Agriculture Reference
In-Depth Information
of organic management. The gradual increase in the amount of supplied organic material,
from the newly to oldest organic cultivations, does not induce gradual modifications in the
structure or the function of the soil microbial communities. Probably, the fact that available
evidence refers to a narrow range of durations of organic management (from 1 to 6 yrs)
introduces bias in our conclusions. By contrary, characteristics of soil microbial communities
are clearly different among conventional and the oldest organic cultivations. Similarly the
structure of microbial communities of the oldest invaded, by plants, areas is much more
distinctive from the communities of the native areas than from the newly invaded areas.
Invasion resulted from the combined effects of human and climatic interventions is a threat
for the function of entire communities. The effects of plants or microbes invasion on soil
microbial populations are principally ought to the modifications of biotic interactions (plant-
plant or plant-microbes interactions). Even in the case of plant invasion, invaders could be
seen as microbial inoculums that induce an array of interactions. The study of invasion's
influences is of special interest in Mediterranean areas since it is generally acknowledged that
abiotic variables than biotic interactions is the principal force shaping communities in these
areas.
Preadaptation of microbes to specific abiotic conditions and to specific substrates inserted
into soil determines their response to newly created conditions. Soil microbial communities
from different ecosystems exhibited characteristics that continue to persist for long after their
establishment in new sites. So, the idea that microbes adapted quickly by altering their
structure or function in response to changing conditions, because of their short life cycles,
does not seem to occur in Mediterranean regions. In these areas where organisms are
developed to cope with the inter- and intra- annual variability of abiotic variables, the changes
in latter variables induced by human impacts are of lower magnitude. Unlike to non
significant effects of small-scale variations described by climate change scenarios, large-scale
(seasonal) variations of temperature and humidity affected significantly all microbial
parameters. Compared to grazing, season exerted the most pronounced effect on microbial
activity and diversity. Variations in microbial community composition between invasive and
native areas were of less magnitude than variations in composition recorded from year to
year. During the summer sunny days soil organisms experience high temperature regimes in
Mediterranean regions. Thus, fires resulting in soil surface temperatures around 40-50
o
C
(soon after their outbreak) have no dramatic effects on soil microbial communities. Moreover,
there are instances were the influence of time and site characteristics is more significant for
microbes than the influence of pesticides.
By ranking the sensitivity of the three components of microbial communities, that is
activity, structure and size, the former proved to be the most sensitive. Changes of respiration
or of enzymatic activities in response to any disturbing factor either physical or anthropogenic
were reported, since in most cases added materials were used as energy sources. On the
contrary, changes in microbial biomass are rarely detected. Although, data on microbial
structure are generally limited, there is evidence indicating shifts in the structure of microbial
community in response to climatic or human impacts.
Finally, a finding with important consequences for the function of Mediterranean soil
ecosystems refers to the correlation between microbial community structure and soil
processes. Microbial groups that carry out more general processes such as CO
2
production are
redundant between ecosystems, while those involved in specific processes (e.g nitrification)
are not. In Mediterranean areas characterized as fine-grained mosaics due to enhanced spatial
