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plants. Soil interstitial species responsible for decomposition are adapted to this particular habitat. Their trophic interactions release complex organic matter into simpler more soluble molecules, which are accessible to plant roots and their symbionts. One by-product of decomposition accumulates as chemically resistant humus. Another by-product of their respiration accumulates in the atmosphere as carbon dioxide, which is required for photosynthesis. The production of biologically useful inorganic molecules from organic compounds, as a result of biological activity, is termed biomineralization (Fig. 2.1).

The soil is responsible for irreplaceable ecosystem services, such as water filtration, food production, recycling of nutrients through decomposition, and detoxification of chemicals. However, soils can be variously abused by agricultural overexploitation, chemical pollution or poor management. As our global human population density increases, our demands from the soil and the impact on soil ecosystems are exacerbated. The complexity of the soil, and of decomposition, is illustrated by the hundreds of species of bacteria, protozoa, fungi and invertebrates which can be found in just a few grams of most soils. Soil processes in nutrient cycling, carbon storage and the return of C as CO2 to the atmosphere sustain primary production upon which organisms, including humans, depend. The challenge before us is to understand soil ecology sufficiently, so as to manage these processes sustainably for the future.

Fig. 2.1. Schema of the ecosystem and flow of water, air and nutrients through soil, across a hill slope to a riparian zone.

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