European Journal of Soil Biology 40 (2004) 113–121
www.elsevier.com/locate/ejsobi
Dynamics of soil microbial biomass C, N and P in disturbed
and undisturbed stands of a tropical wet-evergreen forest
A.R. Barbhuiya a, A. Arunachalam a,*, H.N. Pandey b, K. Arunachalam a,
M.L. Khan a, P.C. Nath a
a
Restoration Ecology Laboratory, Department of Forestry, North-Eastern Regional Institute of Science and Technology, Nirjuli 791109,
Arunachal Pradesh, India
b
Department of Botany, North-Eastern Hill University, Shillong 793022, Meghalaya, India
Received 2 October 2003; accepted 2 February 2005
Available online 12 April 2005
Abstract
To understand the spatial and temporal dynamics of soil microbial biomass and its role in soil organic matter and nutrient flux
in disturbed tropical wet-evergreen forests, we determined soil microbial biomass C, N and P at two soil depths (0–15 and
15–30 cm), along a disturbance gradient in Arunachal Pradesh, northeastern India. Disturbance resulted in considerable increase
in air temperature and light intensity in the forest and decline in the soil nutrients concentration, which affected the growth of
microbial populations and soil microbial biomass. There were significant correlations between bacterial and fungal populations
and microbial biomass C, N and P. Soil microbial population was higher in the undisturbed (UD) forest stand than the disturbed
forest stands during post-monsoon and less during rainy season due to heavy rainfall. Greater demand for nutrients by plants
during rainy season limited the availability of nutrients to soil microbes and therefore, low microbial biomass C, N and P.
Microbial biomass was negatively correlated with soil temperature and pH in all the forest stands. However, there were significant positive relationships among microbial biomass C, N and P. Percentage contribution of microbial C to soil organic C was
higher in UD forest, whereas percentage contribution of microbial biomass N and P to total N and total P was higher in the
moderately disturbed site than in the highly disturbed (HD) site. These results reveal that the nutrient retention by soil microbial
biomass was greater in the selective logged stand and would help in the regeneration of the forest upon protection. On the other
hand, the cultivated site (HD) that had the lowest labile fractions of soil organic matter may recover at a slower phase. Further,
minimum and maximum microbial biomass C, N and P during rainy and winter seasons suggest the synchronization between
nutrient demand for plant growth and nutrient retention in microbial biomass that would help in ecosystem recovery following
disturbance.
© 2005 Elsevier SAS. All rights reserved.
Keywords: Disturbance; Microbial biomass; Soil nutrient pool; Tropical wet-evergreen forest; Northeast India
1. Introduction
Understanding on the dynamics of microbial biomass following forest disturbances is important not only with regard
to the role in soil nutrient turnover, but also to develop syn* Corresponding author.
E-mail address: [email protected]
(A. Arunachalam).
1164-5563/$ - see front matter © 2005 Elsevier SAS. All rights reserved.
doi:10.1016/j.ejsobi.2005.02.003
chronized strategies for reclamation and management of
degraded lands [30]. This is particularly critical in tropical
forest ecosystems that are nutrient poor, but harbor rich plant
species diversity [34]. In the northeastern India, the tropical
wet-evergreen forest at lower elevations has been overexploited for timber and non-timber forest products. In addition, forest clearance for agriculture particularly jhum cultivation have also contributed significantly to the loss of forest
in the region [32]. Presently, the typical wet-evergreen for-
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A.R. Barbhuiya et al. / European Journal of Soil Biology 40 (2004) 113–121
ests that once was a dominant vegetation type in the region
has become pocketed as remnant patches (108 km2) distributed in the Deomali and Joypur forest divisions of Arunachal
Pradesh and Assam, respectively. In these forests, tree regeneration is also slow as the soil is highly leached and run-off
losses are more during monsoon. Several studies have revealed
the role of soil microbial biomass in organic matter and nutrient turnover [5,8,20]. The information on changes in soil
microbial biomass following vegetation removal is valuable,
not only because it provides an indication of slower, less easily detectable soil organic matter changes [30], but also
because it represents an important labile pool of plantavailable nutrients [20] and plays an active role in nutrient
conservation in the tropical soils [35] by preventing nutrient
leaching [40].
