Effect of solid waste compost on microbi
Biol Fertil Soils (2000) 32 : 410–414
Q Springer-Verlag 2000
ORIGINAL PAPER
C. Guerrero 7 I. Gómez 7 J. Mataix Solera 7 R. Moral
J. Mataix Beneyto 7 M.T. Hernández
Effect of solid waste compost on microbiological and physical
properties of a burnt forest soil in field experiments
Received: 24 November 1999
Abstract The restoration of soil microbial activities is
a basic step in the reclamation of burnt soils. For this
reason, the ability of municipal solid waste compost to
accelerate the re-establishment of bacterial and fungal
populations, as well as to re-establish physical properties in a burnt soil, was evaluated in a field experiment.
Four treatments were performed by adding different
doses of compost (0, 0.5, 1 and 2 kg compost m –2 soil)
to a burnt Calcic Rodoxeralf soil, and the changes in
microbial populations, salt content, aggregate stability
and bulk density were evaluated for 1 year. Initially,
the addition of compost had a negative effect on soil
microbial populations, but 3 months after compost addition, the number of viable fungal propagules increased in all the amended soils. This positive effect
lasted until the end of the experiment. From 30 days
onwards, all the amended soils showed a greater total
number of bacterial cell forming units than the unamended burnt soil. Organic amendment increased the
percentage of 2- to 4-mm aggregates, although the effect on the stability of the 0.2- to 2-mm aggregates and
on bulk density was less noticeable.
Keywords Aggregate stability 7 Bacteria 7
Burnt soil 7 Compost 7 Fungi
M.T. Hernández (Y)
Department of Soil and Water Conservation
and Organic Waste Management,
Centro de Edafología y Biología Aplicada del Segura,
CEBAS-CSIC, P.O. Box 4195, 30080 Murcia, Spain
e-mail: mthernan6natura.cebas.csic.es
Tel.: c34-968-215717
Fax: c34-968-266613
C. Guerrero 7 J. Mataix Solera 7 R. Moral 7 J. Mataix Beneyto
Department of Agrochemistry and Environment,
Miguel Hernández University, 03202 Elche, Spain
I. Gómez
Department of Agrochemistry and Biochemistry,
University of Alicante. P.O. Box 99, 03080 Alicante, Spain
Introduction
Wildfires strongly modify ecosystem characteristics,
particularly soil properties, which contribute to the ecosystem’s destruction. Soil microbiota, which is the main
agent responsible for organic matter decomposition
and is involved in the macronutrient cycles (Harris and
Birch 1989) is negatively affected by fire (Hernández et
al. 1997). The importance of bacteria and fungi in natural soil ecosystems has been well recognised, and microbial activities play an important role in the reclamation of disturbed ecosystems, where they contribute to
nutrient turn-over, N and P immobilisation (microbial
biomass) and the formation of relatively stable aggregates (Dinel et al. 1992). In general, bacterial populations often dominate fungi and other microbial groups
after fire (Vázquez et al. 1993). The bacterial population usually returns to normal values very quickly after
burning, while fungi seem to recover more slowly. The
pre-burnt state may not be reached even after 1 year
(Raison 1979). This may be due to an increase in soil
pH, changes in or lack of organic matter, and/or the inhibitory effect of different pyromorphic compounds
(Widden and Parkinson 1975).
In order to recover forest ecosystems after wildfires,
the physical, chemical and microbiological characteristics of the soil have to be re-established. The addition
to the soil of an organic material, with a high macroand micronutrient content and a diverse microbial population, will favour the re-establishment of microbial
biomass and activity and, hence, plant recovery, reducing the time needed to reach suitable levels of soil protection (Villar et al. 1998)
The aim of this work was to evaluate the efficacy of
adding a compost to a burnt soil in order to accelerate
the re-establishment of bacterial and fungal populations and to restore the soil’s physical characteristics.
For this, a municipal solid waste compost was added to
a burnt soil at 3 different dosage rates and changes in
bacterial and fungal populations, as well as in the phy-
411
sical and physico-chemical properties of the soil, were
studied during 1 year.
Materials and methods
The experiment was carried out in a forest soil burnt 5 months
previously, located in Bocairente (Valencia, Spain), at 38744b9nN
and 0742b9nW. The soil is a Calcic Rhodoxeralf (Soil Survey Staff
1990), and the site (730 m above sea level) has a ~2% slope and
SE orientation. No live vegetation was observed, and the soil had
a Munsell colour of 7.5 YR 4/4 (dry) and 10 YR 3/2 (wet) due to
the ash cover. During the experimental period, the average temperature was 15.3 7C, the average daily minimum temperature being 8.9 7C and the average daily maximum temperature 21.7 7C;
the mean annual rainfall was 440 mm. The study was performed
in 10-m 2 plots randomly located in a burnt area of 3000 m 2. Four
treatments were established by adding different doses of a municipal solid waste compost from an industrial composting plant in
Valdemingómez, Madrid, Spain, to the burnt soil. The application
rates of 0, 0.5, 1, and 2 kg compost m –2 soil,(dry basis) shall be
referred to as control, SW1, SW2 and SW3 treatments, respectively. All the treatments were carried out in quadruplicate. Compost
was applied superficially to the burnt soil, and no attempt was
made to mix it with topsoil. Samples were taken 15, 30, 60, 90,
120, 150, 240, 300, and 360 days after compost application. Each
sample consisted of a mixture of eight randomly collected soil
subsamples, taken from the top 15 cm.
