Agriculture Reference
In-Depth Information
Moreover, as biosolids application increased, the C/N ratio of plant material was reduced
resulting in greater N mineralization activity.
Compost amendment is another way to improve soil conditions. It can stabilize soil
structure (Barzegar et al. 2002), increase the amount of organic material inserted to soil
(Filcheva and Tsadilas 2002), stimulate soil microflora (Ros et al. 2003) or induce
antagonistic effects to native microflora. The impact of compost on microbial communities is
ought to organic and inorganic compounds that they contain and to the interactions that
compost endogenous microflora induced since it is a microbiologically active product
(Schloss et al. 2003). Data concerning the influence of compost as microbial inoculum was
presented in the study of Saison et al. (2006). The hypothesis that compost-borne pathogens,
through their interactions with other soil microorganisms were responsible for the
modifications of the native soil microflora community, was not supported by this study. They
concluded that the changes in microbial characteristics were essentially due to the input of a
compost matrix rich in organic matter. But which were the microbial parameters that were
affected by compost amendment? Size, activity and structure. In all cases, the influences were
dose-dependent. Changes in genetic diversity and activity were detected only when high
amount of compost was added. Under these conditions, differences in genetic diversity were
recorded only 4 days after the application, while the influences on microbial characteristics
persisted for much longer period in comparison with effects induced by low doses.
Apart from materials incorporated into soils, pesticides used in liquid phase to control
vegetation have a considerable influence on microbial communities. Pesticides are
distinguished in herbicides, fungicides and insecticides. For the use of herbicides a lot of bare
soil is needed, while for the others lower exposure of soil is enough. The disadvantages of
pesticides' use, which increased considerably over the last 50 years, are related to the non-
specific toxic influence on microorganisms. In some cases, the toxic effect was not due to the
main constituent but to its secondary metabolites produced during its degradation (Bjørnlund
2000). The bioavailability of each pesticide is of crucial importance and is strongly related to
soil type (Busse et al. 2001).
The effects of pesticides on microbes were examined in terms of overall microbial
activity (Greaves 1982), but this cannot give any insight on what happens in the microbial
communities. Some of the pesticide constituents could suppress the growth of specific
microbes or enhance the proliferation of others, affecting the successional patterns in the
microbial community (Johnsen et al. 2001). The effects of a herbicide (glyphosate) on soil
microbial communities, was studied by Busse et al. (2001). Glyphosate is often the preferred
herbicide in intensive forestry and is used mainly in California forests. There are controversial
results concerning its influence on communities. In artificial media the effects on microbial
growth were negative (Dick and Quinn 1995), while in field studies the effects were either
absent or stimulating (Haney et al. 2000, Gianfreda et al. 1995), a fact that was associated
with the soil characteristics. To test this hypothesis, glyphosate effects on biomass, activity,
community size and metabolic diversity were examined in an array of soils differing in the
amounts of clay, oxide and organic matter (Busse et al.2001). Soil respiration in glyphosate-
treated soils was much higher than in the control implying that microorganisms can use
glyphosate as energy source or for acquisition of nutrients. The variation in respiration
between soils followed the soils differences in absorption potential which is related to their
oxide and clay content. The other aspects of microbial community did not respond to
glyphosate application. It is interesting to notice that the impact of time and site proved more