Tree cutting is one of the prominent anthropogenic disturbances in the forest ecosystems of the northeastern India [6].
Felling results in opening up of forest canopy and alterations
in the forest-floor microenvironment that deteriorates the soil
nutrient level [24,28]. Changes in the soil physico-chemical
and microbial biomass due to various disturbances has been
studied by different workers in the humid subtropical region
of northeast India [3,5,26] and in some tropical soils of India
[34,37]. It is hypothesized that the dynamics of soil microbial biomass is confounded by the vegetal cover that is altered
during tree cutting in forests [5]. We, therefore, determined
the seasonal variations in microbial biomass C, N and P and
soil physico-chemical properties to understand the role of
microbial biomass in organic matter and nutrient dynamics
in tree-cut and undisturbed (UD) stands of a tropical rain forest in the northeastern India.
2. Study sites
An UD tropical wet-evergreen forest and two disturbed
sites were selected in Deomali Reserve Forest and its adjoining areas in the Tirap district of Arunachal Pradesh (latitude
27°05′ to 27°28′N; longitude 95°20′ to 95°38′E; altitude
220 m asl). The average annual rainfall ranges between
2500 and 3600 mm with maximum rainfall during July–
September (Fig. 1). Mean monthly minimum and maximum
temperatures were 7–36 °C, respectively. The year has four
distinct seasons viz., cold-dry winter (December–February),
warm pre-monsoon period (March–May), warm-wet monsoon period (June–September) and cool post-monsoon
autumn (October–November).
The soil is brown, loamy, lateritic ultisol derived from parent pegmatite rock. Although, soils in the three selected stands
have developed from the same parent material, the UD forest
has a well-developed soil profile with a distinct litter layer
and an organic horizon extending to a depth of 5 cm. In the
disturbed sites, the organic horizon was very thin and less
distinct because of the absence of a litter layer. Such variations are attributed to the nature and intensity of disturbance
in the site. For instance, the moderately disturbed (MD) site
had experienced selective logging until 1994, and the system
is regrowing since then. This site (MD) (ca. 2 ha) is dominated by tree species such as Mesua ferrea, Terminalia myriocarpa, Alangium begonifolia, Tetrameles nudiflora, Duabanga grandiflora, Sapium baccatum, etc. with individuals
distributed between 10 and 70 cm DBH. The highly disturbed (HD) site (ca. 2 ha) has been massively felled before
6–8 years for settled cultivation. During summer (June–
August), paddy is cultivated and during winter (December–
March) mustard is mixed with vegetables. For the past 5 years,
the site has been under continuous cultivation. A few intact
trees are interspersed in the HD site along with large number
of cut-stumps measuring 1–2 m cbh. Shrubs and tree saplings were absent in this site as the site is under cultivation.
Biscofia javanica, Dillenia indica, D. grandiflora, Bombax
ceiba and Albizia sp. were the tree species at this HD stand.
For comparison purposes, one UD forest stand (over 5 ha)
was selected in the adjacent Deomali Reserve Forest dominated by mature trees (30–70 cm DBH) of Dipterocarpus
macrocarpus, Shorea assamica, M. ferrea, T. nudiflora,
Castanopsis indica and Vatica lanceaefolia.
3. Materials and methods
Soil sampling was done both in the UD and disturbed forest stands during January, April, July and October 2002, rep-
Fig. 1. Monthly variation in rainfall (—) number of rainy days (—) at Deomali, Arunachal Pradesh, India.
A.R. Barbhuiya et al. / European Journal of Soil Biology 40 (2004) 113–121
resenting winter, spring, rainy and autumn seasons, respectively. From each stand, 10 soil cores (5.5 cm inner diameter)
were collected randomly from 0 to 15 cm and 15–30 cm soil
depths after clearing the litter layer on the ground and mixed
depthwise to obtain composite samples. After removing
stones, pebbles and large pieces of plant material, the sample
was sieved to 2 mm mesh size and divided into two parts.
