FEMS Microbiology Ecology 13 (1993) 93-104
© 1993 Federation of European Microbiological Societies 0168-6496/93/$06.00
Published by Elsevier
93
FEMSEC 00487
Soil microbial populations after wildfire
Francisco J. Vfizquez, Maria J. Acea and Tarsy Carballas
Instituto de Investigaciones Agrobioldgicas de Galicia (CSIC), Campus Universitario, Santiago de Compostela, Spain
(Received 14 March 1993; revision received 2 August 1993; accepted 24 August 1993)
Abstract: Population fluctuations were increased by burning, which also modified the incubation patterns and the densities of
several microbial groups, although without changing the order of their population sizes. In the short term, fire produced a sharp
increase in microbes but affected the groups studied differently. Aerobic heterotrophic bacteria, including the acidophilic and
sporulating ones, were stimulated by fire while cyanobacteria, algae and fungi (propagules as well as hyphae length) were clearly
depressed. In the long term, the positive effect of fire on bacteria was nullified except on the sporulating ones; fungal propagules,
but not mycelium, reached the unburned soil values, cyanobacteria and algae also increased. Soil incubation both improved the
beneficial and diminished the negative fire effect on the microbiota.
Key words: Burning; Bacteria; Cyanobacteria; Fungi; Algae; Forest ecosystem
Introduction
Soil microorganisms are important agents in
nutrient cycling and energy flow and they are
extremely sensitive to environmental changes; in
spite of this, studies on postfire microbial ecology
are rare [1-3]. The action of both heat and ash
modify the soil chemical and physical status [4-7]
and thus alterations in microbial populations may
be expected following soil burning. Partial or
total soil sterilization immediately after fire has
been reported [1,8-10]; however, rapid soil recolonization by microorganisms present in water, air
or unburnt soil takes place [1,2] and, conse-
Correspondence to: M.J. Acea, Instituto de Investigaciones
Agrobiol6gicas de Galicia (CSIC), Apartado 122, E-15080
Santiago de Compostela, Spain.
quently, microbial response to burning probably
will depend mainly not on the initial effect but on
the substrate changes induced by fire.
Soil changes caused by fire are largely related
to fire intensity [2,4,11] which is usually low or
moderate in prescribed or controlled burning and
high in uncontrolled burning which normally occurs in the dry summer-fall season. Most research
on fire effects on microrganisms has been devoted to controlled fires [1,12,13] but concern
about wildfires has arisen because they are progressively more numerous and widespread [1416].
This investigation aims to evaluate changes in
soil microbes after a forest wildfire. One month
and one year after burning, the changes in microbial populations, bacteria, fungi and algae, were
studied both immediately and during a 30 days
soil incubation.
94
burned and the burned soil at 90% of field capacity were separately incubated in 500-ml bottles
for 30 days at 22°C. Incubation was carried out in
a sunlight chamber (about 300 lx) with 8 h periodic cycles of light and dark. Microbial counts
were made at 0, 1, 3, 5, 7, 10, 15, 20 and 30 days
of incubation.
Materials and Methods
The study area
An Atlantic forest of 30 years old Pinus pinaster
Sol. located in Costa Requeixo (N.W. Spain,
U.T.M. 29TNH2919, elevation 140 m a.s.1.) was
used for this study. One wildfire in the summer
killed all the trees and ground vegetation and
also consumed part of the litter and humus layers
to a depth of 5 - 7 cm, leaving the soil surface
covered with white ash. Two plots of 10 × 10 m,
one burned and another just outside the perimeter of the fire, were established. The area was
nearly level ( 0 - 2 % slope range) [17]. The annual
rainfall was 1822.6 mm and the average temperature varied from 8.0 to 18.0°C; the mean air
temperature and the precipitation during the
week before the first sampling were 16.5°C and
122.8 ram, respectively, the equivalent values one
year after the fire being 22.0°C and 47.6 mm.
