Agriculture Reference
In-Depth Information
since regional variation may be very high: British and Scandinavian tundra soils, for
example, have densities 3 to 100 times higher than equivalent North American ecosys-
tems (Kjöller and Struwe, 1982). Furthermore, microbial biomass seems to be roughly
proportional to the amount of organic matter (Figure III.3). Since there is no clear geo-
graphical pattern for organic matter accumulation in the soils of vegetated regions
(Sánchez
et al
., 1982), regional and local factors are more likely to determine microbial
abundance than factors operating at larger scales.
On a regional scale, vegetation, soil characteristics and types of land-use influence the
abundance and composition of the microflora by modifying water and temperature
regimes and oxygen and energy supply. In Swedish agricultural soils, the density of
bacteria increased with soil organic matter concentration in cultivated fields (Figure III.6)
(Schnürer
et al
., 1985). The effect of soil type has also been observed by Kaczmarek
(1984) in Polish cultivated fields with mean densities of 5
and 18
g dry soil
respectively in sandy and muck soils.
The
meso-scale
may be considered as that represented by vertical heterogeneity down
a soil profile or horizontal heterogeneity created by plant distribution or microrelief.
2.4.1.1
Vertical distribution
Bacterial population densities diminish with depth in parallel with organic matter contents.
However, other factors such as the water-logging of deep horizons or differences in
the physical structure of successive horizons may confound this pattern leading to
different patterns of decrease with depth (Dommergues and Mangenot, 1970).
Population densities of most micro-organisms decrease regularly with depth (type I):
maximum density occurs close to the surface and the rate of decrease may vary depending
on the species. However, some micro-organisms attain their maximum densities at