Soil chemical and physical analysis
Electrical conductivity (EC) was measured in a 1 : 5 (w/v) aqueous
solution at 20 7C. Chlorides were determined in the above extract
by HPLC. Sodium was extracted by 1 N ammonium acetate in a
1 : 10 soil : solution ratio (Knudsen et al. 1982), and determined by
emission spectrophotometry. Parameters were analysed in 2-mmsieved soil samples. Aggregate stability was determined in 0.2- to
2-mm and 2- to 4-mm-diameter soil aliquots, following the method of Benito et al. (1986) modified by Roldán et al. (1996). Bulk
density was determined in soil clods of 2–3 cm diameter by the
method of Barahona and Santos (1981).
Table 1 Changes with time in the electrical conductivity (EC) of
the amended and unamended soils. C Burnt soil, SW1 0.5 kg compost m –2 burnt soil, SW2 1 kg compost m –2 burnt soil, SW3 2 kg
compost m –2 burnt soil, LSD least significant difference
(P^0.05), ns not significant at P^0.05
Sampling
time
(days)
15
30
60
90
120
150
240
300
360
LSD
F-ANOVA
EC (mS m –1)
LSD
F-ANOVA
17.9
28.0
27.1
12.3
17.1
9.9
5.3
4.6
4.6
***
***
***
***
*
***
***
n.s.
n.s.
Treatments
C
SW1
SW2
16.1
16.0
20.5
13.4
12.5
16.3
11.4
16.6
15.4
1.2
***
28.8
32.6
41.5
22.8
21.8
21.4
20.7
20.1
14.5
8.5
***
30.2
65.3
52.2
23.7
27.6
23.3
21.0
17.0
16.9
13.7
***
SW3
80.1
121.8
131.8
58.7
43.9
46.0
31.4
22.0
20.5
26.5
***
* P^0.05, ** P^0.01, *** P^0.001
the compost and inorganic species formed by organic
matter mineralisation. After heavy rainfall, the EC decreased in all the treatments due to salt leaching, as
confirmed by the increase in EC observed in the 15- to
30-cm-deep soil samples after rain (data not shown).
High correlation coefficients were obtained between
EC and the chloride (r 1 0.96; P~0.001) and sodium
(r 1 0.0.84; P~0.001) content, particularly in amended
soils. No statistically significant differences were detected between treated and untreated soils after
300 days.
Soil microorganisms
Soil microbiological analysis.
Twenty grams of fresh soil were diluted in tenfold series in sterile
water. Viable bacterial numbers were determined by plate colony
counts (Schmidt 1973) after 5 days of incubation at 28 7C on Petri
dishes containing solid media. The composition of this agar medium in 1 l was: 1.15 g bacteriological peptone; 0.15 g sodium citrate; 0.025 g sodium nitrate; 0.015 g iron (II) sulphate 7-hydrate;
0.015 g magnesium sulphate 7-hydrate; 0.015 g potassium chloride; 0.015 g calcium chloride; agar technical n73,11 (Oxoid). Viable fungus propagules (CFU) were determined in rose bengal
agar (Oxoid) (Martin 1950) with the same dilution plate method
(Schmidt 1973).
Data were analysed using one-way ANOVA and significant
differences between means were determined by Tukey’s test.
Results and discussion
The addition of compost increased soil EC in a dosedependent manner (Table 1). These values increased
during the first 60 days following the addition of compost, an increase attributable to the solubilisation of
soluble ions like chlorides, sulphates and sodium from
During the first 60 days of the experiment the number
of fungal propagules in the amended soils was lower
than in the control (Table 2). This negative effect was
probably due to the presence in the compost of high
amounts of salts and other substances toxic to the soil’s
native fungi. García and Hernández (1996) indicated
that sodium sulphates and chlorides have a negative effect on soil microbial activity and microbial biomass,
this negative effect being much more noticeable in the
case of sodium chloride. However, it was observed that
during the first 60 days, the number of fungal propagules in the treated soils increased with increasing
amounts of added compost: SW3 1 SW2 1 SW1, a fact
explained by the high level of fungi in the compost
(6800 10 3 CFUs g –1). These compost fungi are adapted
to living in this medium, and are more tolerant than the
natural soil fungi to inhibitory substances, such as sodium chlorides and sulphates. It is therefore supposed
that the fungus propagules initially found in the
amended soils mainly came from the compost. Between
60 and 90 days, 40 mm rainfall was registered, a suffi-
412
Table 2 Changes with time in the micro-organism populations
(cell forming units; CFUs) of the amended and unamended soils.