Microbial analyses
Sterile distilled water (360 ml) was added to
each 40-g soil subsample and mixed for 10 min in
a magnetic stirrer operating at half speed. The
soil suspension was diluted in 10-fold series in
250-ml bottles and five Petri dishes containing
solid media or five test-tubes containing liquid
media were inoculated from each dilution. Microbial populations were counted by the Most Probable Number method as described by Alexander
[19]. Populations were grown in a medium containing 1.0 g dextrose, 0.5 g KNO 3, 1.0 g K2HPO4,
0.2 g MgSO4.7H20, 0.1 g CaCI2, 0.1 g NaC1, 0.01
g FeC13, 1.0 g yeast extract and 1 1 distilled water.
Cyanobacteria were cultured in nitrogen- and or.
gamc carbon-deficient B G l l medmm, as is recommended by Rippka [20], and algae in mineral
salt medium; the presence of either cyanobacteria
or algae in the positive tubes was confirmed microscopically and their numbers were estimated
by the most-probable-number technique. Counts
of aerobic heterotrophic bacteria and aerobic
sporeformer bacteria (Bacillus spp.) were made
by the spread-plate method on agar (1.5%) containing yeast extract-mineral salts medium at pH
7.0 [21]. Selective cultures of acidophiles and
spore-forming bacteria were achieved by acidification of the isolation medium to pH 4.5 and by
Soil sampling and preparation
The soil was a temperate zone Humic Cambisol [18] over granite with a relatively high organic matter content and an acid pH. Table 1
shows the main characteristics of both the unburned and the burned soil samples. From each
plot a gross sample, comprising 50 subsamples
each of about 200 g, was collected from the top
0-5 cm of the umbric horizon. In the field, samples were placed in sterile flasks to prevent drying or heating. Sampling and sample manipulation were performed aseptically. The soil was
sieved (4 ram) and the fraction < 4 ram, thoroughly homogenised, was used for all the subsequent determinations. Portions (40 g) of the un-
,
.
•
,
,
Table 1
Main characteristics of the soils
Time after
Sample
fire
pH
Water
H 20
Total C
Total N
Inorganic N
(%)
Total P
Available P
(/.~g/g dry soil)
One month
Unburned
Burned
4.5
4.9
25
36
9.8
8.7
0.6
0.7
9.6
130.0
531
600
33
118
O n e year
Unburned
Burned
4.9
4.9
15
11
9.8
8.7
0.6
0.7
16.1
91.5
607
507
32
56
Particle fractions = u n b u r n e d soil: 59% sand, 7% silt, 13% clay; burned soil: 58% sand, 9% silt, 18% clay.
95
pasteurization [22] of the soil dilutions before
plate inoculation, respectively. Fungi propagules
were counted on Czapeck-Dox medium (30.0 g
sucrose, 3.0 g NaNO3, 1.0 g K2HPO4, 0.5 g KCI,
0.5 g MgSOa.7H20, trace FeSO4, 15.0 g agar and
1 1 distilled water) at pH 4.5, and the length of
fungal hyphae estimated by staining the soil with
0.1% Trypan blue and following the membrane
filter technique as described previously [23]. The
inoculated tubes and plates were incubated at
28°C for 4 to 6 weeks and for 7 to 10 days,
respectively.
All results were obtained by triplicate determinations and are expressed on the basis of oven-dry
(ll0°C) weight of soil (dry soil). The data were
processed by a standard analysis of variance and,
in cases of a significant (P < 0.05) F-statistics,
Tukey's minimum significant difference test [24]
at the 95% confidence level was used to separate
the means.
Growth techniques such as those described
above reflect viability and biochemical potential
of microbiota and are good indicators of gross
changes in the population [25]. Most available
studies on soil microorganisms, including the references used in the present work, are based on
such techniques.
