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The impact of disturbance on detrital dynamics and soil microbial biomass of a Pinus kesiya forest in north-east India

The impact of disturbance on detrital dynamics and soil microbial biomass of a Pinus kesiya forest in north-east India

Pores~~~ology Management ELSEVIER Forest Ecology and Management 88 (19%) 273-282 The impact of disturbance on detrital dynamics and soil microbial b...

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Pores~~~ology Management ELSEVIER

Forest Ecology and Management 88 (19%) 273-282

The impact of disturbance on detrital dynamics and soil microbial biomass of a Pinus kesiyyaforest in north-east India A. Arunachalam *, Kusum Maithani, H.N. Pandey, R.S. Tripathi Deparhnent

of Botany, North-Eastern

Hill University,

Shillong

793 022, India

Accepted 28 March 19%

Abstract Detrital dynamics and microbial nutrient flux due to disturbances such as treefall and tree cutting were studied in a subtropical Pinus kesiya Ro$e Ex. Gordon forest in north-east India. Disturbance has substantially altered community structure, and therefore soil nutrient status. Natural gap formation has not resulted in significant changes in dry matter, C and N accumulation in litter and fine roots, or in microbial nutrient concentrations. However, there was a significant reduction in all functional parameters in the selectively logged site and soil heap. Soil microbial C, N and P were maximum in the understorey and minimum in the heap. Fine roots and microbial biomass contributed more to nutrient recycling in the ecosystem. N-mineralization was generally higher in the disturbed sites. Keyworrls: Disturbance; Fine roots; Litter; Microbial biomass; N-mineralization; Pinus kesiya

1. Introduction

The structure and function of plant communities in terrestrial ecosystems are largely determined by disturbances (Armesto and Pickett, 1985). Studies on forest clearing suggest that net loss of soil organic matter occurs with additional disturbance to soil or with long-term removal of forest canopy. Natural gap formation in the forest canopy due to single or multiple treefalls also perturbs the productivity and Corresponding author. Present address: Lecturer in Forestry, Department of Applied Sciences, North Bastern Regional Institute of Science and Technology. Nirjuli-791109, Arunachal Pradesh, INDIA. Tel: (0360) 47434. Fax: (0360) 44302, 44307. E-mail: forest @ nerist.emet.in. l

0378- 1127/%/$15.00 P/I SO378-1127(96)03801-7

nutrient cycling patterns of a given forest (Chandrashekara and Ramakrishnan, 1994). Recovery of the disrupted nutrient cycling in a degraded ecosystem is closely linked with vegetation regrowth. This enhances input of organic matter and nutrients to top soil through litter and detrital root mass, helps nutrient conservation by reducing losses, and increases nutrient availability by favourably altering the hydrology and physico-chemical and biological properties of the soil. Singh et al. (1989) reported that soil microorganisms also play an important role in nutrient conservation in terrestrial ecosystems. In north-east India, tree cutting and shifting agriculture have resulted in the conversion of primary broadleaved forest into several seral communities.

Copyright 0 1996 Elsevier Science B.V. All rights reserved.

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Ecolo,qy and Manugement

Pinus kesiyu, a rare species of the native forest,

grows well in degraded sites, resulting in the formation of monospecific secondary forests, especially at higher altitudes (800-2000 m above sea level (a.s.1.)). Pine is utilized for timber, and urbanization in the region has also contributed to forest clearing. The objectives of this study were to assess the impact of natural gap formation and tree cutting on the detrital and microbial nutrient dynamics of a Pinus kesiyu forest.