One part of the composite sample was retained for the estimation of soil physico-chemical parameters and the other
fresh part was used for the analysis of microbial biomass.
Soil texture was determined by Bouyouncos [10] hydrometer method and bulk density was determined by soil core
method [9]. Water holding capacity (WHC) was determined
according to Keen’s box method given in Piper [29], while
soil moisture content was measured gravimetrically by incubating 10 g of field moist soil sample in a hot-air oven at
105 °C for 24 h. Organic C content of soil was determined by
dichromate oxidation and titration with ferrous ammonium
sulfate [43]. Total N was estimated following semi-micro
Kjeldahl procedure by acid-digestion, distillation and titration [1]. For determination of total P, the soil samples were
digested using a triacid mixture, followed by the colorimetric reaction (molybdenum-blue method) with ammonium
molybdate and stannous chloride [19]. The pH of the soil
sample was measured in a 1:2.5 soil/water suspension.
Chloroform fumigation-extraction method was used to
estimate microbial C (MBC), N (MBN) and P (MBP). MBC
and MBN were determined in fresh soil by chloroformfumigation extraction method [14,39]. For the CFE, 15 g fresh
soil samples were placed in 50 ml beaker and kept in a vacuum
desiccator containing a 100 ml beaker with 25 ml alcoholfree chloroform. Another desiccator was maintained without
chloroform, and then the desiccator was kept in the dark for
72 h at room temperature. The fumigated treatments were
evacuated using a vacuum pump until the chloroform boiled
rapidly. The soil samples were transferred to 250 ml conical
flask and extracted with 0.5 M K2SO4 after shaking for 20 min
in a rotatory shaker at 110 rpm. The extracts were filtered
through a Whatman No. 42 filter paper and the filtrates (10 ml)
were digested using H2SO4 in a block digestor at 145–
155 °C for 30 min. The digest was titrated against ferrous
ammonium sulfate (0.2 N) using 1,10 phenanthroline monohydrate as indicator. For MBN, the digested filtrate was distilled by steam distillation using semi-micro Kjeldahl and
titrated against hydrochloric acid (0.05 N). MBP was determined by chloroform fumigation-extraction method [12]
using 0.5 M NaHCO3 as extracting solution. The MBC, MBN
and MBP were calculated as follows: Microbial C = (EC of
fumigated soil – EC of unfumigated soil) × 2.64; Microbial N
= (EN in fumigated soil – EN in unfumigated soil)/0.54 and
Microbial P = (EP of fumigated soil – EP of unfumigated
soil)/0.40, where EC, EN and EP are extractable C, N and P,
respectively.
For estimation of microbial population, soil samples were
collected separately from the two soil depths using a sterilized corer. The samples were brought to the laboratory in
115
sealed containers and the microbial population was estimated within 24 h of sampling. Soil bacterial population was
estimated by Waksman’s [42] method using nutrient agar
medium at 105 dilution and fungal population were determined by dilution plate method [23] using rose bengal agar
medium at 104 dilution in water. The inoculated Petri dishes
were incubated for 24 h (30 ± 1 °C) for bacteria and for 5 days
(25 ± 1 °C) for fungi. The colonies developed were counted
using the digital colony counter.
Tukey’s test and LSD at P < 0.05 level have been used to
compare seasonal and annual mean across disturbance gradient and soil depths. Correlation analysis was done following
Zar [44] to study the relationship between microbial biomass
C, N and P and soil characteristics.
4. Results
In the UD forest stand air temperature, soil temperature
and light intensity were significantly (P < 0.05) lower than
the disturbed sites (Table 1). Clay content, WHC and moisture content were greater in the UD site, and declined along a
disturbance gradient. Soil was acidic (pH 4.29–6.59), and
showed no spatial variation between the two depths studied
(Table 2). Whereas, the depthwise variation was significant
(P < 0.05) in case of soil organic C (F = 6.38) and total N (F
= 4.31); all were high in the top 0–15 cm soil layer. There
was 40–50% decline in the concentration of organic C in the
HD forest as compared to the UD stand (Table 2). Total N
showed little variation between moderately and HD sites. Soil
organic C was greater during spring, decreased sharply during autumn through rainy season and again increased during
winter (Table 2), while total N concentration was minimum
during rainy season and maximum during spring in the UD
forest. In moderately and HD stands, it was higher during
winter. On the contrary, total P concentration was high during rainy season and low during autumn in the UD stand and
during spring in the disturbed forest.
