Chapter 2: LITERATURE REVIEW
2.2 THE NATURE AND SIGNIFICANCE OF BIODIVERSITY
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
conditions in relatively few microhabitats are suitable to sustain microbial life (Giller et al., 1997).
The concept of three levels of diversity, namely: α-diversity, which distinguishes between species within the community of a habitat; β-diversity, or the rate and extent of species change along habitat gradients; and γ-diversity, or species richness over a range of habitats, (i.e. total biodiversity in a landscape), was proposed by Whittaker (1972). Gamma-diversity is a function of α-diversity of habitats and the differences in β-diversity between them (Giller et al., 1997; Lynch et al., 2004). Although this approach may be useful for above-ground ecosystems, it is inadequate for interpreting soil diversity as the soil biota are also characterised by spatial diversity. This includes differences between bulk soil and the rhizosphere, macro- and microaggregates, macro- and micropores and different soil horizons (Lynch et al., 2004).
Soil organisms can change the physical, chemical and biological properties of soil in innumerable ways, with the composition and structure of the biota at one hierarchical level influencing the spatial heterogeneity at another level. Consequently it was proposed that soil could be regarded as possessing at least five zones of concentrated activity (Beare et al., 1995). These spheres of biotic activity with their various incumbent organisms interact in many different ways (Haynes and Graham, 2004).
They include:
1. the detritusphere, or the zone of recognisable plant and animal detritus undergoing decay, where litter and humus above the soil surface show considerable saprophytic, mycorrhizal and root fungal activity and colonisation by fungivorous fauna;
2. the drilosphere, or the zone influenced by earthworms by means of their nitrogenous waste excretions and mucilage;
3. the porosphere, or the milieu occupied by organisms ranging from bacteria, protozoa, and nematodes inhabiting water films, to microarthropods, fungal mycelia and larger soil biota occupying channels between aggregates;
4. the aggregatusphere, or the area encompassing all the organic matter constituents, primary particles and voids, where microbial activity is concentrated in the interstices between and within macroaggregates often in association with particulate organic matter; and
5. the rhizosphere, or the zone of primary root influence by means of exudates and exfoliates (Beare et al., 1995), where the numbers and types of microbes are greater than in the adjacent soil (Marshall, 1976). Rhizospheres are dynamic environments where microorganisms compete for resources such as plant-derived organic carbon/energy sources (Piceno and Lovell, 2000) and where plants have a strong influence on soil microbial communities by rhizodeposition and the decay of litter and roots (Nannipieri et al., 2003;
Maharning et al., 2009). The population in the rhizosphere is composed primarily of non-pathogenic organisms. However, because of the density and increased microbial interactions (competitive, antagonistic and beneficial) in this zone, the elimination or suppression of pathogens may result (Alexander, 1977).
The composition of the microflora of any habitat is controlled by the biological equilibrium resulting from the association and interaction of all individuals within a population. Environmental changes may temporarily upset this equilibrium although it will be re-established as the population shifts to accommodate the new circumstances.
In soil, many microbes occur in close proximity and interact in a unique way. The combination of all these individual interactions, establishes the equilibrium population in a given habitat (Alexander, 1977). As no single organism can respond to all environmental contingencies, the assemblage of microbial populations in a typical ecosystem most likely contains a mixture of ‘generalists’ and ‘specialists’. This results in an overall community sensitivity to environmental conditions (Balser et al., 2001).
Soil macrofauna, including earthworms and termites, have direct effects on nutrient cycling by vertical and horizontal redistribution of plant litter, which creates patches of organic matter that act as substrates for microbes and fauna. Some organisms reduce structural and functional diversity in soils by fragmentation of the mosaic of
patches into a relatively homogeneous mixture. By physically rearranging soil particles, the macrofauna change pore size and pore distribution, and consequently patterns of infiltration and gaseous emission (Beare et al., 1995). It is apparent, therefore, that soil is both a heterogeneous medium and a dynamic, ever-changing habitat.