For other abbreviations, see Table 1
Sampling
time
(days)
Fungi (10 3 CFUs g –1 soil)
LSD
F-ANOVA
18
7
16
7
24
10
16
7
25
**
***
***
***
***
***
***
***
***
23
13
45
46
17
8
11
49
27
***
***
***
***
*
***
***
*
***
Treatments
C
15
60
30
84
60
117
90
84
120
92
150
57
240
70
300
82
360
88
LSD
13
F-ANOVA ***
SW1
SW2
SW3
26
44
75
86
80
121
123
126
166
13
***
51
63
79
91
140
149
102
73
169
15
***
55
71
96
143
180
150
128
119
170
18
***
Bacteria (10 5 CFUs g –1
15
75
38
66
30
64
121
111
60
55
259
167
90
97
211
44
120
32
18
14
150
68
136
222
240
101
142
180
300
112
126
137
360
75
95
104
LSD
25
29
33
F-ANOVA ***
***
***
soil)
17
109
280
276
37
248
223
190
194
28
***
* P^0.05, ** P^0.01, *** P^0.001
cient quantity to leach the sodium chlorides and sulphates from the topsoil. As a direct consequence of the
diminished levels of these toxic substances, the number
of fungal CFUs in the SW1 and SW2 treatments at
90 days reached the levels found in the untreated soil,
while the numbers in the SW3 treatment exceeded
those of the unamended soil. A great increase in fungal
CFUs was again detected in treated soils 120 days after
soil amendment, particularly in the intermediate (SW2)
and high (SW3) dose treatments. This fact can be explained by:
1. The leaching of salts caused by two consecutive intense rainfall events (140 mm in 48 h) which took
place some time before sampling.
2. The decomposition of toxic substances, other than
salts, in the medium.
3. The positive effects of compost addition on soil
characteristics (increased organic matter, nutrients,
etc.), which would have stimulated fungal growth.
The fungal CFUs detected at this stage would now
have been both native (from the soil) and exogenous
(from the compost).
The bacterial populations evolved differently from
the fungi. At the first sampling date (15 days), the number of viable bacterial CFUs in the amended soils was
in general lower than that observed in the unamended
soil (Table 2), probably due to the presence in the com-
post of substances which inhibited or were toxic to viable micro-organisms, or due to the anaerobic conditions caused by the mineralisation of the added organic
matter. This negative effect had disappeared by the
next sampling date (30 days), when the amended soils
presented a greater number of viable bacterial CFUs
than the control soil (Table 2). This could have been
due to:
1. The beneficial effect of the nutrients and organic
matter added with the compost counterbalancing the
negative effect of the toxic substances.
2. The ability of the soil bacterial population to adapt
to the new soil conditions and their developing resistance to the toxic substances.
3. The natural degradation or leaching of the toxic substances.
The number of bacteria in the unamended soil followed seasonal changes, although the variations in this
soil were wider than in the amended soils, which were
enriched with bacteria added with the compost. This
may have been due to the reduction in microbial community diversity caused by fire (Staddon et al. 1998),
which was not completely restored in the unamended
soil.
Aggregate stability (0.2–2 mm)
Initially, the addition of compost did not seem to influence soil aggregate stability, the percentage of stable
aggregates remaining statistically similar in both
amended and unamended soils (Table 3). However, at
60 days, amendment was seen to have a negative and
dose-dependent effect on the stability of the 0.2- to 2mm aggregates, the soils amended with a high dose
(SW3) showing the lowest percentages of stable aggregates. The soils amended with a low dose (SW1)
showed percentages of stable aggregates which were
statistically similar (with some exceptions) to those of
the control, while the high dose (SW3) produced percentages, which were significantly lower than those observed in the control soil between days 60 and 240. The
SW2 treatment showed an intermediate behaviour.
This pattern could be due to: (1) the dispersion of soil
colloids as a result of the high salinity produced by high
doses of compost, or (2) the fractionation of long-chain
carbohydrates (responsible for soil aggregation processes), resulting from the stimulation of microbial activity by compost, particularly at the SW3 dosage. Lax and
García-Orenes (1993) found that fractionated organic
matter had less of an effect on soil aggregation than
non-fractionated organic matter. By the end of the experiment (300 days after adding the compost) the percentages of stable aggregates in SW3 soils were again
similar to those of unamended soils. This can be explained by the decrease in salinity with time and by the
formation of new aggregates as a consequence of the
activity of the newly developed microbial populations
in the amended soils.