2. Study area

The study was conducted in a Pinus kesiyu Royle Ex. Gordon forest at Shillong (latitude 91”56’E, longitude 25”34’N, altitude 1500 m a.s.l.1, the capital of Meghalaya, India. The climate is monsoonal with an average annual rainfall of 2500 mm, 85% of which occurs during mid-May to September (rainy season). Autumn is in October and November, a transitional period between rainy and winter seasons. The winter season (December to February) is characterised by low temperatures (mean minimum 3°C; mean maximum 16°C) and occasional frost. March to mid-May (spring season) is warmer, with average maximum and minimum temperatures of 23°C and 16”C, respectively. The soil is lateritic (oxisol), sandy loam and slightly acidic (pH 5.5-6.3). The forest is 22 years old and covers approximately 50 ha. The only tree species in the forest is Pinus kesiya. The biomass of P. kesiya is 331.44 Mg ha- ’ , calculated from allometric relationships obtained by Das and Ramakrishnan (1987). There are neither shrubs nor tree saplings in the forest. The dense ground layer is dominated by Eupatorium adenophorum, Luntana camera and grasses such as Imperata cylindrica and Arundinella benghalensis. A large number of pine seedlings were also found. There is a heavy growth of epiphytic lichens, mosses and ferns. An experimental area of about 10 ha was surveyed to locate treefall gaps in the forest. A gap was considered as an “opening in the forest extending down through all foliage levels to an average height of 2 m above ground” (Brokaw, 1982). Three gaps

88 (19961273-282

(average area 263.3 m2) originating from multiple treefails (not less than three) were identified, and the investigation was carried out in those gaps only. In the gaps, seedlings of Pinus kesiya, and grasses such as lmperata cylindrica and Arundinellu benghalensis dominated the forest floor. However, species such as Rubus ellipticus, Osbeckia stellata and Lantana camera were also present. Nearby a portion of the forest was selectively logged for building construction during September 1993. As a consequence, patches of uncut pine trees (biomass 160.3 Mg ha-’ 1 were interspersed in the area. A few cut-stumps were left behind in between the uncut trees The ground vegetation was dominated by Imperata cylindrica, Eupatorium adenophorum, Luntana camera and ferns. A dense growth of tree seedlings was observed. Three such patches (average area 289 m*) were also selected for the present study. The logging also resulted in whole tree harvesting and total removal of top soil. This soil was dumped aside in heaps, which occupied about 5-10% of the total forest area. These heaps are now mostly dominated by pine seedlings. A few young dicotyledonous species such as Eupatorium adenophurum, Lantana camera etc. were also present. Three plots (25 m X 25 ml of understorey with a dense canopy were presumed to be undisturbed (UD), three gaps formed by treefalls were presumed to be slightly disturbed (SD), three sites disturbed by selective logging were presumed to be moderately disturbed (MD), and three soil heaps, representing extreme stages of soil degradation, were presumed to be highly disturbed (HD) sites. These gradients were based on the intensity, size and duration of the disturbances.

3. Methods 3.1. Vegetation analysis

In each replicated site, the vegetation was analyzed during October 1994 in 5-10 randomly placed 10 m X 10 m quadrats for trees and in 1 m X 1 m quadrats for herbaceous vegetation. Nomenclature of plant species follows Hooker (1872- 1897). Density,

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frequency and basal area were determined according to Misra (1968). 3.2. Forest floor litter, root and soil sampling and analysis

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cedures and the buried bag technique, as outlined by Anderson and Ingram (1993). The data presented are all the means of the three replicated plots in each site. 3.3. Statistical analysis

Litter samples were collected from each plot during October 1994, from the 1 m X 1 m quadrats used for the ground vegetation analysis. Soil cores (diameter 6.5 cm) were sampled from all these quadrats at O-10 cm depth for the determination of physicochemical properties, fine-root biomass and microbial nutrients. Polythene bags containing soil cores from 0- 10 cm depth were also buried in these quadrats for in situ N-mineralization. The litter, root and soil samples were brought to the laboratory in polythene bags and stored at 4°C. The soil samples from each site (i.e. three plots) were bulked, air-dried and used for the determination of texture, pH, water-holding capacity (WHC), organic-C, total Kjeldahl nitrogen (TKN) and available-P following standard procedures (Allen et al., 1974). The litter samples were grouped into leaf (pine needles and leaves of monocots and dicots) and miscellaneous (woody litter of < 20 mm diameter, bark and reproductive organs) fractions, oven-dried at 80°C and weighed. The roots were retrieved from the soil cores following wet-sieving (Bohm, 1979) and roots of < 2 mm diameter (fine roots) only were considered. The fine roots were classified into < 1 mm (finer roots) and 1-2 mm diameter classes. Live roots (biomass) were distinguished from dead roots (necromass) by visual and textural characteristics (Persson, 1983; Arunachalam et al., 1996). The fine roots were washed with a gentle flow of tap water to overcome soil contamination, oven-dried at 80°C and weighed. Ash content was determined by igniting the oven-dried material at 550°C for 6 h in a muffle furnace. Carbon (C) content was calculated as 50% of the ash-free mass (Allen et al., 1974). Total Kjeldahl nitrogen (TKN) was estimated in the litter and root samples using a Kjeltec Auto 1030 Analyser. The carbon and nitrogen contents of litter and fine roots were calculated by multiplying the dry mass by their respective concentrations. Microbial biomass-C, -N and -P and in situ N-mineralization were estimated following fumigation-extraction pro-

