Agriculture Reference
In-Depth Information
2.4.1.2
Horizontal distribution
The horizontal distribution of micro-organisms on a mesoscale is determined by geological
and pedological factors. Plant distribution also greatly influences the distribution of soil
micro-organisms on the soil surface by the litter effect, and within the predominantly
inorganic soil, by the rhizosphere effect. Large invertebrates such as termites and earth-
worms may also significantly affect the distribution of micro-organisms in their respec-
tive functional domains (see Chapter IV). The development of bacterial populations in
leaf litter is limited and they are most abundant within the upper mineral soil horizon
where their horizontal distribution is influenced by the availability of energetic resources.
They are most concentrated in the rhizosphere where they live on readily assimilable
root exudates and dead root material. Other sites in the soil with concentrations of
organic matter may be preferentially colonised by bacteria and include structures built
by soil invertebrates, such as the linings of earthworm burrows or the walls of termite
galleries and fungal chambers (see Chapter IV).
In leaf litter, fungal populations often show patchy distribution patterns. This is
especially clear for white rot fungi (Basidiomycota), essential decomposers of the litter
system, which form patches of a few square centimetres (Bartolozzi, 1984 in Garay, 1989).
The
microscale distribution
of bacteria is that at the microaggregates,
i.e.,
small
structural units of a few tens of micrometres in size. Bacteria may live inside microag-
gregates or outside, on their surfaces or in the pores
between them (Hattori and
Hattori, 1976; Balkwill
et al.,
1977).
Outside the aggregates, the flux of water is rapid which facilitates the supply of
energy, nutrients and oxygen, although conditions may quickly become limiting when
soils dry out. The microflora is mainly composed of fungi, spore-forming Gram-positive
bacteria and Actinobacteria (Figure III.8). Members of these groups are opportunistic
organisms whose populations can vary rapidly depending on microclimatic conditions
and their ability to resist adverse conditions.