413
Table 3 Changes with time in the percentage of stable aggregates
in amended and unamended soils. For abbreviations, see Table 1
Sampling
time
(days)
Aggregate stability (%)
(0.2–2 mm)
LSD
F-ANOVA
Table 4 Changes with time in the. bulk density of the amended
and unamended soils. For abbreviations, see Table 1
Sampling
time
(days)
Treatments
C
SW1
SW2
SW3
15
30
60
90
120
150
240
300
360
LSD
F-ANOVA
54.8
59.6
65.8
54.7
50.8
52.2
53.2
53.3
54.3
6.0
***
47.3
59.8
53.5
50.5
46.5
50.4
52.6
52.5
56.3
6.5
**
46.0
54.5
50.9
42.1
46.9
53.3
51.5
52.5
54.9
2.4
***
47.9
53.4
49.5
43.1
45.0
41.9
46.6
47.9
47.3
1.9
*
15
30
60
90
120
150
240
300
360
LSD
F-ANOVA
Aggregate stability (%) (2–4 mm)
90.0
87.4
79.6
70.1
1.4
93.0
88.0
90.8
85.5
3.4
94.7
94.6
92.4
91.3
2.3
81.6
93.7
85.9
90.4
1.4
77.5
84.9
86.1
84.9
3.4
82.9
89.8
90.8
85.6
2.8
88.4
93.2
91.7
91.8
2.2
90.8
94.7
91.6
94.7
3.1
85.7
91.4
90.1
88.0
3.1
3.1
2.6
2.4
1.9
***
***
***
***
8.3
5.8
7.5
5.3
2.7
4.8
3.4
6.6
7.5
n.s.
n.s.
**
***
**
***
**
n.s.
n.s.
15
30
60
90
120
150
240
300
360
LSD
F-ANOVA
Bulk density (g cm –3)
LSD
F-ANOVA
0.07
0.10
0.16
0.05
0.11
0.11
0.07
0.09
0.05
*
**
n.s.
***
n.s.
*
n.s.
n.s.
*
Treatments
C
SW1
SW2
SW3
1.24
1.22
1.25
1.16
1.28
1.20
1.30
1.37
1.42
0.10
***
1.29
1.34
1.43
1.32
1.33
1.37
1.39
1.39
1.39
0.07
**
1.26
1.41
1.35
1.43
1.34
1.30
1.37
1.41
1.33
0.08
**
1.35
1.38
1.38
1.40
1.32
1.32
1.33
1.34
1.37
0.11
ns
* P^0.05, ** P^0.01, *** P^0.001
***
**
*
***
***
***
**
*
*
* P^0.05, ** P^0.01, *** P^0.001
Aggregate stability (2–4 mm)
At the first two samplings dates, particularly the first,
the addition of compost was seen to have a slightly negative effect on aggregation (Table 3). After this period, aggregation was significantly higher in the amended
soils at all doses. This increase in the percentage of stable aggregates in the amended soils can be attributed
both to salt leaching and to the increase in microbial
populations. Positive correlation coefficients between
the percentage of 2- to 4-mm stable aggregates in the
amended soils and the number of CFUs of viable bacteria were observed (rp0.8049, P~0.01; rp0.6862,
P~0.05; and rp0.7306, P~0.05, for SW1, SW2 and
SW3, respectively).
Bulk density
Bulk density did not show any clear tendency. Initially,
bulk density values were slightly higher in the amended
soils, which could be explained by the changes observed in the 0.2- to 2-mm aggregate stability, there being a close relationship between aggregation and bulk
density (Clapp et al. 1986). The values of bulk density
in all the soils (amended and control) tended to stabil-
ise at F1.3–1.4 g cm –3. Such a value is probably optimal
for this soil, and the application of compost would only
accelerate the natural evolution of bulk density. This
hypothesis is supported by the fact that bulk density did
not change with time in the SW3 treatment, whereas
significant changes were observed in the control (Table 4).
In conclusion, from the above results it can be concluded that the addition of MSW compost to burnt soils
is a suitable technique for accelerating the natural recovery process of burnt soils, thus shortening the period of time a soil is exposed to potential erosion processes. Although the addition of compost may initially
have a negative effect on soil microbial populations due
to the presence in the compost of large amounts of salts
and other toxic substances, this effect soon disappears
since the toxic substances are degraded and the salts
are leached. Therefore, the addition of compost contributes to the quicker re-establishment of microbial populations, which play an important role in aggregate stability and in the biogeochemical cycles of macronutrients, thus favouring the recovery of plant cover and,
consequently, increasing soil protection. Among all the
compost rates assayed, 1 kg m –2 (SW2) seemed to be
the most suitable for the reclamation of burnt soils in
our experimental conditions, since it stimulated soil
fungal and bacterial development and improved the
stability of the 2- to 4-mm aggregates. At this dosage
the initial negative effects due to the addition of salts or
toxic substances with the compost was less noticeable
than at the high dose (SW3).
