Agriculture Reference
In-Depth Information
significant than that of glyphosate. Very often the effects of pesticides are masked by
variation in temperature, humidity, substrate availability and disturbance (Domsch et al.
1983). This becomes even more pronounced in mediterranean regions because the
heterogeneity in time and space belongs to their inherent characteristics.
T
HE
E
FFECTS OF
G
RAZING
Millenia of human impacts have shaped the highly heterogenous “Mediterraenan mosaic
landscape” (Naveh 1998). The major factor in the shaping of this landscape is the
heterogeneity in grazing pressure by different species of domestic animals (Perevolotsky and
Seligman 1998). For instance, in Israel cattle graze within fenced rangelands at a more or less
constant stocking rate, while goats usually graze at unfenced areas during specific time
periods (Henkin et al. 2006). At a plant community level, grazing could affect soil processes
by altering plant species diversity and the plant dominance patterns (Ovalle et al. 2006, Peco
et al. 2006). Positive relations between plant diversity and culturable soil bacterial activity
and diversity were recorded by Stephan et al. (2000).
In addition, by grazing, a mosaic of microsites from sheltered areas under shrubs to
exposed locations between shrubs is created. In each microsite different plant species can
occur, producing litter of variable quantity and quality. Moreover, due to differences in cover
because of the grazing management, soil micro-abiotic conditions vary from one site to
another (Papatheodorou et al. 1998). All these influences are expected to affect significantly
the soil microbial communities. Monokrousos et al. (2004) examined the extent to which soil
chemical microenvironment - aspects of microbial community included - differed beneath
various plant species. This study was conducted in a Greek grassland where grazing resulted
on the coexistence of five shrub species interspaced by bare soil. Samples were taken from
areas underneath each species in periods coinciding with seasonal changes in climatic
variables. Variables describing the size and activity of microbial community (microbial
biomass C and N, ergosterol as an index of fungal biomass and soil respiration) were affected
significantly by sampling period and plant species. However, the variability in data induced
by temporal variation was greater than that induced by spatial heterogeneity. Due to
differences in microbial biomass, soils under evergreens were discriminated from soils under
seasonal dimorphic species, indicating the importance of plant growth form in shaping
microbial communities. Under the same experimental scheme but only for one sampling
period the functional diversity of microbes was surveyed by Biolog GN plates. The produced
pattern was different than the previously mentioned one. The microbial diversity in each
microsite was completely distinctive from the others while the plant life form didn't seem to
have any significant effect (unpublished data). The conclusion is that in this case the response
of microbial functional diversity was indiosyncratic.
Apart from Monokrousos et al. (2004), the pronounced effect of season on microbial
activity and diversity, compared to that of grazing, was also recorded in soil plots from
Chihuahuan desert (Liu et al. 2000). Grazing exhibited insignificant independent effect but
when it was combined with drought treatments, the effects on diversity became significant in
specific seasons. According to the authors, the key factor for soil microorganisms in this
ecosystem was the availability of litter substrate.
