Agriculture Reference
In-Depth Information
activities while invasion by exotic species alters the rhizosphere microbial communities
with the magnitude of influence to be increased with time after invasion. CO
2
enrichment, through its effect on plant productivity, affects significantly soil structure.
However, no effects on microbial community structure and biomass are detected.
Treatment with herbicides, compost or biosolids changes the activity, size and
composition of microbes due to physicochemical characteristics of the amendments.
Finally, transition from conventional to organic farming does not results in gradual
changes in the characteristics of the microbial community.
I
NTRODUCTION
Apart from regions surrounding Mediterranean sea, areas characterized as Mediterranean
or with Mediterranean type climate are those occupying mainly the western coasts of the five
continents such as parts in south- western South Africa or in south-western Australia. These
areas have a distinctive character that arises from both the physiographic conditions and the
history of local societies' development (Brandt and Thornes 1996). The question is “which
are the special features of these regions in terms of climate and in terms of their
biocommunities' organization in time and space?”
Mediterranean climate is characterized by a hot summer with a high rate of
evapotranspiration and a mild winter with rather high precipitation (Aschman 1973). The
rainfall varies from about 1000 mm in the more northernly areas and areas above 800 m, to
250 mm in southern drylands. Mediterranean drylands, those with preciptation less than 500
mm per year, exhibit strong inter -annual variability (from 150 to 600 mm; Brandt and
Thornes 1996). The principal limited factor for life in these areas is summer dryness.
Mediterranean-type ecosystems appear to be highly heterogeneous in time and space.
Variations of temperature and humidity conditions in time exhibit interannual, seasonal and
diurnal patterns (Stamou 1998). These variations are the more or less predictable components
of the mediterranean climate while sudden catastrophic events such as heavy rainfalls (100
mm per hour or even more) could be of crucial importance (Lammott and Bladin 1989).
Concerning space, a large-scale spatial heterogeneity is due to the inherent characteristics of
the landscape enabling the coexistence of different biocommunities nearby each other, while
a small-scale fragmentation of the biocommunity derived from human impacts exerted for
millennia. Grazing, burning and cutting resulting to spatially fragmented habitats, are the
main anthropogenic pressures exerted for at least the last 10000 yrs while intensive
agriculture, invasion of exotic species and atmospheric CO
2
enrichment are factors recently
involved. Although the influence of grazing and fire on the structure and function of
Mediterranean regions has been extensively exploited (Perevolotsky and Seligman 1998,
Henkin et al. 2006), the influence of the latter factors is still under investigation.
Traditionally, the vast majority of studies conducted in Mediterranean areas focus on the
structure and function of vegetation (Stamou and Pantis 1995, Kazanis and Arianoutsou 1996,
Arianoutsou and Thanos, 1996, Puerto and Rico 1997, Kazanis and Arianoutsou 2004, Tzialla
et al. 2006, Ovalle et al. 2006, Peco et al. 2006) while those related to soil sub-system are
rather limited.
Likewise vegetation, the soils in Mediterranean regions are the product of interactions
between the natural processes of pedogenesis and the activities of human societies. Brown