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Q Springer-Verlag 2000
ORIGINAL PAPER
C. Guerrero 7 I. Gómez 7 J. Mataix Solera 7 R. Moral
J. Mataix Beneyto 7 M.T. Hernández
Effect of solid waste compost on microbiological and physical
properties of a burnt forest soil in field experiments
Received: 24 November 1999
Abstract The restoration of soil microbial activities is
a basic step in the reclamation of burnt soils. For this
reason, the ability of municipal solid waste compost to
accelerate the re-establishment of bacterial and fungal
populations, as well as to re-establish physical properties in a burnt soil, was evaluated in a field experiment.
Four treatments were performed by adding different
doses of compost (0, 0.5, 1 and 2 kg compost m –2 soil)
to a burnt Calcic Rodoxeralf soil, and the changes in
microbial populations, salt content, aggregate stability
and bulk density were evaluated for 1 year. Initially,
the addition of compost had a negative effect on soil
microbial populations, but 3 months after compost addition, the number of viable fungal propagules increased in all the amended soils. This positive effect
lasted until the end of the experiment. From 30 days
onwards, all the amended soils showed a greater total
number of bacterial cell forming units than the unamended burnt soil. Organic amendment increased the
percentage of 2- to 4-mm aggregates, although the effect on the stability of the 0.2- to 2-mm aggregates and
on bulk density was less noticeable.
Keywords Aggregate stability 7 Bacteria 7
Burnt soil 7 Compost 7 Fungi
M.T. Hernández (Y)
Department of Soil and Water Conservation
and Organic Waste Management,
Centro de Edafología y Biología Aplicada del Segura,
CEBAS-CSIC, P.O. Box 4195, 30080 Murcia, Spain
e-mail: mthernan6natura.cebas.csic.es
Tel.: c34-968-215717
Fax: c34-968-266613
C. Guerrero 7 J. Mataix Solera 7 R. Moral 7 J. Mataix Beneyto
Department of Agrochemistry and Environment,
Miguel Hernández University, 03202 Elche, Spain
I. Gómez
Department of Agrochemistry and Biochemistry,
University of Alicante. P.O. Box 99, 03080 Alicante, Spain
Introduction
Wildfires strongly modify ecosystem characteristics,
particularly soil properties, which contribute to the ecosystem’s destruction. Soil microbiota, which is the main
agent responsible for organic matter decomposition
and is involved in the macronutrient cycles (Harris and
Birch 1989) is negatively affected by fire (Hernández et
al. 1997). The importance of bacteria and fungi in natural soil ecosystems has been well recognised, and microbial activities play an important role in the reclamation of disturbed ecosystems, where they contribute to
nutrient turn-over, N and P immobilisation (microbial
biomass) and the formation of relatively stable aggregates (Dinel et al. 1992). In general, bacterial populations often dominate fungi and other microbial groups
after fire (Vázquez et al. 1993). The bacterial population usually returns to normal values very quickly after
burning, while fungi seem to recover more slowly. The
pre-burnt state may not be reached even after 1 year
(Raison 1979). This may be due to an increase in soil
pH, changes in or lack of organic matter, and/or the inhibitory effect of different pyromorphic compounds
(Widden and Parkinson 1975).
In order to recover forest ecosystems after wildfires,
the physical, chemical and microbiological characteristics of the soil have to be re-established. The addition
to the soil of an organic material, with a high macroand micronutrient content and a diverse microbial population, will favour the re-establishment of microbial
biomass and activity and, hence, plant recovery, reducing the time needed to reach suitable levels of soil protection (Villar et al. 1998)
The aim of this work was to evaluate the efficacy of
adding a compost to a burnt soil in order to accelerate
the re-establishment of bacterial and fungal populations and to restore the soil’s physical characteristics.
For this, a municipal solid waste compost was added to
a burnt soil at 3 different dosage rates and changes in
bacterial and fungal populations, as well as in the phy-
411
sical and physico-chemical properties of the soil, were
studied during 1 year.
Materials and methods
The experiment was carried out in a forest soil burnt 5 months
previously, located in Bocairente (Valencia, Spain), at 38744b9nN
and 0742b9nW. The soil is a Calcic Rhodoxeralf (Soil Survey Staff
1990), and the site (730 m above sea level) has a ~2% slope and
SE orientation. No live vegetation was observed, and the soil had
a Munsell colour of 7.5 YR 4/4 (dry) and 10 YR 3/2 (wet) due to
the ash cover. During the experimental period, the average temperature was 15.3 7C, the average daily minimum temperature being 8.9 7C and the average daily maximum temperature 21.7 7C;
the mean annual rainfall was 440 mm. The study was performed
in 10-m 2 plots randomly located in a burnt area of 3000 m 2. Four
treatments were established by adding different doses of a municipal solid waste compost from an industrial composting plant in
Valdemingómez, Madrid, Spain, to the burnt soil. The application
rates of 0, 0.5, 1, and 2 kg compost m –2 soil,(dry basis) shall be
referred to as control, SW1, SW2 and SW3 treatments, respectively. All the treatments were carried out in quadruplicate. Compost
was applied superficially to the burnt soil, and no attempt was
made to mix it with topsoil. Samples were taken 15, 30, 60, 90,
120, 150, 240, 300, and 360 days after compost application. Each
sample consisted of a mixture of eight randomly collected soil
subsamples, taken from the top 15 cm.
