Agriculture Reference
In-Depth Information
particular depths (type II). This may be due to the leaching of toxic substances from
the surface litter
e.g.,
the distribution of
Azotobacter
in forested rendzinas (mollisols).
Micro-organisms may also have a concave type of distribution. This has been observed
in soils where chemical compounds toxic to micro-organisms, have accumulated in
intermediate soil layers thus depleting the microbial community. The deeper horizons of
higher pH in calcareous soils are favourable for the greater development of populations
of micro-organisms.
The depth distributions of bacteria also change seasonally. In the upper 8 cm of
a forested rendzina (mollisol), Proth (1978) observed three patterns of distribution,
depending on the season (Figure III.7). Kjöller and Vestberg (1985) made similar obser-
vations on seasonal variation in the distribution of bacteria in the holorganic (L+F +H)
layers of an alder forest soil waterlogged for most of the year. At their study site,
variation in environmental conditions during decomposition was responsible for these
changes and differences in the representation of particular functional groups of
micro-organisms.
The abundance of fungi generally decreases with depth, especially in forest soils
where they preferentially colonise the litter and humus layers (
e.g.,
reviews by Kjöller and
Struwe, 1982; Bissett and Parkinson, 1979; Rodriguez
et al.,
1990). Within the O horizon,
abundance increases from the surface to the F or H layer and then decreases sharply.
In grasslands, the decrease with depth may be far less clear and Harris (1971) has
reported greater abundances at 10-30 cm than in the upper ten centimetres.
