Agriculture Reference
In-Depth Information
the microbial communities. Increases in temperature are known to affect Gram-positive
bacteria altering either the amount of branched relative to straight-chain fatty acids and/or the
relative proportion of
iso
and
anteiso
branched fatty acids. Also, the amounts of cyclopropyl
fatty acids increased with increasing temperature (Peterson and Klug 1994). Concluding
Steinberger and his colleagues (1999) mentioned that microbes in desert ecosystems could
exhibit one of the following responses: significant decreases of their biomass during the dry
period which is attributable mainly to the decrease of Gram-negative bacteria, increases of
biomass during the dry period due to the increase of Gram-positive and Gram-negative
bacteria or no changes of biomass between rainy and dry periods. This latter response
characterized microbes of the more arid zones.
C
ONCLUSION
The climatic variables, the plant rhizosphere products and the material used to improve
soil fertility or to control vegetation are the selective forces influencing the size, activity and
structure of soil microbial communities. However, in most instances soil type proved to exert
the greatest influence on these communities. Differences in soil type could mask differences
induced by climatic or human impacts. Therefore, only under similar soil types comparisons
among microbial communities are feasible.
The contents of rhizosphere products are strongly correlated with the occurrence of
specific plant species. Changes in plant diversity patterns are induced by grazing and fire
regimes, by small or large-scale changes in climatic variables and by the increase of CO
2
in
atmosphere. Moreover, plant dominance patterns are modified due to changes in land uses
(e.g. from grasslands to cultivations). Areas occupied by plant communities belonging to
similar successional stages are characterized by microbial communities of similar structural
organization. Moreover, plant growth forms (evergreens or seasonal dimorphic) affect
considerably the size of the microbial biomass while the microbial functional diversity
responds idiosyncratically. It is worth noticing that irrigation with salt water exerts a negative
influence on all aspects of microbial communities while the magnitude of influence is
analogous to that exerted by plant type. The effects of fires on microbial communities could
be dramatic or not because in different cases different driving factors such as the temperature
of the applied fires, the duration of the burning or the water status of the humus are involved.
However, especially for fungi, their responses to fire parameters are always reported negative.
Biosolids and composts are amended to soils for improving soil fertility and facilitating
their recycling. The effects of former on microbial communities depend on the amount, the
chemical content and the frequency of application. Microbial diversity increases linearly as
the rate of biososlids' application increases. Also, due to their chemical content, biosolid
effects on microbial populations vary considerablly from toxic to undetectable ones. Dose-
dependent effects are also produced by compost amendment and pesticides application.
Pesticides can alter the successional patterns of soil microbial communities whereas their
influences are strongly related to soil type. Soil structure is decisive for their bioavailability.
Compost influences strongly all aspects of microbial communities since it could act both as a
microbial inoculum and at the same time as a source of organic matrix. The amount of
organic material inserted into soil differentiated between cultivations with various durations