Soil chemical and physical analysis
Electrical conductivity (EC) was measured in a 1 : 5 (w/v) aqueous
solution at 20 7C. Chlorides were determined in the above extract
by HPLC. Sodium was extracted by 1 N ammonium acetate in a
1 : 10 soil : solution ratio (Knudsen et al. 1982), and determined by
emission spectrophotometry. Parameters were analysed in 2-mmsieved soil samples. Aggregate stability was determined in 0.2- to
2-mm and 2- to 4-mm-diameter soil aliquots, following the method of Benito et al. (1986) modified by Roldán et al. (1996). Bulk
density was determined in soil clods of 2–3 cm diameter by the
method of Barahona and Santos (1981).
Table 1 Changes with time in the electrical conductivity (EC) of
the amended and unamended soils. C Burnt soil, SW1 0.5 kg compost m –2 burnt soil, SW2 1 kg compost m –2 burnt soil, SW3 2 kg
compost m –2 burnt soil, LSD least significant difference
(P^0.05), ns not significant at P^0.05
Sampling
time
(days)
15
30
60
90
120
150
240
300
360
LSD
F-ANOVA
EC (mS m –1)
LSD
F-ANOVA
17.9
28.0
27.1
12.3
17.1
9.9
5.3
4.6
4.6
***
***
***
***
*
***
***
n.s.
n.s.
Treatments
C
SW1
SW2
16.1
16.0
20.5
13.4
12.5
16.3
11.4
16.6
15.4
1.2
***
28.8
32.6
41.5
22.8
21.8
21.4
20.7
20.1
14.5
8.5
***
30.2
65.3
52.2
23.7
27.6
23.3
21.0
17.0
16.9
13.7
***
SW3
80.1
121.8
131.8
58.7
43.9
46.0
31.4
22.0
20.5
26.5
***
* P^0.05, ** P^0.01, *** P^0.001
the compost and inorganic species formed by organic
matter mineralisation. After heavy rainfall, the EC decreased in all the treatments due to salt leaching, as
confirmed by the increase in EC observed in the 15- to
30-cm-deep soil samples after rain (data not shown).
High correlation coefficients were obtained between
EC and the chloride (r 1 0.96; P~0.001) and sodium
(r 1 0.0.84; P~0.001) content, particularly in amended
soils. No statistically significant differences were detected between treated and untreated soils after
300 days.
Soil microorganisms
Soil microbiological analysis.
Twenty grams of fresh soil were diluted in tenfold series in sterile
water. Viable bacterial numbers were determined by plate colony
counts (Schmidt 1973) after 5 days of incubation at 28 7C on Petri
dishes containing solid media. The composition of this agar medium in 1 l was: 1.15 g bacteriological peptone; 0.15 g sodium citrate; 0.025 g sodium nitrate; 0.015 g iron (II) sulphate 7-hydrate;
0.015 g magnesium sulphate 7-hydrate; 0.015 g potassium chloride; 0.015 g calcium chloride; agar technical n73,11 (Oxoid). Viable fungus propagules (CFU) were determined in rose bengal
agar (Oxoid) (Martin 1950) with the same dilution plate method
(Schmidt 1973).
Data were analysed using one-way ANOVA and significant
differences between means were determined by Tukey’s test.
Results and discussion
The addition of compost increased soil EC in a dosedependent manner (Table 1). These values increased
during the first 60 days following the addition of compost, an increase attributable to the solubilisation of
soluble ions like chlorides, sulphates and sodium from
During the first 60 days of the experiment the number
of fungal propagules in the amended soils was lower
than in the control (Table 2). This negative effect was
probably due to the presence in the compost of high
amounts of salts and other substances toxic to the soil’s
native fungi. García and Hernández (1996) indicated
that sodium sulphates and chlorides have a negative effect on soil microbial activity and microbial biomass,
this negative effect being much more noticeable in the
case of sodium chloride. However, it was observed that
during the first 60 days, the number of fungal propagules in the treated soils increased with increasing
amounts of added compost: SW3 1 SW2 1 SW1, a fact
explained by the high level of fungi in the compost
(6800 10 3 CFUs g –1). These compost fungi are adapted
to living in this medium, and are more tolerant than the
natural soil fungi to inhibitory substances, such as sodium chlorides and sulphates. It is therefore supposed
that the fungus propagules initially found in the
amended soils mainly came from the compost. Between
60 and 90 days, 40 mm rainfall was registered, a suffi-
412
Table 2 Changes with time in the micro-organism populations
(cell forming units; CFUs) of the amended and unamended soils.