The data were analyzed using ANOVA (fixed effects model). Linear regressions were also used where necessary, according to Zar (1974). Tukey’s test was used to compare the mean values across the sites.

4. Results 4.1. Microenvironment

and soil nutrient status

Light intensity was significantly higher in disturbed plots than in the understorey. Air and soil temperatures were significantly lower in the understorey (UD) and gaps (SD) than in the selectively logged plots (MD) and soil heaps (I-ID), where they were almost the same (Table 1). Bulk density, water-holding capacity (WHC) and moisture content (SMC) of the soil showed a decreasing trend, and pH showed an increasing trend with increasing degrees of disturbance. The concentrations of organic-C and TKN in soil were highest in the understorey and lowest in the soil heap (Table 2). The C/N ratio in soil was highest (11.6) for the understorey and lowest (6.3) for the selectively logged plot. There was a significant decline in the level of soil ammonium-N in the MD and I-ID plots,

Table 1 Microenvironmental

variability

( f SE) in the study sites (n = 9)

Variables

UD

SD

MD

HD

Light intensity (lux x 100) Air temperature Soil temperature Soil moisture content (%o)

30af2

236bf10

242bf3

250bk5

(“C) PC)

23 a f 1 19 a f 1 2ga+3

25’+2 21ak2 27=f2

29b+l 26bjz2 lgb+l

29bkl 26b*2 16b*l

UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap. Within each row, means with the same superscripts are not significantly different at P < 0.05.

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Table 2 Physico-chemical properties (k SE) of soil samples from the undmtorey aud disturbed plots of a Pinus kesiya forest (n = 9) Properties UD SD MD HD SL SL SL SL Soil textural class Bulk density (g cm3) 1.12 a 1.01 a 0.99 a 0.92 a i 0.09 f 0.02 kO.01 + 0.02 WHC (o/o) 55.12 * 55.01 a 35.19 b 30.62 b i 1.12 5.5 a kO.2 2.78 a rtO.09 0.24 a

k2.31 5.8 a *0.1 1.98 a kO.23 0.20 =

Nitrate-N (p,g g- ’ )

+0.65 6.45 a +0.07

+ 1.32 6.65 =

Available-P (p,g g- ’ )

5.95 a

PH Organic-C (%)

+ 1.11

5.9a *0.1 0.74 = 50.1 I 0.12 a

+ 4.39 6.3 a kO.0 1.04a rto.01

0.10 a *0.01 i:O.Ol + 0.01 kO.01 C/N 11.58 a 10.10 a 6.27 a 10.83 it Ammonimum-N (kg g- ’ ) 16.55 a 16.44 a 6.64 b 7.05 b TKN (%)

kO.13

kO.01 6.12 a *0.15

& 1.17 5.0 a I!z0.1 5.37 a f 0.02

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Table 3 Density of trees (ha- ’ ) and ground vegetation &h#sX lou0 ha-’ ) in the understorey CUD), gaps (SD), selectively logged plots (MD) and soil heaps (HD) in a Pinus kesiya forest ecosystem Vegetation component UD SD MD HD Pinus kesiya

Monocots Dicots

2233 a (77.3) 22.4 a (0.44) 22.6 = (0.4)

12.0 b (0.2) 6.7 b (0.1)

1080”

-

(37.4) 8.9 b

--

(0.2) 4.1 ’ (0.1)

3.2 ’ (0.1)

Values in parentheses are basal area (m* ha- ‘). Mean values with different superscripts within a row are significantly different at P < 0.05 between sites. -. no vegetation recorded.

f 0.99 6.05 a

zko.01 4.76 a + 0.03

UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap; SL, sandy loam. Mean values with the same superscripts across sites are not significantly different at P < 0.05.