Aerobic heterotrophic bacteria, which increased
16-fold, were particularly favoured by the burning
and, among them, Bacillus spp. and the acidophilic bacteria increased 4-fold and twice, respectively, above the control soil values. By contrast, photoautotrophic microorganisms and fungi
were clearly reduced in the short term after the
fire. Thus, the burned soil showed between 100and 33-fold lower cyanobacteria and algae densities, respectively, than those found in the unburned soil; similarly, a 20-fold decrease in
propagules and a 75% (from 105.5 to 26.5 m g-I
soil) reduction in total hyphal length was the
resulting effect of the fire on the fungal population.
One year after the fire, the microbial population density in the burned soil had diminished
significantly compared with that found one month
after the fire (Table 2). The decrease was most
evident for the aerobic heterotrophic bacteria,
whose numbers were reduced by more than two
orders of magnitude; the acidophiles decreased
by more than one order of magnitude while the
spore formers did not change significantly. In
contrast, fungi, whether propagules or hyphae,
increased slightly and cyanobacteria and algae
increased by three and two logarithmic orders,
respectively. The differences between counts one
month and one year after the fire were greater at
the burned than at the unburned site. Also, except for that heterotrophic bacterial group, these
changes were opposite in the burned and unburned soil. As a result, fire-induced soil alterations did not affect the overall microbial population density but the effects were adverse for aero-
Results
Microbial population in the field
One month after the fire, the burned soil
showed a microbial population density 25-fold
greater than that of the unburned soil (Table 2).
Table 2
Microbial populations in the soils ( m i c r o o r g a n i s m s / g dry soil)
Time after
fire
Sample
Microbial
population
( x 106)
Aerobic
heterotrophic
bacteria
( X 10 6)
Acidophilic
bacteria
( x 105)
Sporeforming
bacteria
( X 104)
One month
Unburned
Burned
6.3 a
158.5 c
5.0 c
79.4 d
15.8 c
31.6 d
5.1 a
20.2 b
O n e year
Unburned
Burned
20.0 b
25.5 b
1.5 b
0.4 a
6.0 b
1.3 a
6.9 a
17.4 b
a-d Values within columns with dissimilar letters are significantly different ( P < 0.05).
Cyanobacteria
( x 102)
Algae
( x 102)
Fungi
( x 104)
12.6 c
0.1 a
13.3 c
04 a
38.8 c
2.0 a
3.8 b
195.1 d
5.4 b
52.5 d
6.4 b
3.6 a,b
96
bic heterotrophic bacteria and the acidophiles.
On the other hand, fungal propagules showed
similar densities in both soil samples whilst fungal
mycelium (45.7 and 30.2 m g-I in the unburned
and the burned soil, respectively) was significantly decreased by fire, although the negative
effect was notably reduced after one year. Conversely, spore-forming bacteria, cyanobacteria and
algae responded positively to burning and, consequently, were significantly more abundant in the
burned than in the unburned soil one year after
the fire.
Microbial population during soil incubation
Both one month and one year after the fire,
the unburned and the burned soil population
development patterns were not consistent during
the controlled incubation (Fig. 1). Incubation of
the one month, after-fire, soil samples initially
stimulated microbe growth in the unburned soil
but not in the burned soil, whose soil microbial
counts remained essentially constant during the
first five days of incubation and it was not until
week two that the population was significantly
greater than initially. However, after this peak
(4.0 × 108 microorganisms g-~ soil), the population tended to decline and at the last count
(4.0 N 107 microorganisms g- 1 soil) was one quarter of the initial value. Throughout the incubation
period the microbial population was greater in
the burned than in the unburned soil, although
after 30 days of incubation the positive effect of
burning was not significant, the maximum difference (1.5 orders of magnitude) having been attained after two weeks. Differences in counts and
trends between both soils were reflected in an
average population density in the burned soil
(1.5 × 10 8 microorganisms g-lsoil) being one order of magnitude greater than that in the unburned soil.
