animals that inhabit the soil. Soil fauna includes representatives of many groups of terrestrial and aquatic animals. Protozoans, rotifers, and tiny nematodes (nanofauna) inhabit capillary and even film water. Small arthropods, including mites, Collembola, Symphyla, and other microfauna, occupy spaces between solid soil particles not filled with water. Larger invertebrates—earthworms, myriopods, and insect larvae that tunnel into soil (mesofauna)—use all the soil as their habitat. Burrowing vertebrates, such as moles, palaearctic mole rats, mole lemmings, and mole mice, constitute the macrofauna, or megafauna.
One sq m of soil contains from ten to several hundred animals of the mesofauna group and from several thousand to several hundred thousand individuals of the microfauna group. There are thousands of protozoans in 1 g of soil. Soil fauna may be subdivided into animals that permanently inhabit the soil (geo-bionts), organisms that pass through one of the active (feeding) stages of their life cycle in soil (geophils), and species that use the soil only as a refuge (geoxenes). Soil fauna occupies mainly the upper horizons, to a depth of 20–40 cm. In arid localities, only a few species penetrate to a depth of several meters.
Soil fauna, an important factor in soil formation, influences all the properties of soil, including fertility. Its activity accelerates the humification and mineralization of plant residues, alters the salt regime and soil pH, increases the soil’s porosity and permeability to water and air, helps to deepen the accumulative horizon and mix the soil layers, and creates the soil’s water-stable granular structure. Many soil inhabitants, especially insect larvae (such as cockchafers, click beetles, and ground beetles) and nematodes, are dangerous crop and forest pests. Agrotechnical and chemical methods are used to control them.
In many countries, to increase soil fertility, especially of land newly brought under cultivation, the soil fauna is enriched by the introduction of useful species and by the application of compost abounding in useful species.
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M. S. GILIAROV