as compared with the UD and SD plots. Concentrations of nitrate-N and available-P were maximum in the gaps, but differences between the understorey and the gaps were not significant. 4.2. Community characteristics

and in the gaps (101320 ha-’ 1. In the understorey and soil heaps it was 56000 and 64000 ha-‘, respectively. 4.3. Biomass, C and N contents of forest-floor litrer

The mass of forest-floor litter was reduced significantly by disturbances (Fig. 1). Litter mass was maximum (1.5 Mg ha-’ 1 in the understorey and minimum in the soil heaps (0.02 Mg ha-’ >. Pine needles contributed 32-758 of the total litter mass in all sites. Both monocotyledonous and dicotyledonous species contributed significantly (5-25%) to

The total basal area of P. kesiya (the only tree species) was 77 m2 ha - ’ in the undisturbed forest and 37 m* ha-’ in the selectively logged plot. The total tree density was maximum in the understorey and minimum in the selectively logged plot; no trees were observed in the gaps and soil heaps (Table 3). Monocotyledons such as Imperata cylindrica and Arundinella benghulensiswere dominant in the gaps and selectively logged plots. However, in the understorey, dicotyledons such as Eupatorium adenophorum, Lantana camera, Rubus ellipticus, Ambrosia urtimissifolia and Osbeckia stellata contributed

equally with monocotyledons to the total density of herbaceous vegetation. In soil heaps, only sparsely distributed shade-intolerant dicotyledonous species were present. The density of pine seedlings was highest in the selectively logged plots (113 530 ha- ’ )

Fig. 1. Forest floor litter mass in understorey KJDJ, gaps (SD), cut-tree stands (MD) and heaps (HD). Open bIocks, pine needles; diagonal shading, dicotykdon leaves; solid blocks, monocotyledon leaves; vertical shadhtg, miscellaneous fraction. vertie&baFs represent the standard error (n = 30).

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Table 4 Concentration (%) of nitrogen in litter samples from storey and three disturbed plots in the pine forest Litter fraction Leaf litter Pine needles

UD

SD

MD

1.23 a (14.19 ")

0.97 a (4.77 b,

0.91 a (6.91b)

0.37 a (Oxsa)

0.84' (0.34')

Monocots Dicots

Miscellaneous

;iY3aa, 1.29 a

0.79

(0.41a)

(0.12

0.70 a (2.00 ")

(5.28

0.75

a

0.93

88 (19%)

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277

the underHD

a

1.20

a

(0.14 -

')

0.91

(0.11 “)

(0.04 “)

0.90

-

b,

(2.66

a

(l-2mm)

a

“1 a

and Management

UD

MD

HD

Fig. 2. Fine-root mass ( < 1 mm or l-2 mm in diameter) in the understorey (UD), gaps (SD), cut-tree stands (MD) and heaps (HD). Open blocks, biomass; diagonal shading, necromass. Vertical bars represent the standard error (n = 30).

"'1

UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap. Values in parentheses are the nitrogen content (kg ha- ’ ). Within a row, values with different superscripts are significant P < 0.05. -, no litter recorded.

SD

at

total litter accumulation. The miscellaneous fraction of the litter was 46% of the total litter mass in the gap, 27% in the selectively logged plot and 19% in the understorey. The carbon content of the forest floor litter was highest in the understorey and lowest in the soil heaps (see Table 7). The contribution of the litter to the total stock of C in the soil was two to three times greater in the selectively logged plot compared with the understorey and the gaps. N concentration was higher in the leaf litter components of the understorey than those of the other three disturbed plots (Table 4). N stock was maximum in the understorey (17.33 kg ha-‘) and minimum in the soil heaps (0.18 kg ha-’ ). N content in the leaf litter was higher than that of the miscellaneous fraction in all the study sites (Table 4). However, the contribution of litter to total N capital in the soil was insignificant.

to 0.7. The total fine-root mass (5.12-0.71 Mg ha-’ ) also followed the trend of the BM:NM ratio. Finer roots contributed 43-100% of the total fine root mass, while the roots in the l-2 mm diameter class contributed up to 57% only. The fine roots accumulated 21.8% of the total soil C stock in the selectively logged plots, followed by gaps (10.2%), understorey (7.3%) and soil heaps (3.5%). N concentration in the fine roots from different sites did not show any definite trend (Table 5), but was generally greater (0.6-l%) in the necromass than the biomass (0.4-0.8%).

