Agriculture Reference
In-Depth Information
obtained based on the organisms that are actively metabolizing the substrates (Roslev et al.
1998). Significant information about soil microbial community is also revealed by the
consideration of genetic diversity. As significant components of living cells, DNA and RNA,
are excellent signature molecules (Paul and Clark 1996). The DNA from environmental
samples is obtained either by direct lysis of cells within the soil or by isolation of bacterial
cells and then lysis of the bacterial fraction (Orveas 2000). From the different fractions of
RNA, the most interest in microbial ecology is rRNA, and especially 5S and 16S rRNA.
There is an array of methods used to analyse the microbial genetic diversity in environmental
samples but I don't refer to it, since it is beyond the scope of this study.
For biomass assessment, the methods that are widely used are the fumigation-incubation
method of Jenkinson and Powlson (1976) and the fumigation-extraction method (Vance et al.
1987
)
. The exposure of soil organisms to chloroform vapor destroys their cells and allows
cytoplasmic C, N, P, and S to leak into the soil. By the use of appropriate conversion factors
these substances are converted to microbial biomass-C, N, P and S respectively. Alternative
methods for biomass estimations are that of microscopy based on direct counts as well as that
of substrate-induced respiration (SIR; Anderson and Domsch 1978). This latter can be used
for assessing microbial activity called “basal respiration” as well. Direct counts provide
limited information since only the culturable part of microbes is considered while the
culturing environment differs from the soil environment. Soil microbial activity is estimated
by the CO
2
-release, when soil is incubated under aerobic conditions for a specific time period,
or through the assessment of enzyme activities. In the case of enzymes, the activity is specific
and refers to the degradation of known substrates.
It has been established that soil biota in Mediterranean regions are well adapted to face
the adversity of environment induced by the oscillations of abiotic variables and the impacts
of human interventions (Stamou 1998). In this context, the aim of this review is to explore the
responses of soil microbial communities to the above mentioned influences. Therefore, the
review is organized in a way that each section discusses the responses of microbial
communities in terms of structure and function to grazing, fire, agriculture, invasion of exotic
species, change of climatic variables. If relevant information is available, the responses of
microbes to these interventions will be compared to those induced by the predicted
oscillations of abiotic variables that constitute an inherent characteristic of mediterranean
climate.
M
ICROBIAL
C
OMMUNITIES IN
A
GRICULTURAL
S
OILS
The Effect of Management Practices
In studies focusing on the soil quality in agricultural systems, the community of soil
microorganisms is examined in order to detect its ability to reflect changes induced by
different management practices or by different cropping systems. Nowadays a substantial
interest is devoted to the way organic and conventional cultivation affects soil properties. For
most of these properties (e.g. soil organic matter) significant changes occur only after decades
(Johnston 1986), while for microbiological properties few years are enough (Drinkwater et al.
1995).