For other abbreviations, see Table 1
Sampling
time
(days)
Fungi (10 3 CFUs g –1 soil)
LSD
F-ANOVA
18
7
16
7
24
10
16
7
25
**
***
***
***
***
***
***
***
***
23
13
45
46
17
8
11
49
27
***
***
***
***
*
***
***
*
***
Treatments
C
15
60
30
84
60
117
90
84
120
92
150
57
240
70
300
82
360
88
LSD
13
F-ANOVA ***
SW1
SW2
SW3
26
44
75
86
80
121
123
126
166
13
***
51
63
79
91
140
149
102
73
169
15
***
55
71
96
143
180
150
128
119
170
18
***
Bacteria (10 5 CFUs g –1
15
75
38
66
30
64
121
111
60
55
259
167
90
97
211
44
120
32
18
14
150
68
136
222
240
101
142
180
300
112
126
137
360
75
95
104
LSD
25
29
33
F-ANOVA ***
***
***
soil)
17
109
280
276
37
248
223
190
194
28
***
* P^0.05, ** P^0.01, *** P^0.001
cient quantity to leach the sodium chlorides and sulphates from the topsoil. As a direct consequence of the
diminished levels of these toxic substances, the number
of fungal CFUs in the SW1 and SW2 treatments at
90 days reached the levels found in the untreated soil,
while the numbers in the SW3 treatment exceeded
those of the unamended soil. A great increase in fungal
CFUs was again detected in treated soils 120 days after
soil amendment, particularly in the intermediate (SW2)
and high (SW3) dose treatments. This fact can be explained by:
1. The leaching of salts caused by two consecutive intense rainfall events (140 mm in 48 h) which took
place some time before sampling.
2. The decomposition of toxic substances, other than
salts, in the medium.
3. The positive effects of compost addition on soil
characteristics (increased organic matter, nutrients,
etc.), which would have stimulated fungal growth.
The fungal CFUs detected at this stage would now
have been both native (from the soil) and exogenous
(from the compost).
The bacterial populations evolved differently from
the fungi. At the first sampling date (15 days), the number of viable bacterial CFUs in the amended soils was
in general lower than that observed in the unamended
soil (Table 2), probably due to the presence in the com-
post of substances which inhibited or were toxic to viable micro-organisms, or due to the anaerobic conditions caused by the mineralisation of the added organic
matter. This negative effect had disappeared by the
next sampling date (30 days), when the amended soils
presented a greater number of viable bacterial CFUs
than the control soil (Table 2). This could have been
due to:
1. The beneficial effect of the nutrients and organic
matter added with the compost counterbalancing the
negative effect of the toxic substances.
2. The ability of the soil bacterial population to adapt
to the new soil conditions and their developing resistance to the toxic substances.
3. The natural degradation or leaching of the toxic substances.
The number of bacteria in the unamended soil followed seasonal changes, although the variations in this
soil were wider than in the amended soils, which were
enriched with bacteria added with the compost. This
may have been due to the reduction in microbial community diversity caused by fire (Staddon et al. 1998),
which was not completely restored in the unamended
soil.
Aggregate stability (0.2–2 mm)
Initially, the addition of compost did not seem to influence soil aggregate stability, the percentage of stable
aggregates remaining statistically similar in both
amended and unamended soils (Table 3). However, at
60 days, amendment was seen to have a negative and
dose-dependent effect on the stability of the 0.2- to 2mm aggregates, the soils amended with a high dose
(SW3) showing the lowest percentages of stable aggregates. The soils amended with a low dose (SW1)
showed percentages of stable aggregates which were
statistically similar (with some exceptions) to those of
the control, while the high dose (SW3) produced percentages, which were significantly lower than those observed in the control soil between days 60 and 240. The
SW2 treatment showed an intermediate behaviour.
This pattern could be due to: (1) the dispersion of soil
colloids as a result of the high salinity produced by high
doses of compost, or (2) the fractionation of long-chain
carbohydrates (responsible for soil aggregation processes), resulting from the stimulation of microbial activity by compost, particularly at the SW3 dosage. Lax and
García-Orenes (1993) found that fractionated organic
matter had less of an effect on soil aggregation than
non-fractionated organic matter. By the end of the experiment (300 days after adding the compost) the percentages of stable aggregates in SW3 soils were again
similar to those of unamended soils. This can be explained by the decrease in salinity with time and by the
formation of new aggregates as a consequence of the
activity of the newly developed microbial populations
in the amended soils.
