Agriculture Reference
In-Depth Information
structure is expected the direction of changes is unknown. There is an equal possibility for
increases of fast growing microbes that could exploit the labile substrates produced by
microbial death or for dominance of gram-positive bacteria and fungi with enhanced
osmoregulatory capabilities, due to their cell walls characteristics. In accordance with
Warldop and Firestone (2006), the bacterial communities in oak and grasslands soils, studied
by Fierer et al. (2003), were distinctive in terms of genetic structure. Their response to
frequency of drying-rewetting events differed between the vegetation types. In oak soils the
higher the frequency of stress that soils were exposed, the more the bacterial communities
differed from the unstressed control. On the contrary, the grass soil shows a high degree of
community-level variability between samples and no apparent separation of bacterial
communities on the basis of stress frequency was detected. The differences in the responses
of the bacterial communities were well explained if their adaptation to field abiotic conditions
was taken into account. For not preadapted to high variability in field soil moisture oak
communities, the exposure to drying-rewetting cycles was more stressful. As Balser and
Firestone (2005) mentioned the response of soil processes to environmental changes was also
a function of the preexisting adaptation of the microbial community and its physiological
plasticity.
The study of Balser and Firestone (2005) aimed at investigating the relation between
composition and function. Specific functions (N-mineralization, C-mineralization,
nitrification potential) and their relation to parameters of microbial communities were studied
in an annual grassland and a mixed-conifer forest. Biomass and lipid diversity were unrelated
to soil processes, while the opposite occurred with community structure, as this latter was
depicted on the first principal component analysis of PLFAs. The production of N
2
O
,
the
gross N mineralization and the nitrification potential were related to composition of the
community, the production of NO
3
-
to structural and functional organization of the
community, while the CO
2
production to temperature. It was revealed that community
structure was strongly related to functions carried out by a narrow group of microbes while
functions that are most common between soils such as CO
2
production, were rather controlled
by abiotic variables than by the composition of the microbial community. These findings are
in accordance with those of Warldrop and Firestone (2004) concerning the lack of
redundancy of microbes mediating specific soil processes. The presence, absence or
abundance of these microbes proved crucial for soil functioning. Cyclopropyl fatty acids,
indicative of gram-negative bacteria were associated with N
2
O production and nitrification
potential while branched fatty acids (biomarkers for gram-positive bacteria) were associated
with nitrate concentration. The study of Balser and Firestone (2005) illustrated the ability of
fatty acids analysis to reveal the link between structure and function.
Another ecosystem in which the variability of soil microbial community parameters in
time and space was studied, was a desert in Israel (Steinberger et al. 1999). The chosen sites
represented a gradient from semi-arid to arid climate (rainfall varied from 650 to 110 mm and
temperature from 13 to 20
o
C). The main characteristic of desert ecosystems is the
unpredictability of moisture availability that forces organisms to be active only for short time
periods. The selective force of rainfall was proved by the positive relation between
precipitation and microbial biomass (Steinberger et al. 1999). Apart from biomass, the
number of fatty acids was also higher in sites with higher precipitation. Soil microbial
communities in terms of their structural organization were distinctive between sites and
among sampling periods. Changes in climatic variables induced shifts in the composition of
