11 - Decomposition

Summary

Building nutrients into organic molecules is one half of the cycle of life; disassembling the molecules is the equally important other half. The rate of material cycling within an ecosystem can be controlled by either process. Primary production in many ecosystems is limited by the rate at which organically bound elements such as nitrogen, phosphorus and sulphur are released for uptake by plants, through the processes of decomposition of plant litter and the mineralisation of soil organic matter. Most of the energy fixed by primary production is not passed to herbivores and so on up the trophic ladder, but is liberated by decomposing organisms. The process of decomposition, and the organisms which accomplish it, are therefore key components in any ecosystem.

The disappearance of organic litter is a result of two processes: comminution, in which the particles are broken up into smaller and smaller pieces and mixed with the soil through the action of the weather and animals; and oxidation, in which the long-chain organic molecules are converted to carbon dioxide and inorganic residues (Swift, Heal & Anderson 1979). Oxidation releases the nutrient elements bound in the organic molecules, and is therefore also referred to as mineralisation. Since comminution can have important effects on the site and rate of oxidation the two processes must be considered together as part of the overall decomposition process, but failure to distinguish them conceptually can lead to misinterpretation of field and laboratory data.