During incubation of the one year soil samples, the burned soil microbial population initially
decreased and thereafter increased somewhat, the
maximum value (4.0 × 10 7 microorganisms g-1
soil) being found at the end of incubation when
the population size was slightly greater than the
original one (Fig. 1). Until three days of incubation there were no significant differences between the unburned and the burned soil, the
highest positive effect of fire (1 logarithmic order)
having been reached at the first week of incubation. The average population density in the
burned soil (2.5 × 10 7 microorganisms g-1 soil)
was about 4-fold in excess of that in the unburned
soil.
Bacteria
In the one month samples, the burned and the
unburned soil aerobic heterotrophic bacteria
populations did not follow the same pattern during incubation (Fig. 2). In the burned soil, the
maximum bacterial number (1.0 × 108 cells g-~
soil) was attained after five days of incubation,
the final bacterial density (7.9 x 10 6 cells g-1
log MPN/g soil
log MPN/g soil
9
y
8
7
7-
6
'
'
0
1
,
3
5
,
7
10
days
,
,
,
14
20
30
unburned
6
0
1
3
5
7
10
days
14
20
30
[ ] burned
Fig. 1. Overall microbialpopulation during incubation of soil samples taken one month (M) and one year (Y) after the fire.
97
soil) being one order of magnitude lower than the
initial value. Except at the end of incubation, the
burned soil had significantly greater counts of
aerobic heterotrophic bacteria than the unburned
soil; nevertheless, differences tended to reduce
after two weeks, the highest positive effect of the
burning (1.7 logarithmic orders) being found at
day five. In the burned soil, the aerobic het-
erotrophic bacteria averaged 4.4 × 107 cells g-1
soil, which represented an increase of one order
of magnitude compared to the unburned soil
mean. Acidophile or sporeformer populations did
not change consistently in the unburned and the
burned soil samples. The acidophilic and the
spore-forming bacteria in the burned soil reached
maximum values (2.0 × 107 cells g-1 soil and 1.3
A, in
5
'
0
'
-
3
7
5
10
14
20
log CFU/g soil
-
,
30
i
0
r
1
i
3
F
5
days
/
log CFU/g soil
7-
6
6-
5 /
0
1
3
5
7 10
days
14 20
5 /~
0
30
log CFU/g soil
M
i
14
i
20
log CFU/g soil
AcB /
7
i
7 10
days
1
3
5
,
30
y
7 10
days
14
20
30
7 10
days
14
20
30
log CFU/g soil
SpB
_
F
654-
3
0
1
3
,
5
7 10
days
14
20
30
[]unburned
3
0
[]
1
3
5
burned
Fig. 2. Aerobic heterotrofic bacteria (AHB), acidophilicbacteria (AcB) and spore-formingbacteria (SpB) during incubation of soil
samples taken one month (M) and one year (Y) after the fire.
98
X 10 6 cells g-~ soil, respectively) after two weeks
of incubation, these peaks representing also the
highest rise (16-fold the acidophilic and 97-fold
the sporeformers) compared to the unburned soil.
At the end of the incubation, the acidophiles in
the burned soil showed a density similar to their
initial value whereas the sporeformers were more
than twice the initial count, the average density
of the former being 8.7 × 10 6 cells g-~ soil and
that of the spore-forming 6.1 x 105 cells g-~ soil.
Counts of both bacterial groups were significantly
greater in the burned than in the unburned soil at
all the sampling times, the result being an increase in their average density of about one logarithmic order for the acidophiles and 15-fold for
the sporeformers. On the other hand, in the
unburned soil, but not in the burned soil samples,
the densities of both bacterial groups were positively correlated with that of the aerobic heterotrophic bacteria (acidophiles: r = 0.70, P <
0.05; spore-forming: r = 0.75, P < 0.05).
