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mineralization, by comminution of plant debris, and via faecal deposition for
subsequent microbial attack (Beare et al., 1995).
2.2.2 Diversity of habitats
Soils are the most complex of microbial habitats as the characteristics of solid soil
components are spatially and temporally variable. This influences the physical and
chemical properties of soils, thus affecting both plant and microbial growth (Marshall,
1976). Soil differs from other habitats in that it possesses a solid phase comprising
approximately half the soil’s volume and consisting of particulate matter of varying
size, which can bind biological molecules (Nannipieri et al., 2003). The remainder of
the soil volume consists of pores filled with air and water. The amount of pore space
depends on the texture, structure and organic matter content of the soil, with
individual pore size, total pore space and pore continuity affecting water movement
and retention (Alexander, 1977).
The most important interfaces affecting microbial behaviour in natural habitats are
those of the solid-liquid type. Moisture availability at this interface can limit the
movement of soil microorganisms (Marshall, 1976), with water and air movement
regulating the activities of the microflora. Bacteria are rarely free in the liquid phase
of soil, with most cells (approximately 80–90%) adhering to solid surfaces such as
clay particles and humus (Alexander, 1977). Soil microhabitats are dynamic systems
since environmental factors are constantly changing (Nannipieri et al., 2003). Within
soil, several microhabitats exist such as the rhizoplane, the rhizosphere, aggregates,
decaying organic matter and the bulk soil itself (Lynch et al., 2004).
Nannipieri et al. (2003) described soil as a structured, heterogeneous, discontinuous
system that is generally poor in energy sources and nutrients (compared with nutrient
concentrations required for optimal in vitro microbial growth) where microbes occupy
discrete microhabitats. Although available space in soil is extensive, less than 5% is
generally occupied by living microorganisms (Alexander, 1977). The heterogeneous
nature of soil results in the presence of so-called ‘hot spots’ (micro-sites) or areas of
increased biological activity, where microflora and fauna are concentrated because
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