Bacterial and fungal populations (Table 3) and microbial
biomass C, N and P (Table 4) were significantly (P < 0.05)
greater in the UD forest than the disturbed stands. All the
values were greater in the topsoil layer (0–15 cm). The microbial populations were minimum during rainy season and maximum during autumn in the UD forest stand (Table 3). In the
moderately and HD stands, fungal populations were higher
during spring and lower during rainy season.
Microbial biomass C was minimum during rainy season
in all stands and the maximum value (1135 µg g–1) was
recorded during autumn in the UD stand and during winter in
the disturbed forests (Fig. 2a). Microbial N was also low during rainy season at both soil depths in all the three stands
(Fig. 2b). The maximum values were generally recorded during autumn in UD and HD stands, but in the MD stand the
value was greater during winter (77 µg g–1). The peak values
for microbial P were 44, 31 and 13 µg g–1 in UD, MD and
HD stands, respectively (Fig. 2c). Its trough was recorded
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Table 1
Vegetation, microclimate and soil (0–15 cm) physical characteristics of disturbed and UD forest sites
Study sites
Physical parameters
Vegetation
Density (ha–1)
Trees
Shrubs
Herbs
Basal area (m2 ha–1)
Trees
Shrubs
Herbs
Disturbance index (%)
Plant detritus
Litter (kg ha–1)
Fine roots (<2 mm) (kg ha–1)
Microclimate
Air temperature (°C)
Relative humidity (%)
Light intensity (Lux)
Soil temperature (°C)
Litter depth (cm)
Soil properties
Texture
Sand (%)
Silt (%)
Clay (%)
WHC (%)
Bulk density (g cm–3)
Moisture content (%)
UD
MD
HD
658
7500
15,900
369
3740
38,917
41
1160
40,250
85.55
2.61
0.27
0
20.83
0.60
0.44
54
5.02
0.37
0.27
88
754.02 ± 115.15
1462.39 ± 257.60
146.91 ± 14.78
762.23 ± 18.54
109.67 ± 42.01
735.01 ± 45.55
22.76 ± 1.50
75.04 ± 6.31
2258.29 ± 127.02
20.47 ± 1.32
2.51 ± 0.23
26.37 ± 3.57
67.62 ± 1.07
10364.68 ± 221.70
23.70 ± 2.72
1.24 ± 0.33
25.82 ± 3.20
69.13 ± 3.40
17270.75 ± 411.28
22.76 ± 2.51
0.80 ± 0.02
58.30
12.00
29.70
59.99 ± 1.96
0.67 ± 0.008
30.06 ± 2.38
66.95
10.51
22.54
45.77 ± 6.85
0.83 ± 0.02
20.04 ± 1.46
78.77
7.64
13.59
40.02 ± 1.16
0.88 ± 0.008
18.48 ± 1.37
± S.E. (n = 20); values are the means of four seasonal samplings. UD-undisturbed, MD-moderately disturbed and HD-highly disturbed sites.
WHC-water holding capacity.
Basal area of cut stumps
×100 (Rao et al. [31]).
Disturbance index =
Total basal area of all the trees
during rainy season and peak during autumn. The C/N ratio
in microbial biomass (12.11–14.54) was higher in the subsurface soil layer than the surface layer (10.92 and 13.25). MD
stand had higher value than the UD and HD stands; the latter
had the lowest value. The microbial C/P ratio declined along
the disturbance gradient from UD to HD stand (Table 4). The
microbial N/P ratio varied between 1.33 and 2.01. The percentage contribution of microbial biomass C to total soil
organic C (4.17–6.08%) was greater in the UD forest, but the
contribution of microbial biomass N and P to total N and
total P, respectively, was greater in the MD forest (Table 4).
