By Porteous, James; Davidson, Steve
Ecos , No. 134
The carbon hidden away in soil originates from plants and is the unglamorous and often forgotten part of the cycle of carbon between the land, oceans and atmosphere. Yet it is a fundamental component of soil's organic matter, integral to healthy soils and the productivity of agricultural and wider ecological systems, and an important sequester of the carbon in C[O.sub.2] that contributes to global warming.
Soil organic matter comprises all the living, dead and decomposing plants, animals and microbes in the soil along with the organic residues and humic substances they release.
'It includes carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur and it is a small but vital part of all soils,' says Mr Jan Skjemstad, formerly of CSIRO Land and Water. 'We now recognise four different types--crop residues, particulate organic matter, humus (usually the largest pool) and recalcitrant organic matter like charcoal--so it is a complex mixture of materials that vary in size, chemistry, degree of decomposition and interaction with soil minerals.'
Dr Ram Dalal, of the CRC for Greenhouse Accounting and the Queensland Department of Natural Resources, has also studied soil organic matter in croplands for many years and has reviewed its role. (1) He describes soil organic matter as containing both living and nonliving organic components.
'Soil microbes, especially fungi and bacteria, and larger soil fauna are the living component while plant residues and various other organic materials represent the non-living portion" says Dalal, 'and it is the microbes that are the main driving agent for organic matter turnover.'
The amount of organic matter in soil results from the balance of carbon inputs (vegetation and roots) and outputs (decomposition, leaching and erosion). This offers the prospect of fiddling with the rates of these inputs and outputs to manage or manipulate soft organic matter levels.
While this sounds promising, the biological, physical and chemical processes involved are complex, so boosting organic matter by, say, altering cropping systems isn't always that simple. For example, there are thresholds of soil organic matter content above which the various benefits for productivity or soil function level off, and clay softs respond differently to more sandy soils.
Soil organic matter is now universally regarded as important stuff. In a GRDC report, (2) Dr Evelyn Krull, of CSIRO Land and Water, and her colleagues, said that many farmers see increases in soft organic matter as desirable because this is usually directly related to better plant nutrition, ease of cultivation and seedbed preparation, greater aggregate stability, reduced bulk density, improved water holding capacity, enhanced porosity and earlier warming in spring. At a fundamental level, soft organic matter provides a reservoir of metabolic energy that enables a range of biological processes to occur in the soil.
A disappearing act
Many studies have documented the seemingly inevitable decline of soil organic matter following land clearing and use of conventional cropping systems.
To give an idea of this, researchers (3) found that percentage losses of organic carbon from Australian soils, already low in carbon compared to European soils for example, varied from 10-60 per cent over 10-80 years of cultivation, and other studies have recorded similar losses. (4)
Why, then, are land clearing and cropping so detrimental to soil organic matter?
'The declining organic matter levels in croplands" says Dalal, 'are mostly due to changes in temperature, moisture, soil aeration, exposure of new soil surfaces as aggregates are disrupted, reduced input of organic materials relative to grassland or forest returns, increased erosion, and the export of carbon and nutrients, such as nitrogen, phosphorus and potassium, from farms in produce.'
Large losses of soil carbon can lead to land degradation while restoration of soil organic matter improves soil quality and enhances ecological sustainability. …