Soil carbon
Soils are an important global carbon pool, storing more carbon than the atmosphere and vegetation combined. As part of the carbon cycle, soils can store, cycle and emit different forms of carbon. Some of these forms may be very stable and stay in the soil for thousands of years, while others may be broken down in just a few hours.
The global carbon cycle is the cycling of carbon between the atmosphere, the oceans and land. Human activities affect the distribution of carbon between the different pools of the global carbon cycle. In the terrestrial (land) pool, carbon can be found stored in the biomass of vegetation and in soil.
Soil carbon forms
Soil carbon occurs as organic and inorganic forms. Organic carbon is derived from soil organisms, microbes and decaying plant matter. These make up the main component of soil organic matter. Organic carbon can also be found in sugars, starches, proteins, carbohydrates, lignins, waxes, resins and organic acids.
Inorganic carbon is derived from the weathering of rocks or minerals reacting with the atmosphere. The most common form of inorganic carbon found in soil is calcium carbonate.
Soil organic matter
Soil organic matter is an important part of a healthy soil, influencing many soil functions and properties. Soil organic matter is made up of several ‘pools’ which last for different lengths of time in soil and affect different soil functions.
- The 'active pool' is formed by recently added plant and animal debris and partially decomposed materials.
- The 'slow pool' is formed as the organic residues from plants, soil organisms and microbes are further decayed, becoming humus.
- The 'passive pool' is made up of inert organic matter or charcoal, which are very stable but not biologically active.
Soil organic matter has many functions that are beneficial, including:
- acting as an energy source for micro-organisms
- providing a supply and storage of nutrients (such as nitrogen, phosphorous and sulfur)
- supporting soil structure by binding particles together into aggregates
- increasing water infiltration and water holding capacity
- increasing pH buffering capacity (the ability of soil to resist changes in pH), making soils less vulnerable to soil acidification.
Managing soil carbon
The use and management of land determines the movement of carbon between the pools of carbon. Vegetation and soil rehabilitation improve the capture of carbon from the atmosphere, whereas deforestation and burning of vegetation release carbon from the terrestrial (land) pool into the atmosphere.
Soil degradation and cultivation also results in the loss of soil organic carbon, released into the atmosphere in the form of carbon dioxide. Land clearing and over-grazing disturb the soil and can contribute to soil organic carbon loss by removing plant residues and accelerating the decomposition of organic matter.
Storing carbon in soil
The most common question about soil carbon is how much can be stored in a soil. The best guide to how much carbon your soil can store is to look at how much carbon exists(ed) under native vegetation. Without the addition of more water, the biological productivity of your soils (which determines the amount of soil carbon), will be limited first by rainfall and temperature, and second, by management. In simple terms, higher rainfall leads to more carbon being stored and higher temperatures lead to less.
Even under the best management, it is unlikely that any agricultural practices (cropping or grazing) will lead to soil organic carbon levels equal to or higher than was found prior to disturbance. Organic carbon stocks in Australian soils are generally much lower than the global average, because much of Australia is arid. Data on soil carbon in Queensland soils can be accessed from the Queensland Globe.
Soil carbon and grazing systems
Around 85% of Queensland is used for grazing of pasture land. This area includes woodlands, forests, natural grasslands and areas planted to exotic pasture species. General practices for improving soil carbon in grazing lands include:
- growing pastures with perennial species, as these pastures have greater soil carbon inputs than annual pastures
- managing pastures well so they are not overgrazed or eroded and can produce a higher rate of plant growth, which increases or maintains the level of carbon in the soil.
Soil carbon and cropping lands
Queensland farmers have been very selective in the land they choose to grow crops on—only 2% of the state is used for cropping.
Practices that may help maintain soil carbon levels in cropping lands include:
- minimising soil compaction by the adoption of practices such as controlled traffic farming
- using green and brown manure crops
- adopting a pasture phase in a crop rotation system (preferably including a legume)
- adopting zero till or conservation farming practices.
Of these, adopting a perennial pasture phase is the most likely practice to increase soil carbon levels in cropping lands.