Agriculture Reference
In-Depth Information
was also recorded in plots subjected to experimental fires (De Marco et al. 2005). Enhanced
microbial biomass is an uncommon response to burning, because the soil structure is
destroyed, the pH increases while the forms of N and C remaining after fire are more
recalcitrant to microbial attack (Rutigliano et al. 1995, Diaz-Ravina et al. 2006). In this vein,
Liu et al. (2000) mentioned decreasing of functional diversity and microbial activity due to
burning, in spring and summer. The authors concluded that it was due to reduced carbon
inputs into soil because of the delimitation of plants due to burning. It seems that most of the
results reported in literature are quite controversial. One of the reasons might be related to the
water status of the burned humus. In fact, when humus is dry an application of a prescribed
burning of 150
o
C could result in an immediate decrease of microbial biomass but when the
humus is moist, the relevant effects could be different. Only after water evaporation - which
can last for a certain time period- increasing temperature would result in decreasing microbial
biomass (Pietikainen et al. 2000).
The work of Guerrero et al. (2005) related the response of microbes to the temperatures
of applied fires. They studied the fate of organic C and microbial C in temperatures varying
from 100 to 700
o
C. For temperatures below 400
o
C, the increase of extractable C enabled the
recolonization of bacteria with consequent changes in respiration. In higher temperatures,
heating caused a reduction in extractable C followed by negative impact on microbial
biomass. For fungi, temperatures above 200
o
C caused an almost 100% reduction in their
biomass. The high sensitivity of fungi to fire is supported also by the findings of D' Ascoli et
al. (2005). As shown, fires of relatively high temperatures (> 500
o
C) have considerable
impact on microbial population. In the same vein, Palese et al. (2003) found that the
enzymatic activities decreased by fire, but a peak of activity was recorded in soil burned at
309
o
C and it was related to higher amounts of soluble C. Another interesting finding is
provided by the work of Fioretto et al. (2005) who examined the effects of prescribed fires on
soil microbiota under three shrub species typical of mediterranean maquis (
Phyllirea
angustifolia
L.,
Cistus incanus
L. and
Myrtus communis
L.). They reported that although the
microbial biomass and the activities of selected enzymes differed among soils collected
underneath different plant species, in general the effects of fire were not dramatic. The
impacts of fire on the size of microbial biomass exhibited less variation, while that on activity
was largely dependent on vegetation mosaic and species. The lack of dramatic effects was
attributed to the fact that temperature in soil surface did not increase extremely. Actually,
with the application of fire, the surface temperature raised from 100 to 400
o
C, for a few
minutes, while during the next two hours dropped to 40-50
o
C. In Mediterranean areas, soil
biota are adapted to face temperatures in this range (40 - 50
o
C) because such temperatures
are frequently attained within the soil surface layers during the summer sunny days.
Diaz-Ravina et al. (2006) examined the effects of heating, fire retardant and soil texture
on the composition of microbial communities. Retardants are usually polymers applied in
order to combat wildfires and to control prescribed burnings. Among the examined
parameters, heating at 350
o
C provoked drastic changes in soil properties followed by
decreasing microbial biomass and alteration of community structure. Although most of the
microbes were killed by fire, Gram negative bacteria were able to use the high availability of
substrate while they were also favored by pH increases. Bacteria recolonized rapidly burned
areas, whereas the recovery of fungi was extremely slow. The effects of soil texture and
retardant were of minor importance. The influence of retardant depended on dose and was
differentiated between heated and unheated soils.
