Decomposition is where complex organic materials such as those from plant material such as leaves, branches, roots and root exudates break down in to simpler molecules to be used by other organisms. It is therefore one of the most important services performed by soil organisms (Barrios, 2007). Decomposition is carried out at many levels, from the initial physical breakdown to smaller particles during feeding by small invertebrates and then colonisation and enzymatic digestion by microorganisms. The ultimate end point of the decomposition process is the production of carbon dioxide which is re-cycled back to organic materials via plant photosynthesis and other autotrophic organisms.

Organic matter decomposes at different rates depending on the complexity of the molecule. Simple sugars such as glucose are readily used by a very wide range of organisms. Polymers, being more complex, need a succession of enzymes to effect their breakdown. For example, cellulose is a homopolymer of glucose. Its initial depolymerisation to shorter subunits units is carried out by a range of microorganisms including the fungal saprophytes Aspergillus, Trichoderma and Penicillium and bacteria such as Bacillus and Clostridium.

Much of the soil cellulose is associated with other more complex polymers such as lignin. Lignin is the most resistant polymer from plants and is the third most abundant of the plant residues after cellulose and hemicellulose. The composition of the lignin molecule is dependant upon the plant source with relatively few microorganisms such as some Basidiomycetes fungi able to completely break down the molecule. Soil animals help in the degradation of lignin by dispersing lignin degrading organisms through the soil.

The rate of decomposition is dependant upon the carbon:nitrogen (C:N) ratio , temperature, pH amount of water. Decomposition is generally limited by the amount of nitrogen available although when this is not limiting other minerals such as phosphorous may become the limiting factor. Fungi prefer a higher C:N ratio than bacteria (200:1 for fungi compared to 20:1 for bacteria). Low C:N ratio are associated with high turnover rates, high C:N ratio with slow decomposition.

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