5. Discussion
Tree cutting resulted in considerable changes in the microclimate and soil physico-chemical and microbiological properties of the forest. Thinning of the canopy in the disturbed
stands was responsible for reduced litter depth and greater
light intensities, and greater soil and air temperatures on the
forest floor. Greater WHC of the soil in the UD forest is attributed to high clay and organic matter contents. Low soil organic
matter in the disturbed stands may be attributed to less litter
production and faster decomposition rate [41], while low soil
pH in the UD forest as compared to the disturbed sites could
be the result of lower rate of leaching leading to greater accumulation of reaction products in the soil [4]. Low concentration of total N during rainy season indicates that this element
is highly leacheable [27]; therefore, it has been reported as a
limiting nutrient in tropical rain forests [40]. On the contrary,
higher P concentration during monsoon signals its relatively
immobile nature, as compared to N.
Though the soil microbial populations varied seasonally
at all sites and soil depths, their over all populations were
greater in the UD forest where soil was rich in organic matter
than in the disturbed stands. Low microbial populations during rainy season in the UD and disturbed stands may due to
run-off losses of microbial propagules along with the plant
materials due to heavy rainfall [36]. Except for this, the fungal population did not show significant variations during other
A.R. Barbhuiya et al. / European Journal of Soil Biology 40 (2004) 113–121
117
Table 2
Soil pH and nutrient concentrations in disturbed and UD forest stands
Study sites/soil depth (cm)
Soil chemical parameters/seasons
pH (1:2.5 w/v H2O)
W
S
R
A
Organic C (%)
LSD
W
S
R
A
Total N (%)
LSD
W
S
R
A
Total P (%)
LSD
W
S
R
A
LSD
UD
0–15
5.05
± 0.03
5.37
± 0.06
4.29
± 0.03
5.02
± 0.04
0.39
1.65
± 0.13
1.84
± 0.07
1.51
± 0.22
1.33
± 0.23
0.18
0.63
± 0.04
0.80
± .01
0.45
± 0.08
0.77
± 0.04
0.13
0.30
± 0.02
0.28
± 0.08
0.43
± 0.03
0.22
± 0.07
0.07
15–30
5.06
± 0.11
5.50
± 0.05
4.47
± 0.01
5.13
± 0.08
0.36
1.28
± 0.11
1.79
± 0.09
1.24
± 0.24
0.90
± 0.06
0.31
0.50
± 0.04
0.52
± 0.03
0.44
± 0.07
0.34
± 0.01
0.07
0.21
± 0.02
0.23
± 0.05
0.41
± 0.02
0.21
± 0.01
0.08
MD
0–15
5.03
± 0.04
5.86
± 0.05
5.75
± 0.02
5.00
± 0.04
0.39
1.02
± 0.03
1.76
± 0.04
0.76
± 0.05
0.68
± 0.05
0.42
0.70
± 0.01
0.40
± 0.001
0.18
± 0.005
0.41
± 0.02
0.18
0.19
± 0.04
0.16
± 0.007
0.19
± 0.003
0.17
± 0.008
0.01
15–30
5.27
± 0.18
5.77
± 0.03
5.41
± 0.03
5.10
± 0.05
0.25
0.74
± 0.13
1.41
± 0.11
0.79
± 0.02
0.55
± 0.11
0.32
0.50
± 0.02
0.18
± 0.001
0.17
± 0.001
0.15
± 0.001
0.14
0.14
± 0.01
0.13
± 0.01
0.19
± 0.001
0.17
± 0.001
0.02
HD
0–15
5.88
± 0.07
6.35
± 0.04
5.79
± 0.04
6.49
± 0.03
0.29
1.17
± 0.08
1.42
± 0.0 8
0.56
± 0.03
0.54
± 0.04
0.38
0.40
± 0.01
0.27
± 0.001
0.14
± 0.002
0.24
± 0.05
0.09
0.17
± 0.08
0.11
± 0.004
0.18
± 0.07
0.12
± 0.07
0.03
15–30
6.01
± 0.18
6.43
± 0.07
5.89
± 0.21
6.59
± 0.06
0.28
0.84
± 0.02
0.89
± 0.05
0.41
± 0.06
0.35
± 0.02
0.24
0.30
± 0.01
0.25
± 0.001
0.11
± 0.004
0.15
± 0.006
0.07
0.14
± 0.004
0.11
± 0.001
0.13
± 0.01
0.10
± 0.001
0.02
LSD
(P < 0.05)
0–15 15–30
0.39 0.40
0.40
0.39
0.69
0.58
0.69
0.69
0.26
0.23
0.18
0.36
0.45
0.33
0.34
0.22
0.12
0.09
0.22
0.15
0.14
0.14
0.22
0.09
0.06
0.03
0.07
0.05
0.11
0.12
0.04
0.05
± S.E. (n = 5). W - winter, S - spring, R - rainy, A - autumn.
