Agriculture Reference
In-Depth Information
They also explain why, at low moisture potentials, non-spore forming cells may survive
in moist microsites. Finally, they illustrate how opposing processes (
e.g.,
nitrification
and denitrification) may occur at any time in closely adjacent microsites (see Fig. I.15,
p40), as emphasised by Berg (1986).
Different types of micro-aggregation, and therefore soil structure, may influence the
composition of the overall microbial community. Some soils will have more opportunistic
micro-organisms located in non-protected microsites whereas others with a well-developed
micro-aggregate structure will have large amounts of Gram-negative specific bacteria
protected in micropores (see Section III 2.4.3).
2.4.2
TEMPORAL VARIATION
Microbial populations may change rapidly. In a few hours, or days, rapid changes in
environmental conditions,
e.g.,
a sudden input of assimilable organic matter or moisten-
ing of a dry soil, may trigger a flush of microbial activity and rapid cell division.
On a seasonal scale, progressive changes in climatic or energy supply conditions
may be reflected in changes to the densities and biomasses of microbial populations.
Over longer time-scales, organic matter transformations during decomposition may
be followed by changes in micro-decomposer populations resulting in very different
microbial successions.
2.4.2.1
Short-term variation
Dormant micro-organisms may rapidly reactivate when conditions again become
favourable. Bacteria which are protected inside microaggregates may leave this shelter
24 h after a sucrose solution has been added to the soil (El Bahlki
et al.,
1978) and it takes
a similar time for fungal or bacterial spores located outside the aggregates to germinate.
Consequently, bacteria can increase their numbers dramatically within a few days, although
little variation occurs within 24 h (see,
e.g.,
Zaitzeva and Zviagintzev, 1978; Clarholm
and Rosswall, 1980).
In the humus layer of a Swedish conifer forest, Clarholm and Rosswall (1980)
observed increases in population density and biomass of, respectively, 2 to ind
and 4.2 to 17.6 mg dwt dry wt soil over three days following rainfall. An analysis
of the structure of bacterial populations showed that, under such conditions, 15 to 30 %
of the formerly-dormant population grew actively with a generation time of 14 to 17
hours. After a maximum has been reached, both density and biomass decrease following
intensive grazing by protists and nematodes (Figure III.9).The initial rise in biomass
resulted from an increase in the size of individual cells, as shown by the significant rise
in the number of large rods observed in the population. Numbers subsequently declined,
probably because they had started to divide into small coccoid cells.
Bottner (1985), simulated the alternation of 8 to 10 day dry and 15 to 20 day moist
periods in a Mediterranean soil. Drying killed 25 to 33 % of the biomass (bacteria +
fungi) although during the wet periods, biomass returned again to the same level present
before drying.