Table 5 Nitrogen concentration (%) in fme roots in the understorey and three disturbed plots (SD, MD, HD) in the pine forest Fine root fraction.

UD

< 1 mm diameter Biomass 0.82

MD

a ")

0.62 (3.6

a b,

1.06 a

0.62 (9.9

a b)

0.35 (5.7

a b)

(8.5

Necromass

SD

(12.1')

0.75

CUD)

HD

0.77

a

(8.9 '7 0.77 a (10.5 "'1

a

(2.3 0.83 (3.4

bd) a d)

0.63

a

-

(1.9 '1 1.04 a (5.6 "1

-

4.4. Biomass, C and N contents of$ne roots Fine-root biomass was maximum in the understorey (3.4 Mg ha-‘) and minimum in the soil heaps (0.3 Mg ha-’ ). Generally, necromass was greater in the disturbed plots than the forest understorey (Fig. 2). An average of 86% of the necromass was in finer roots (< 1 mm diameter). The biomass (BM) to necromass (NM) ratio of fine roots showed a decreasing trend (i.e. UD > SD > MD > HD) from 1.9

l-2 mm diameter Biomass 0.39 Necromass

a (9.0 ") 0.64a (3.9

"1

0.78 a (5.6 "1

UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap. Values in parentheses am nitrogen content (kg ha- ’ 1. Values in the same row with different superscripts ate not signiticant at P < 0.05. -, no fme roots recorded.

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Table 6 Microbial C, N and P concentrations (p,g g- ’ ) ( jI SE) in soils at the study sites (n = 9) Parameter UD SD MD HD MB-C MB-N MB-P

294.4 a + 9.3 118.3 a rto.9

19.6 a +0.2

MB-C/MB-N MB-C/MB-P

2.5 a 15.0 a

287.9 a *3.1 104.6 b f0.3 8.2 b dco.3

2.8 ’ 35.1 b

126.3 b + 3.9 72.3 ’ *0.1 5.5b t- 0.9 1.8 b 23.1 ’

55.7 c +4.1 35.6 d 50.1 4.9b + 0.7 1.6 b 11.4 ad

UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap. Mean values in the same row with the same superscripts are not significant at P < 0.05.

4.5. Microbial C, N and P

Concentrations of microbial C (B,), N (BN 1 and P (B,) in soil were maximum in the understorey and minimum in the soil heaps (Table 6). The difference between B,, B, and UD, SD was not significant, while the differences between UD and MD, and UD and HD were significant (P < 0.05). C/N ratio in Table 7 C and N stocks (kg ha-‘) in soil, litter, fine roots and microbial biomass in a disturbed Pinus ksiva forest Nutrient/category UD SD MD HD Carbon Soil Litter Roots

31136a 732 a (2.4) 2268 a (7.3) 330 a (1.1)

19998 b 695 b (3.5) 2039 b (10.2) 291b (1.5)

7326 ’ 516’ (7.1) 1595 c (21.8) 125’ (1.7)

9568 d 7d

(0.001) 338 d (3.5) d &05,

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88 (1996)

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the microbial biomass of the understorey and gaps was significantly higher than that in the selectively logged plots and soil heaps. C/P ratio did not show a similar trend: the highest ratio was obtained in the gaps and the lowest in the soil heaps. B, represented 1.1-I .7% of the total soil organic-C stock in the understorey, gaps and selectively logged plots. The contribution of B, in the soil heaps was negligible. The contribution of B, to total soil N content was more or less the same in the understorey and gaps, the highest contribution was in the selectively logged plots and the lowest in the soil heaps (Table 7). 4.6. In situ N-mineralization

Ammonium-N and nitrate-N in the soil increased significantly (P < 0.01) after 30 days of field incubation. Ammonification rate ranged from 0.2 to 0.4 pg g-’ day-’ and nitrification rate ranged from 0.1 to 0.2 p,g g- ’ day- ‘. The net N-mineralization varied significantly between the understorey and the other three disturbed sites (Table 8). pH did not show any significant variation, but the soil moisture content declined significantly (P < 0.01) after 30 days of incubation. However, the reduction was much more pronounced in soils from the disturbed sites compared with the forest understorey.

