Symposium on “Protozoan Ecology”: The Role of Protozoa in Litters and Soils1

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Abstract
Soil is the focus of organic matter turnover in terrestrial ecosystems and is an interstitial mosaic of microsites composed of particle aggregates and pore spaces, where transformation, decomposition, mineralization, and humification of organic matter takes place. Microorganisms and animals are scattered discontinuously in these microsites. Microarthropods and larger fauna increase the rate and amount of mineralization by comminution of organic matter and by redistribution of microsites through movements of earthworms and large arthropods; however, mineralization and return of nutrients to plants occurs in the community of bacteria, fungi, protozoa, and nematodes living in the water films covering aggregates and filling pore spaces. Protozoa, especially small amoebae, are important bacterial grazers because they can enter tiny spaces unavailable to nematodes. The latter graze bacteria, fungi, and protozoa. Protozoan and nematode predation increase the amounts of soluble nutrients and decrease the competitive abilities of bacteria, thus making these nutrients more available to plants. Protozoa enhance nutrient recycling out of proportion to their biomass.