise be warming the atmosphere. Increasing SOC has production benefits, as well as opportunities to diversify income through carbon markets and sustainability credentials.

 

How is soil organic carbon increased?
Adding carbon, reducing losses of carbon and protecting existing carbon in the soil are the three main pathways for increasing SOC. Management strategies to increase SOC act on these pathways by enhancing the inputs and minimising the losses. Plant-based carbon is the main input of SOC in agricultural systems. This includes roots, shoots, leaves, flowers, crop residues and root exudates. Soil microbes convert fresh organic matter into more stable (protected) forms of SOC, such as humus. Decomposition of SOM by microbes and erosion are the main losses of SOC from the system. Increasing SOC can be slow and the rate will vary depending on soil type, climate, vegetation, level of SOC and management. Generally, soils with a low SOC content will increase SOC faster than soils high in SOC. Soil type will also determine the amount of carbon a soil can potentially hold onto. In turn, best management strategies will depend on the soil type, climate, enterprise type, land use history, farm plans and the condition of the soil and environment.

 

Published: 2021

Author(s): Greater Sydney Local Land Services