Table 8 pH, SMC (%), concentrations (pg g-‘) of ammonium-N and nitrate-N after 30 days of field incubation and N-mineraliiation rate (pg g-’ day- ‘) in soils from the disturbed sites and understorey in the pine forest Parameter UD SD MD HD PH 5.8 a 6.0 a 6.0 a 6.5 a k0.7

Nitrogen Soil Litter

2688 a zo606)

1188 c

b

b

920 d c

&06~

td.m,

GoO2~ b

b

Roots Microbes

2020 b

z, 132 = (4.9)

(:5.2, 106b (5.2)

Fi.3,

0.1 a

0.1 a

19.9 a f 1.2

Ammonium-N

22.4 a * 1.3

Nitrate-N

10.6 a

Net N-mineralization rate Ammonification rate Nitrification rate

0.3 =

(41Ioo6,

UD, tmderstorey; SD, gap; MD, cut-tree stand; HD, soil heap. Values in parentheses are percentages of total soil C and N stocks. Different superscripts in the same row indicate that means differ at P < 0.05.

0.2 a

f 0.0 17.9 a f 0.9 28.1 b * 2.2 9.8 a +0.2 0.5 a 0.4 =

SMC

It 0.6

to.2 10.0 b

5 1.0 18.7 ’ k2.1 9.6 a i:O.2 0.6 if 0.4 a 0.2 a

f0.1 8.8 b i0.3 16.2 ’ i3.4 12.0 = + 1.3 0.5 a 0.3 = 0.2 *

UD, understorey; SD, gap; MD, cut-tree stand; HD, soil heap. Values are mean * standard error (n = 9). In each row, differentsuperscripts indicate that the means differ at P < 0.05.

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5.2. Litter dynamics

5. Discussion 5.1. Microenvironmental

and Management

variability,

soil and vegeta-

tion

There were a number of changes in the physicochemical characteristics of the soil subsequent to different intensities of disturbance, caused by microenvironmental changes in the sites. The reduction in the soil moisture content in gaps, selectively logged plots and soil heaps as compared with the forest understorey (see Table I> could partly be due to increased light intensity and soil temperature, and in part to a decline in the bulk density and waterholding capacity of the soil. Soil organic-C and nutrient concentrations (see Table 2) are less than those reported for natural humid subtropical broadleaved forests of the region (Arunachalam et al., 1994; Maithani et al., 1996), and this is attributable to the species composition. The soil fertility of the pine forest did not decline significantly due to natural gap formation, but selective logging (stem harvesting) and soil degradation (whole tree harvesting) have significantly altered the soil nutrient status. This could be due to excessive erosion/leaching of top soil as the area receives high rainfall. Similar findings have also been reported by Chandrashekara and Ramakrishnan (1994) in a humid tropical forest in Kerala, India. Community characteristics such as density and basal area showed marked differences among the four sites (see Table 3) due to discernable variation in soil conditions and biotic influences. The forest understorey, with its optimum soil nutrient status and microenvironment, has favoured the growth and regular distribution of both dicots and monocots. Generally, the disturbed sites were dominated by the monocots, as only they could withstand the prevailing high light intensity and soil temperature. Tree seedling recruitment was very pronounced in the gaps and selectively logged plots, and very low in the understorey. This is attributable to the light availability and soil moisture regimes, as influenced by canopy opening. Competition for various resources among seedlings and herbaceous species, especially for light and space in the relatively spacelimited understorey, might have hampered successful recruitment of seedlings (Whitmore, 1984) and therefore seedling density.