Cb
log MPN/g soil
4
M
2
o
o
1
3
5
One year after the fire the dissimilarity between the unburned and the burned soil had
diminished (Fig. 2). During incubation, both soil
samples showed related patterns for aerobic heterotrophic bacteria (r = 0.70, P < 0.05) as well as
for acidophiles (r = 0.80, P < 0.05) and sporulating microbes (r = 0.70, P < 0.05). The bacterial
changes in the burned soil were less marked than
those observed during incubation of the soil collected one month after burning (Fig. 2). The
bacterial group size increased slightly or remained constant until day 20 of incubation, when
the population was maximal (2.1 × 106 cells gsoil), and declined thereafter to a final density
(6.3 × 105 cells g-~ soil) similar to the initial one.
Throughout incubation the burned soil had a
slightly less aerobic heterotrophic bacteria than
that of the unburned soil, which was reflected in
an average population (1.3 × 10 6 cells g-1 soil)
half that in the unburned soil. The acidophiles in
the burned soil increased to 7.9 × l0 s cells g-1
7 lO
days
log MPN/g soil
:t
14 20 30
o
AL"
4-
2
2- I
1
3
5
7 10
days
3
5
7
10
14
2'0 30
log MPN/g soil
4.
0
1
days
log MPN/g soil
o
Y
14 20 30
[]
unburned
o-~
0
1
3
[]
burned
5
7 10
days
-
14 20 30
Fig, 3. Cyanobacteria(Cb) and algae (ALG) during incubation of soil samples taken one month (M) and one year (Y) after the fire.
99
soil within the first week of incubation and thereafter declined, its final density (1.2 x 105 cells
g-1 soil) being similar to the initial one. At all
the sampling times the acidophile abundance in
the burned soil was significantly lower than that
in the unburned soil; consequently, their average
number (2.6 x 106 ceils g-1 soil) decreased 5-fold
due to the burning. The spore-forming bacteria
did not markedly change in the burned soil during incubation; their maximum density (4.3 x 105
cells g-t soil) was after 20 days of incubation
and, after decreasing, the final density (1.1 x 105
cells g-~ soil) was similar to the initial one. As a
result, the average density of the sporulating bacteria (2.7 x 105 cells g-1 soil) was 3-fold that in
the unburned soil. In the burned soil as in the
unburned soil the population changes between
the three bacterial groups during incubation were
positively and significantly correlated: namely
aerobic heterotrophic bacteria with both acidophiles (r = 0.85, P < 0.01) and sporeformers
(r = 0.80, P < 0.05), and acidophiles with sporeformers (r = 0.70, P < 0.05).
Cyanobacteria and algae
One month after the fire there was a positive
correlation between the unburned and the burned
soil for cyanobacteria (r = 0.78, P < 0.05) and
algae (r = 0.80, P < 0.05) (Fig. 3). In the burned
soil, cyanobacterial as well as algal populations
decreased to less than 10 microorganisms g - t soil
during the first days of incubation, remaining
thereafter essentially constant. Throughout the
incubation period the densities of both groups in
the burned soil were significantly reduced compared with those in the unburned soil and, although the differences between both soils tended
to attenuate during incubation, the average number of cyanobacteria (7 cyanobacteria g-t soil)
and algae (11 algae g-1 soil) were lowered 27and 20-fold, respectively, by the burning.
One year after the fire the similarity between
the unburned and the burned soil for cyanobacterial (r = 0.85, P < 0.01) and algal (r = 0.93, P <
0.01) patterns of population change were closer
than one month after the burning (Fig. 3). In the
burned soil both groups proliferated significantly
during incubation, their final density (2.0 x 105
cyanobacteria g-t soils; 3.7 x 106 algae g-1 soil)
being one logarithmic order greater for cyanobacteria and 650-fold for algae than their initial
values. During incubation the populations of both
these photoautrotrophic microorganisms were
significantly greater in the burned than in the
unburned soil and thus the mean numbers of
cyanobacteria (4.9 x 104 cyanobacteria g - t soil)
and algae (2.2 x 105 algae g-1 soil) in the burned
soil were more than two orders of magnitude and
5-fold greater, respectively, than those in the
unburned soil.