seasons. However, bacterial population was greater during
post-monsoon period and declined during spring (Table 3).
In the HD stand, it was higher during winter as the site was
not under cultivation and lower during rainy season when it
was under rice cultivation. Vegetation characteristics also
influenced soil microbial population as evident from a strong
positive correlation (P < 0.05) between plant density and bacterial (r = 0.937) and fungal (r = 0.931) populations in the
soil. Results further suggest that bacterial (r = 0.909) and fungal (r = 0.902) populations were positively correlated
(P < 0.05) with microbial biomass C. The correlations
between plant density and microbial biomass with soil tem-
perature were negative (r = –0.715 and –0.845, respectively). In general, microbial biomass C, N and P were low
during the rainy season when temperature and soil moisture
conditions were favorable for the microbial activity. Sarathchandra et al. [33] reported that relatively greater demand for
nutrients by plants during the rainy season when the majority
of them are at their peak vegetation growth further limited
the availability of nutrients to soil microbes, thereby reducing their immobilization in microbial biomass. Nonetheless,
greater accumulation of litter and fine roots favored the growth
of microbial population and also accumulation of microbial
biomass C, N and P in the UD stand (Table 1). Evidently the
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Table 3
Mean seasonal bacterial and fungal populations in the disturbed and UD forest soil
Microbial Soil
Populationdepth
(cm)
Bacte0–15
ria*
15–30
Fungi**
0–15
15–30
UD forest
R
W
S
132.66a
± 5.75
100.50a
± 8.00
29.55a
± 0.18
25.77a
± 0.50
98.53b
± 2.05
72.22b
± 0.58
32.11b
± 1.65
23.15a
± 0.27
A
70.06c
± 3.05
64.85c
± 1.25
28.79a
± 0.72
20.10b
± 0.85
149.27d
± 2.45
128.35d
± 4.45
27.33a
± 1.76
24.33a
± 0.45
W
89.48a
± 2.25
28.55a
± 1.23
20.01a
± 0.66
16.25a
± 0.33
Site/seasons
MD forest
S
R
A
W
S
84.50a
± 2.00
77.20b
± 4.15
24.22b
± 1.04
22.15b
± 0.45
72.31c
± 3.07
63.05d
± 4.55
22.05b
± 1.64
20.00b
± 1.85
136.12a
± 6.26
92.55a
± 5.90
19.67a
± 0.19
13.56a
± 0.17
68.00b
± 3.07
65.55b
± 1.01
20.14a
± 1.00
16.45b
± 0.77
44.27b
± 1.65
40.15c
± 0.15
13.05c
± 0.15
10.25c
± 0.13
HD forest
R
39.65c
± 1.02
30.55c
± 0.72
13.00b
± 0.58
12.86a
± 0.40
A
94.55d
± 4.21
88.75d
± 3.55
16.53c
± 0.31
15.02b
± 0.34
± S.E. (n = 5). * Number of colonies × 105 g–1 dry weight of soil. ** Number of colonies × 104 g–1 dry weight of soil. W - winter, S - spring, R rainy, A - autumn. Values with different alphabets as superscripts are significantly different at P < 0.05 level across seasons in each site.
relationships between detrital mass (litter and fine roots) and
microbial parameters (population and biomass) were positive and significant (r = 0.460–0.806, df = 11, P < 0.05).