The standing crop of litter in the undisturbed pine forest (1.5 Mg ha- ’ ) was well below the reported range (2.2-22.6 Mg ha-’ ) for various tropical and subtropical forests (Vogt et al., 19861, possibly because it was sampled just after the rainy season when the decomposition rate is at its peak (Maithani et al., 1996). Meentemeyer et al. (1982) reported that leaf litter accounted 70% of the total litter mass on the forest floor. In the present study, the leaf litter averaged ca. 77% of the total litter mass. In the soil heaps, leaf litter was the only fraction, of which 25% was contributed by the incited dicotyledonous species and tree seedlings, and the rest was from the nearby pine forest as an external input into the system. Relatively higher miscellaneous litter (46%) in the gaps (Fig. 1) could be due to the accumulation of branchfall or treefall residues. Leaf litter had higher N concentration (see Table 4) than the miscellaneous litter. This is in agreement with the observations of Gosz et al. (1972) and Arunachalam et al. (1994) that perennial tissues have lower concentrations of nutrients, especially N. N content in litter generally reflected the trends in the litter mass. The least accumulation of N in soil heaps through litter was due to a very low plant biomass. 5.3. Fine root dynamics

The mean standing crop of fine roots in the understorey is very close to the value reported from a 7-year-old subtropical forest regrowth dominated by P. kesiya (Arunachalam et al., 19961, but more than those reported from Picea sitchensis (3.5 Mg ha- ’ ) and Pinus sylvestris (1.8-4.6 Mg ha- ’ ) forests in temperate regions (Deans, 1981; Persson, 1983). Despite these variations, the values obtained from understorey, gaps and selectively logged plots of the pine forest (3.4-5.1 Mg ha-’ ) were within the reported range (1.0-17.7 Mg ha-’ ) of fine root mass for various forest ecosystems of the world (Vogt et al., 1986). The least standing crop of 0.7 Mg ha-’ obtained from the soil heaps was close to that of a l-year-old stand of a premontane wet forest (0.9 Mg ha-‘) in Costa Rica (Berish, 1982). It has been reported that low availability of water and nutrients promote high production and accumulation af fine

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roots (Vogt et al., 1986). However, our results do not agree with these findings, since root mass decreased as the WHC, organic-C and TKN decreased in soil, along a disturbance gradient. We have also reported similar findings from a disturbed humid subtropical broadleaved forest of this region (Arunachalam et al., 1996). Nambiar (1987) reported that nutrients are not retranslocated from fine roots before senescence. We agree with this report because the N concentration in biomass and necromass of the finer roots (< 1 mm diameter class) did not vary significantly. However, this was not the case with roots of l-2 mm diameter, where the variation was significant (P < 0.05). The reduction in N concentration with the increase in root diameter simply reflects the increase of woody tissues which usually contains fewer nutrients (Khiewtam and Ramakrishnan, 1993). Decline in elemental concentrations in fine root biomass could also be related to soil nutrient status. These results are in contrast with those of Coults and Philipson (1976), who found that nutrients could be internally translocated in roots, resulting in similar concentrations throughout the root system. 5.4. Microbial

nutrient dynamics

Methodologies to quantify Bc in soil are still controversial. Our results for Bc (56-294 P,g g- ’ ), obtained following fumigation-extraction procedures, were lower than the values reported by Arunachalam et al. (1994) for soils of a disturbed climax forest (1040-1532 pg g-l), but well within the lower range of the reported values for various terrestrial ecosystems (61-1900 pg g- ‘) (Vance et al., 1987; Srivastava and Singh, 1988). There was a significant positive relationship between soil organic matter and Bc (r = 0.903, P < 0.05). A relatively higher stand density and soil organic matter, and greater accumulation of litter and fine roots could have favoured the growth of microbial populations, and therefore Bc in the understorey. The dynamics of N in the mineral soil is intimately linked with that of C, because most of the N exists in organic compounds and heterotrophic microbes, which utilize organic-c for energy. As a result, B, showed a positive correlation with Bc (r = 0.958, P < 0.01). Joergensen et al. (1995) re-

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ported that above soil pH 5.0.. microbial C/N ratios vary within a narrow range. Our results fully corroborate this point. The microbial C/N ratios (see Table 6) were smaller compared with those in most of the literature. This could be related to low soil organic-C. Joergensen et al. (1995) also suggested that forest soils with comparatively low C and N availability may give microbial C/N ratios which are well below the optimum values. i.e. 5-8. However, soilspecific differences in growth form and survival strategy are also important in regulating the C/N ratio in microbial biomass. According to the hypothesis of Bremer and van Kessel (1992), the microbial biomass of our study sites was dormant because of a very small microbial C/N ratio. The values obtained for B, (4.9-19.6 Pg gee! i were very low as compared with other reported values for grasslands and woodlands (12.0-67.2 p-g g- ’ ) (Brookes et al., 1984) but were comparable to those of arable lands (5.3-27.5 pg gg ‘). In the soils studied, the contribution of B, to available-P content in soil varied between 1.4 and 4.7%. Brookes et al. (1984) measured B, and B, contents in soils of grasslands, pastures and woodlands and found them to be positively correlated. Our results in the pine forest soils are also in accordance with that study. The microbial C/P ratios (see Table 6) were well within those reported by Brookes et al. (1984). The significant increase in microbial C/P ratios in the gaps and tree-cut plots is due to a relatively sharper decline in microbial P. 5.5. N-mineralization