Fungi
In the one month samples, the pattern of
change for the fungal propagules was similar (r =
0.80, P < 0.05) in the burned and the unburned
soil (Fig. 4). In the burned soil the propagule
log CFU/g soil
log CFU/g soil
6
.
.
.
.
.
M
4
2
2
0
1
3
5
7 10
days
,
,
14 20 30
0
1
3
[ ] unburned
[]
burned
5
,
,
7 10
days
,
i
,
14 20 30
Fig. 4. Fungalpropagulesduringincubationof soil samplestaken one month(M) and one year(Y) after the fire.
100
Table 3
Fungal hyphal growth during incubation of soil samples taken
one month after fire (m/g dry soil)
Days
Soil
unburned
105.0
123.4
186.0
112.9
0
10
20
30
burned
26.3
29.2
19.2
21.2
number slightly oscillated during the first week of
incubation and thereafter declined constantly attaining a density (5 × 102 U g-1 soil) 40-fold
lower than the initial value at the last day of
incubation. At this time, there was also the highest drop (more than 2 logarithmic orders) in
relation to the unburned soil. The negative response of fungi to fire was reflected in an average
density of propagules (1.0 × 10 4 U g-1 soil) lowered by 200-fold by the burning; similarly, the
length of fungal mycelium (Table 3) was reduced
by between 75% and 90% after the burning.
One year after the fire, the fungal propagules
followed the same pattern (r = 0.96, P < 0.01) in
the burned and unburned soil (Fig. 4). In the
burned soil the propagule numbers were slightly
lower or similar to those of the unburned soil
during two weeks of incubation and thereafter
exceeded the unburned soil counts. Consequently, its final density (1.6 × 10 6 U g-1 soil)
was 3-fold the unburned soil propagules density.
The average density of fungus unities in the
burned soil (3.9 x 10 5 U g-1 soil) was not signifim/g soil
50-
[]
unburned
40302010
,
0
,
1
3
5
7 10
days
14
20
30
Fig. 5. Fungal hyphae during incubation of soil samples taken
one year after the fire.
cantly different than that in the unburned soil.
There was also a positive correlation (r = 0.05,
P < 0.01) between the mycelium length changes
in the unburned and the burned soil (Fig. 5). The
burned soil hyphae constantly fell to half (15.8 m
g-1 soil) the initial value. Differences between
both soils tended to decrease; thus, the burned
soil had significantly less mycelium than that of
the unburned soil during the first two weeks of
incubation, but thereafter fungi biomass in the
burned soil was the same as, or greater than, that
in the unburned soil; consequently, hyphae length
(22.41 m g - l soil) was reduced by 18% by the
fire.
Discussion
Burning stimulated microbial changes, but did
not alter the relationship between the population
sizes of the various microbial groups. As is common in most soils [25-27], bacterial numbers
clearly predominated over those of fungi and
algae, and there was a relatively high density of
acidophiles or acid-tolerant microbes, followed by
the spore-formers and much lower numbers of
cyanobacteria. Nevertheless, the nonbiological
and biological fire-induced soil changes affected
the different groups in different ways.
In the short term, burning brought about a
great rise in microbes with a density close to the
upper values given for many soils [25,27-30]. The
relative increase in nutrients observed in the
burned area together with favourable moisture
and temperature conditions may be the main
reason for this microbial proliferation. Similar
factors have been indicated, by other authors, as
responsible for microbial increase after controlled burning [1,2,9]. The improvement of nutrient status also explains, at least in part, why the
saprophytic microbes were promoted by the burning whilst the autotrophic microorganisms,
cyanobacteria and algae, were clearly reduced.