The microbial biomass N values were lower as compared
to the findings reported from coniferous forest (52–125 µg g–1)
[28], broadleaved deciduous forest (132–240 µg g–1) and evergreen forest (42–242 µg g–1) [17]. This may be due to rapid
mineralization rate confounded by greater microbial activity
and N leaching in the tropical soils [25]. Significant positive
correlation between microbial biomass N and soil total N suggests that it can be used as an indicator of soil fertility [7].
Microbial biomass P values (Table 4) were, however, well
within the reported range (5–67 µg g–1) for the arable land,
grassland and woodland soils [13].
Mean C/N ratio (11–15) in soil microbial biomass of the
three sites was higher than the values (6–9) reported by Martikainen and Palojarvi [28] for various forest soils, but was
similar to disturbed chaparral soils (7–13) reported by Fenn
et al. [18]. This is probably due to low N availability and
relatively higher organic matter availability to soil microbes
at these forest sites. Similar observations have been made in
incubation experiments with complex soil microbial populations [15,21]. This may be explained by differential growth
of the fungal and bacterial populations confounded by
Table 4
Mean annual microbial populations and biomass C, N and P (µg g–1) and percentage contribution of microbial biomass to total soil nutrient pool
Parameters
Bacterial population*
Fungal population**
Microbial biomass C
(µg g–1)
Microbial biomass N
(µg g–1)
Microbial biomass P
(µg g–1)
MBC/MBN
MBC/MBP
MBN/MBP
Percentage contribution of
MBC to organic C
MBN to total N
MBP to total P
0–15
112.63a
± 20.44
29.45a
± 1.01
809.45a
± 115.63
74.12a
± 9.95
36.85a
± .86
10.92a
21.97a
2.01a
5.59a
± 1.08
1.27a
± 0.18
1.18a
± 0.006
UD forest
15–30
91.48a
± 16.75
23.34a
± 1.20
732.54a
± 140.64
53.35a
± 10.78
28.66a
± 4.14
13.73a
25.56a
1.86a
6.08a
± 0.28
1.16a
± 0.37
1.06a
± 0.003
Site/soil depth (cm)
MD
0–15
15–30
72.64b
52.24b
± 11.70
± 12.69
19.83b
17.16b
± 2.41
± 2.61
574.71b
498.48b
± 65.23
± 73.62
43.35b
34.28b
± 5.93
± 4.58
27.19b
25.70b
± 2.68
± 2.21
13.25b
14.54a
21.14a
19.39b
1.59b
1.33b
0–15
79.55c
± 8.26
17.33c
± 1.65
368.07c
± 51.94
31.34c
± 3.17
18.9c
± 1.64
11.74a
19.41a
1.65b
5.34a
± 0.84
1.44b
± 0.12
1.59b
± 0.05
5.20c
± 0.56
1.49b
± 0.11
1.35c
± 0.06
5.81a
± 0.12
1.72 b
± 0.23
1.71b
± 0.27
HD
15–30
69.35c
± 14.24
14.49c
± 2.05
295.50c
± 46.47
24.40c
± 3.65
16.74c
± 1.80
12.11a
17.65c
1.46b
4.17b
± 0.24
1.32c
± 0.27
1.39c
± 0.04
± S.E. (n = 20); values are the seasonal means. MBC – microbial biomass C; MBN – microbial biomass N; MBP – microbial biomass P.
* Number of colonies × 105 g–1 dry weight of soil. ** Number of colonies × 104 g–1 dry weight of soil. Values with different alphabets as
superscripts are significantly different at P < 0.05 level in each soil depth across sites.
A.R. Barbhuiya et al. / European Journal of Soil Biology 40 (2004) 113–121
119
Fig. 2. Seasonal variation in microbial biomass (a) carbon, (b) nitrogen and (c) phosphorus (µg g–1) at two soil depths (0–15 and 15–30 cm) in
disturbed and UD forest stands. Vertical bars represents the standard error (n = 12).
enhanced root growth and increased root exudates that stimulate the growth of bacteria [16] and the ratio of fungal to
microbial populations were also higher in the UD forest stand.