413
Table 3 Changes with time in the percentage of stable aggregates
in amended and unamended soils. For abbreviations, see Table 1
Sampling
time
(days)
Aggregate stability (%)
(0.2–2 mm)
LSD
F-ANOVA
Table 4 Changes with time in the. bulk density of the amended
and unamended soils. For abbreviations, see Table 1
Sampling
time
(days)
Treatments
C
SW1
SW2
SW3
15
30
60
90
120
150
240
300
360
LSD
F-ANOVA
54.8
59.6
65.8
54.7
50.8
52.2
53.2
53.3
54.3
6.0
***
47.3
59.8
53.5
50.5
46.5
50.4
52.6
52.5
56.3
6.5
**
46.0
54.5
50.9
42.1
46.9
53.3
51.5
52.5
54.9
2.4
***
47.9
53.4
49.5
43.1
45.0
41.9
46.6
47.9
47.3
1.9
*
15
30
60
90
120
150
240
300
360
LSD
F-ANOVA
Aggregate stability (%) (2–4 mm)
90.0
87.4
79.6
70.1
1.4
93.0
88.0
90.8
85.5
3.4
94.7
94.6
92.4
91.3
2.3
81.6
93.7
85.9
90.4
1.4
77.5
84.9
86.1
84.9
3.4
82.9
89.8
90.8
85.6
2.8
88.4
93.2
91.7
91.8
2.2
90.8
94.7
91.6
94.7
3.1
85.7
91.4
90.1
88.0
3.1
3.1
2.6
2.4
1.9
***
***
***
***
8.3
5.8
7.5
5.3
2.7
4.8
3.4
6.6
7.5
n.s.
n.s.
**
***
**
***
**
n.s.
n.s.
15
30
60
90
120
150
240
300
360
LSD
F-ANOVA
Bulk density (g cm –3)
LSD
F-ANOVA
0.07
0.10
0.16
0.05
0.11
0.11
0.07
0.09
0.05
*
**
n.s.
***
n.s.
*
n.s.
n.s.
*
Treatments
C
SW1
SW2
SW3
1.24
1.22
1.25
1.16
1.28
1.20
1.30
1.37
1.42
0.10
***
1.29
1.34
1.43
1.32
1.33
1.37
1.39
1.39
1.39
0.07
**
1.26
1.41
1.35
1.43
1.34
1.30
1.37
1.41
1.33
0.08
**
1.35
1.38
1.38
1.40
1.32
1.32
1.33
1.34
1.37
0.11
ns
* P^0.05, ** P^0.01, *** P^0.001
***
**
*
***
***
***
**
*
*
* P^0.05, ** P^0.01, *** P^0.001
Aggregate stability (2–4 mm)
At the first two samplings dates, particularly the first,
the addition of compost was seen to have a slightly negative effect on aggregation (Table 3). After this period, aggregation was significantly higher in the amended
soils at all doses. This increase in the percentage of stable aggregates in the amended soils can be attributed
both to salt leaching and to the increase in microbial
populations. Positive correlation coefficients between
the percentage of 2- to 4-mm stable aggregates in the
amended soils and the number of CFUs of viable bacteria were observed (rp0.8049, P~0.01; rp0.6862,
P~0.05; and rp0.7306, P~0.05, for SW1, SW2 and
SW3, respectively).
Bulk density
Bulk density did not show any clear tendency. Initially,
bulk density values were slightly higher in the amended
soils, which could be explained by the changes observed in the 0.2- to 2-mm aggregate stability, there being a close relationship between aggregation and bulk
density (Clapp et al. 1986). The values of bulk density
in all the soils (amended and control) tended to stabil-
ise at F1.3–1.4 g cm –3. Such a value is probably optimal
for this soil, and the application of compost would only
accelerate the natural evolution of bulk density. This
hypothesis is supported by the fact that bulk density did
not change with time in the SW3 treatment, whereas
significant changes were observed in the control (Table 4).
In conclusion, from the above results it can be concluded that the addition of MSW compost to burnt soils
is a suitable technique for accelerating the natural recovery process of burnt soils, thus shortening the period of time a soil is exposed to potential erosion processes. Although the addition of compost may initially
have a negative effect on soil microbial populations due
to the presence in the compost of large amounts of salts
and other toxic substances, this effect soon disappears
since the toxic substances are degraded and the salts
are leached. Therefore, the addition of compost contributes to the quicker re-establishment of microbial populations, which play an important role in aggregate stability and in the biogeochemical cycles of macronutrients, thus favouring the recovery of plant cover and,
consequently, increasing soil protection. Among all the
compost rates assayed, 1 kg m –2 (SW2) seemed to be
the most suitable for the reclamation of burnt soils in
our experimental conditions, since it stimulated soil
fungal and bacterial development and improved the
stability of the 2- to 4-mm aggregates. At this dosage
the initial negative effects due to the addition of salts or
toxic substances with the compost was less noticeable
than at the high dose (SW3).
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