SOIL ORGANISMS - Soil Ecology - page 225

These estimates are 1,000 to 10,000 times longer than those obtained for isolated
microbial colonies under optimal laboratory conditions and imply that soil micro-
organisms are only active during short periods of time in a limited number of microsites.
Further, cytochemical tests indicate that very few cells are physiologically active (Foster,
1988). Microbial communities thus appear as 'a huge, largely dormant population, with an
enormous richness of species and an ability to survive hard times' (Jenkinson and Ladd,
1981). The first characteristic is considered a response to the heterogeneity of soils and
the second, a response to their inability to search for food. This striking contrast between
the potential for an extremely fast turnover and field reality has been called the 'Sleeping
Beauty Paradox' (Lavelle et al., 1994a). Macro-organisms ( i.e., roots and invertebrates)
that have the ability to move the soil and change environmental conditions at the scale
of micro-organisms can interrupt this dormancy (acting as 'Prince Charming') and hence
appear to be major regulators of microbial activities (see Chapter IV). Interactions
among micro-organisms with high capacity to digest almost all organic substrates
and 'macro-organisms' that have potential for mechanical activities, are the basis of
the biological systems of regulation that determine soil function (see Chapter IV).
These results also emphasise the need to assess microbial processes at much finer
scales than a hectare or a year. Identifying the appropriate scales requires an in-depth
knowledge of the biology and population dynamics of these organisms.

2.4.4 COMMUNITY STRUCTURE

2.4.4.1 Species richness

The total estimated number of microbial species is very high. Fungi are the most diverse
group with an estimated number of 1,500,000 species whereas algae and bacteria may
comprise, respectively, 60,000 and 30,000 species (Hawksworth and Mound, 1991).
Studies of microbial DNA extracted from a Norwegian forest soil showed the existence
of more than 4000 independent genomes of bacteria in a gram of soil (Torsvik et al. ,
1990). Based on the assumption that individuals with more than 70 % DNA homology
pertain to the same species (Wayne et al., 1987), these authors suggest that there may be
as many as 20,000 to 40,000 bacterial species per gram of soil.
The soil microflora is commonly said to be poorly specific to any one environment
as many species of bacteria and fungi are found in a great variety of soil types and
microclimates; some species have world-wide distributions. Comparison of three
fungal communities from forests in southern Quebec (Widden, 1986), heathland from
northern England (Widden, 1987), and tropical montane forests of Mexico (slopes of
Mt Popocatepetl, Rodriguez et al., 1990) showed that four species are common to these
very different environments i.e., ca. 15 % of the species represented at each site.
This assessment however, is a matter of controversy as the concepts of species and
ecological niches are difficult to apply to micro-organisms (especially bacteria and
actinobacteria) because of their high genetic variability and ecological versatility (see e.g.,
Szábo, 1974; Swift 1976). Further, morphological criteria are few and often difficult to
observe. Indeed, such criteria may be entirely misleading since colonies with very different
morphologies have been shown to have the same DNA and, thus belong to the same