ABSTRACT
Without life, soil would remain an assemblage of mineral particles produced by weathering of rocks. Soil is itself essential for supporting terrestrial life, providing a substrate for plants and hosting the complex food web of soil biota. This includes microorganisms and micro-and mesofauna, most of which are reliant ultimately on nutrients supplied directly or indirectly by plants. The microfauna range in size from the smallest singlecelled protozoans (protists) of around 5 μm to small nematodes, unsegmented worms and tardigrades (eight-legged arthropods) up to 1 mm but not readily visible by eye; mesofauna are larger soil invertebrates which move and shred plant residues or predate on other members of the soil biota (Nannipieri et al., 2003). However, the focus of this chapter is on microorganisms: prokaryotic bacteria and archaea (smaller than 5 μm), and eukaryotic fungi with hyphae up to 50 μm diameter but potentially many metres long. Residues from plant, animal and microbial activity provide organic components, making soils fertile and binding together mineral particles into aggregates that, with the associated pore spaces, confer structure. Soil microorganisms are pivotal for nutrient provision in soil systems through mineralization, where simple and complex organic materials are oxidized to labile inorganic compounds, a large proportion of which is recycled via plants.
