Agriculture Reference
In-Depth Information
Grazing not only increases spatial heterogeneity but it could also affect soil processes in
variable ways: by changing the amount and/or the chemical nature of produced litter, by
creating “hotspots” of microbial activity due to animal dugs resulting in redistribution of
organic material and nutrients (Haynes and Williams 1999) or finally by changing the soil
physicochemical properties due to trampling. However, the available evidence in regard to
this latter showed that although the soil physical properties changed significantly, the
microbial communities (size, structure and activity) exhibited broad tolerance to compaction
indicating poor link between physical and biological indices of soil health (Shestak and Busse
2005, Busse et al. 2006). When soil microbial biomass C and N were recorded underneath
ungrazed, moderately grazed and overgrazed
Quercus coccifera
(L.) stands as well as in open
areas between shrubs, similar values of microbial C were recorded all over the experimental
sites (unpublished data). The lack of changes in the size of microbial biomass due to grazing,
is recorded in the literature quite often (Li et al. 2005, Jackson et al. 2006). Non-significant
effects of grazing on the structure of soil fungal communities were reported for grasslands
soils in Spain (Maggi et al. 2005). On the contrary, microbial biomass N in
Q. coccifera
stands was significantly lower in soil under overgrazed shrubs, supporting the hypothesis that
grazing increases mineralization of N instead of N immobilization (Holland and Detling
1990).
T
HE
E
FFECT OF
F
IRE
Fire occurred in the Mediterranean region for millennia and much of the natural
vegetation is adapted either to irregular wildfires or to prescribed fires of low intensity
(Fioretto et al. 2005). Alike the intensity of grazing fire frequency affects pattern and
diversity of plant communities and landscapes. As mentioned by Traubaud and Galtie (1996),
in some parts of the Massif Central (southern France) fires increased the homogeneity of the
landscape, while in other parts they enhanced heterogeneity and diversity of plant
communities.
In a special issue of the International Journal of Wildland Fire, Doerr and Cerda (2005)
pointed out that although fire has an impact on entire ecosystems -flora, fauna, atmosphere
and soil - special attention has been paid only on the first three ecosystem components.
However, burning and the post-fire conditions can alter the physical soil properties
(aggregation, pore size, water movement and runoff response), the chemical characteristics
(pH, humus structure, availability of nutrients and C/N ratios) and the biological properties
(structure of microbial community, biomass productivity e.t.c) as well. Burning of vegetation
and litter causes a dual effect on soil. It induced a transient heat shock in the upper soil
horizon and produces a layer of nutrient rich ash on the forest floor. Both phenomena being
capable of affecting soil microbes (Pietikainen et al. 2000).
D'Ascoli et al. (2005) applied burning of low and high severity in an Italian maquis and
studied immediate influences on microbial properties. Although, one week after the fire the
functional diversity changed due to both burning per se as well as to the intensity of burning,
a rapid recovery of diversity was recorded. On the contrary total microbial biomass was
enhanced by fire. According to authors, the effect of fire on fungi was greater and persisted
for a longer period compared to that on bacteria. Greater microbial biomass and respiration