Joergensen et al. [22] suggested that forest soil with comparatively low C and N availability may result in microbial C/N
ratios, which are well below the optimum values of 5–8. However, our results do not conform to these findings. Higher C/N
ratios in microbial biomass obtained in this study (10.92–
14.54) indicates that fungi dominated the microbial biomass
of our study sites [2] and are active [11], and hence may have
tremendous potential in soil nutrient retention and conservation during community redevelopment in disturbed forests
[8]. The higher microbial C/N ratio in the lower soil layer
than in the upper layer was mainly due to the lower microbial
biomass N in the former, as the organic matter and fine root
accumulation was low in the subsoil layer [26]. Further,
greater microbial C/N ratio also suggests that the full restoration of N level after forest felling and/or cropping to the
120
A.R. Barbhuiya et al. / European Journal of Soil Biology 40 (2004) 113–121
pre-clearing level may take more time for want of enough
detrital material and required types of microbial populations
[8]. The microbial C/P ratios at the three sites were, however,
well within the range (10.6–35.9) reported by Brookes et al.
[13].
In the present study, the microbial biomass contributed
more C to soil organic C (4–6%) than several other tropical
forests (1.5–5.3%, Theng et al. [38]; Luizao et al. [25]). In
northeast India, Maithani et al. [26] have reported 0.7–1.7%
contribution by microbial biomass C to soil organic C in selectively felled subtropical humid forests. Recently, Arunachalam and Pandey [8] reported 2–4.3% in shifting agricultural
fields in the humid tropics of Arunachal Pradesh, India. This
suggests that soil microbial biomass could be a better indicator of ecosystem recovery than other ecological processes such
as litter or root dynamics [8], because it is the most labile and
active fraction of soil organic matter that determines soil
health [22,30]. However, contribution of microbial N to total
soil N was much lower (1.3–1.7%) compared to a range of
forest soils (3.4–5.9%) [28] and forest regrowths (7.3–8.3%)
[26]. This indicates low N immobilization in microbial biomass, thereby increasing the possibility of its loss from the
ecosystem [27]. It could be that soil N cycling and/or conservation is mainly through plant-driven processes such as litter
and fine roots, whereas soil C turnover is microbe driven.
The low percentage contribution of microbial biomass P to
total soil-P (1.06–1.71%) as compared to the values reported
by Brookes et al. [13] in deciduous woodland (4.7%), grassland (2–4.3%), arable land (1.4–3.5%) and Arunachalam et
al. [3] in humid subtropical forest (1.4–4.7%), could perhaps
be due to low availability of P in the soil for microbial immobilization [37]. Nevertheless, significant positive correlations between microbial biomass C, N and P indicate that the
dynamics of these three elements are closely interlinked in
the nutrient-poor tropical soils [7].
In conclusion, felling has altered the vegetation and
physico-chemical characteristics of the soil that confounded
the microbial population and biomass. Microbial biomass C,
N and P decreased from low to high disturbance regime, and
also from surface to subsurface soil layers. There were significant positive correlations between detrital mass and microbial biomass across the disturbance gradient. The biomass
values were generally low during rainy season when vegetative growth was at its peak and high during post-rainy periods due to enhanced microbial immobilization, particularly
of N and P. Overall, stem harvesting through selective logging and soil degradation by clear-felling have exposed the
soil to direct insolation and reduced litter and fine root input,
thereby reducing the soil fertility level. Further, the microbial C, N and P declined with decrease in WHC and concentration of organic C, total N and P in the soil. This indicates
the dynamic nature of C, N and P circulation on the forestfloor and the microbial populations and biomass are important for nutrient conservation, regeneration and management
of the remnant disturbed tropical forests in the high rainfall
areas of northeastern India.
Acknowledgements
Financial support by the Ministry of Environment and Forests, Government of India is thankfully acknowledged. We
thank PCCF, Arunachal Pradesh for permitting us to work in
the Deomali Reserve Forest. Thanks are due to the two anonymous referees whose comments helped in improving the quality of the paper.
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