Agriculture Reference
In-Depth Information
the soil surface litter that in mediterranean areas can undergo frequent drying-rewetting stress.
The experimental field treatments involved regular or irregular watering and a control
receiving natural rainfall. Sampling was conducted in three occasions and data were
compared to that referring to the decomposition of a temperate spruce litter in Germany. The
shifts in microbial composition, during a period of 17 weeks were more pronounced in spruce
than in pine litter. Pine litter seemed insensitive to drought treatments. It exhibited lower
water content and was characterized by fungal dominance and low concentrations of bacterial
PLFAs. Similar were the results concerning the effect of litterbags' mesh size on PLFA
profiles. In pine litter, only microeukaryotic markers were affected by mesh size implying
that microclimate appeared more important than biotic interactions in determining the
structure of microbial communities.
Human activities result in considerable increases in the amounts of produced CO
2
and O
3
.
Under conditions of elevated CO
2
, photosynthesis and plant productivity generally increase
although a large variation among species and ecosystems is also reported (Poorter et al.
1996). The increase of plant productivity is followed by an increase in the amounts of C
inputs via enhanced plant biomass turnover and/or root carbon losses. CO
2
enrichment is
expected to affect the structure and function of soil microorganisms, since rhizodeposition
plays a significant role in shaping soil microbial communities (De Lucia et al. 1999). On the
contrary, O
3
is a phytotoxic pollutant with detrimental effects on productivity. Damage to the
photosynthetic system in O
3
-sensitive plant species could reduce productivity and soil C
inputs, thus counteracting the effects of elevated CO
2
on microbial metabolism in soil. The
response of soil microbial community beneath early- and late-successional plant species,
under conditions of elevated CO
2
and O
3
, were studied by Phillips et al. (2002). Specifically,
soils were amended with two different substrates (cellobiose and N-acetylglucosamine) and
the changes in microbial structure, biomass and activity were investigated. The hypothesis
that increases of CO
2
affect positively microbial metabolism by affecting C inputs to soil was
supported by this study. Respiration was 29% greater beneath plants growing under elevated
CO
2
. The amendment of the two different substrates affected differently the two groups of
microbes; N-acetylglucosamine increased in a similar way the metabolism of bacteria and
fungi, while that of cellobiose increased mainly the metabolism of fungi. Elevated CO
2
altered the composition of bacterial community; gram-negative bacteria became dominant
over gram-positive while their total biomass remained unaffected. After 3 yrs of experiment,
the authors concluded that microbial community developed under early-successional tree
species seemed to be more sensitive than that developed under late-successional species.
M
ICROBIAL
C
OMMUNITIES IN
D
IFFERENT
E
COSYSTEMS
The responses of different groups of soil organisms to seasonal climatic variations in
Mediterranean areas, were shown to be quite predictable (Stamou 1998). But what about the
responses of soil microbial community? Does composition changes with season and are these
changes predictable? Apart from climatic variables, the type of vegetation exerts also an
important influence on microbial community composition. The presence of specific plant
species induced heterogeneity in the availability of resources in time and space. In seasonally
varying climate the effects of vegetation on microbial communities could be modulated or