The significant increase of aerobic heterotrophic
bacteria, including the acidophiles and the spore
formers, contrasts with the enormous fall, both in
propagule number and hyphae length, of fungal
population. These results show that bacteria were
101
able to survive soil heating better than fungi,
which is in accordance with the findings from
other fires [31,32]. The lack of fungal recovery
contrasts with some data from soils affected by
prescribed fires, whose fungal populations were
not altered [33] or reached their original size
within one [8] or two months [34] after the fire.
The increase of pH [35], as well as the destruction of organic matter [36] by fire, were suggested
as the factors that most influence the frequences
with which microfungi occurred. However, the
fact that the soil remained acid after the burning,
together with the rise found in acidophilic bacteria, seems to indicate that pH was not the factor
responsible for fungal decline. Similarly, because
the quantity of organic C in the burned soil was
relatively abundant and enough to support active
bacterial growth, it is unlikely that lack of organic
matter was the main factor causing the fungal
population reductions. Qualitative differences in
the organic substrate, which is drastically changed
by burning [5,6], and growth inhibition by chemical products from the fire [37] were probably
decisive factors in poor fungal development.
One year after the fire, the overall population
and the aerobic heterotrophic bacteria, except
the sporeformers, decreased and consequently the
beneficial effect of burning desappeared. Ahlgren
and Ahlgren [2] found similar results in a prescribed fire, stating that the drop in microbes was
related to the gradual leaching of ash minerals to
lower soil layers. The fall in aerobic bacteria,
which was relatively more dramatic than the fall
in the overall microbial population, could also be
a consequence of the lowered soil aeration due to
the high content of ashes in the small size fractions. Another cause could be soil compactation
and loss of structure after fire [5,6]. The apparent
selection of Bacillus spp. could be due to the
relatively high bacterial spore resistence to heat,
drying and other unfavourable post-fire factors
[25,28]. There was no negative effect on fungal
propagules while hyphal length continued to be
reduced by burning, although less markedly than
in the short term after fire, which indicated a
relatively slow recuperation in fungal biomass.
Conversely, photoautotrophic organisms multiplied actively, being clearly favoured in the long-
term by the burning. There is little available
information concerning postfire behaviour of
cyanobacteria and algae, but some authors
[25,38,39] regarded these organisms as efficient
colonizers of vegetation-free environments and
thus playing a critical pioneering role in burnt
soils [1,38,39]. It is likely that such a notable
increase in photosynthetic microbiota was partially due to the improvement in light conditions
resulting from fire.
Beneficial short-term fire effects could be
greatly improved if good enviromental conditions
were provided, since during incubation of burnt
soil the overall microbial population as well as all
bacterial groups grew significantly and within two
weeks attained the highest densities and the maximum increase attributable to the burning. Nevertheless, the high populations were not maintained
and the return towards similar or even lower
values than those in the unburnt soil was relatively fast. Conversely, the incubation initially did
not nullify the short-term deleterious effect of
fire on either fungal (propagules as well as
biomass) or photoautotrophic populations. However, in the long term the cyanobacteria and the
algae could sharply increase while the fungi
propagules recuperated slowly, and fungal hyphae failed to rise appreciably even in incubation
conditions, although the negative effect of burning was notably reduced. The tendency of the
fire-perturbed soil to return to normal, which has
also been reported in other fires [2,31,40,41],
determined that the drastic alterations observed
in the growth patterns of microbial groups and
fungal biomass one month after the fire, disappeared within one year of the fire. Thus the same
trends were found for all the microbial groups in
the unburned and the burned soil.
Acknowledgements
The authors wish to thank Mrs. Blanca Arnaiz
for her capable, dedicated and kindly technical
assistance. This research was supported by Consellerla de Educaci6n de La Xunta de Galicia, in
Santiago de Compostela, and the Comisi6n Interministerial de Ciencia y Tecnologla (CICYT),
Spain.
102
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