pattern

The increase in the concentrations of ammoniumN and nitrate-N in the field-incubated soils reflects the ammonification and nitrification processes. Ammonification rate was higher than nit&?&on rate in all the four sites studied. Similar findings have also been reported for tropical forests by several other workers (Schimel and Patton, 1986; Singh et al., 1991). The net N-mineralization rates (see Table 8) of the subtropical pine forest soils were within the range reported by Singh et al. (1991) for soils from tropical India (0.007-0.767 p,g g- ’ day- ’ ). The low N-mineralization rate in soils of the understorey could be due to rapid immobilization of the mineralized nitrogen by the microbes as well as plant roots.

A. Arunachalam

et al./Forest

Ecology

This could be a N-conserving mechanism by which the nutrient loss is checked. On the other hand, rapid mineralization of N in the disturbed plots signals potential loss of available N from the system, owing to a little dense vegetation. This might have resulted in a lower concentration of ammonium-N in soils of the tree-cut stand and soil heaps. 5.6. Relative importance of litter, fine roots and microbes in C and N dynamics

Fine roots play more important role in soil C dynamics than the litter and microbial biomass. On the other hand, microbial biomass contributes much to the soil N pool compared with the two detrital fractions (see Table 7). The insignificant role of these three biological processes in the soil heap compared with the understorey indicates the extremity of soil degradation, and the site may not be easily restored in a short time. An increase in the contribution of litter, fine roots and microbial biomass to C and N pools in soils of the slightly and mildly disturbed plots is a measure of nutrient retention on the forest floor, in spite of reduced soil organic C and total N stocks, and would thus sustain the future uptake and productivity of the regrowing forest community.

6. Conclusions

The present study concludes that the community structure, dynamics of litter, fine roots and microbial biomass, and N-mineralization have been substantially altered due to perturbations. Natural gap formation did not alter the soil nutrient status, microbial nutrients and detrital stock significantly when compared with the understorey. Stem harvesting through selective logging and soil degradation by whole-tree harvesting have exposed the soil to direct insolation and excessive leaching; thereby the soil fertility level declined. Fine-root mass and microbial C, N and P declined with the decrease in water-holding capacity, concentrations of organic-C, total nitrogen and available-P in the soil. It was also concluded that fine roots play a crucial role in C-cycling, and microbial biomass in N-cycling, of the disturbed pine forest. This indicates the dynamic nature of C and N circu-

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lation on the forest floor, as these two biological processes have a rapid turnover, and thus the fine roots and microbes are important for nutrient conservation and maintenance of the disturbed secondary forest. It is therefore suggested that the dynamics of fine roots on the forest floor in relation to microbial biomass and nutrient mineralization should receive more attention from ecologists and forest managers. Based on the ecology of secondary forest succession and nutrient cycling processes, the natural and human-impacted Pinus kesiya forest ecosystem can be sustained for at least a fairly high level of productivity by adopting appropriate forest management practices and also through the planting, in gaps and selectively logged plots, of selected tree species which would conserve nutrients and have low nutrient-use efficiency. Although difficult in practice, microbial management can also help in restoring the soil fertility, especially N-status, through synchronization of nutrient mineralization and its uptake by plants, resulting in fairly high-level productivity of the Pinus kesiya forest.

Acknowledgements

Special recognition is given to Dr. A.K. Das and Professor P.S. Ramakrishnan, TSBF, SARNET, for providing us with the TSBF Methodology Book. The authors thank CSIR, Government of India, for financial assistance. The first author is grateful to Professor K.B. Misra, Director, NERIST, for providing the necessary facilities. The authors thank the two anonymous referees for their useful comments for improvement of the paper.

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