Agriculture Reference
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type. For structure assessment microscopy and FAMEs analysis were employed. The results
obtained by these two methods were not in accordance. The more sensitive variable to
changes was the abundances of different FAMEs. When influences were ranked according to
the magnitude of their effects on function and structure, soil type had the greater impact,
while for the same soil type season became more important. A more general study referring to
different types of land uses is that of Steenwerth et al. (2002) in California. Different
grasssland and cultivated ecosystems provide a gradient of increasing intensity of soil
disturbance. How the structure of microbial community changes in relation to the history of
land use and the associated management inputs and practices? Do the communities clearly
discriminate between the different land uses? In order to avoid bias introduced by differences
in soil texture, in this study sites of similar soil texture were chosen. A sharp distinction was
recorded between restored communities with longed lived buchgrasses, communities with
formely-cultivated annual grasslands and cultivated ones. Remarkable was the finding that
the structure of microbial communities of all annual grasslands was similar although the time
since the last tillage event varied from 8 to 50 yrs. The explanation relies on the plant
community composition which remains consistently similar in the same successional stage.
On the contrary, the microbial communities of perennial grasslands varied widely.
Concerning the extent of similarities among microbial communities, Buckley and Schmidt
(2001) concluded that 7 yrs of abandonment after cultivation results in a microbial
community more similar to that of cultivated areas than to never cultivated ones.
The Effects of Biosolids, Compost and Pesticide Application
Since biosolid wastes such as composts, sewage sludges or municipal solid wastes are
recently produced in large amounts, there is an increasing concern about their recycling.
Apart from recycling, the incorporation of these amendments to soil has a significant impact
on soil quality. Biosolids' application could be used as a tool for restoring or improving the
soil fertility in severely degraded areas. Since microbial community has a significant
contribution to soil fertility, in order this latter to change, modifications in composition and
function of soil microbial communities are expected. Most studies focusing on biosolids
examined their effects on arid or semi-arid rangelands (Barbarick et al. 2004, Garcia-Gill et
al. 2004). These ecosystems are considered vulnerable to degradation due to limited
availability of water and the influence of grazing. While biosolids could affect plant
productivity (improving the forage quality for livestock) and soil microbial communities,
their effects depend on the amount, the chemical content and the frequency of application.
The diversity of microbial community increased linearly with increases in the rate of
application (Dennis and Fresquez 1989) while their influence on soil microbes continued to
persist for years after application (Pascual et al. 1999, Barbarick et al. 2004, Sullivan et al.
2006). The rate of colour development in Biolog wells, reflecting microbial activity, was
faster in biosolids-treated than in unamended soils and the rate of development increased as
the dose of application increased (Sullivan et al. 2006). The effects on microbial biomass can
be variable: decrease due to the biosolids toxic chemical concentrations, increase as a
response to increasing plant productivity and to the added amounts of C, N and other nutrients
or no effect. In the study of Sullivan et al. (2006) the lack of effect of biosolids on total
microbial biomass was followed by a shift in the bacterial/fungal ratios in favor of bacteria.
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