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Leaf litter decomposition and nutrient mineralization patterns in regrowing stands of a humid subtropical forest after tree cutting
Forest Ecology and Management 109 (1998) 151Ð161
Leaf litter decomposition and nutrient mineralization patterns in regrowing stands of a humid subtropical forest after tree cutting A. Arunachalama,*, Kusum Maithania, H.N. Pandeyb, R.S. Tripathib a
Restoration Ecology Laboratory, Department of Forestry, North Eastern Regional Institute of Science and Technology, Nirjuli-791109, India b Plant Ecology Laboratory, Department of Botany, North-Eastern Hill University, Shillong-793022, India Accepted 16 December 1997
Abstract Decomposition dynamics, and N and P mineralization patterns of leaf litter of Pinus kesiya, Quercus dealbata, Q. grif®thii, Rhododendron arboreum and Schima khasiana were studied in forest of three different ages in a humid subtropical region of India. The decay pattern varied from species to species. The decay pattern, characterized using a composite linear regression equations, exhibited two to three distinct phases during leaf litter decomposition. Initial lignin, nitrogen (N) and lignin/N showed signi®cant negative correlations with decay rate, whereas soil properties like pH, moisture and total Kjeldahl nitrogen (TKN) and climatic variables, e.g. rainfall and air temperature, showed positive correlations. The annual dry matter decay constants (k) varied from 0.77 in R. arboreum to 1.39 in Q. grif®thii. Nutrient release from the decomposing litter was in¯uenced by the seasonal cycle of mineralization and immobilization processes. Net mineralization was rapid during rainy season, as nitrogen (N) and phosphorus (P) concentrations in the decomposing leaf litter decreased by ca. 20±50% from the preceding season, while immobilization occurred during winter when nutrient concentration increased up to 60%. Annual dry matter decay, net N and P mineralization constants for Q. dealbata were higher in the 16-year old regrowth than in the 13-year old regrowth. # 1998 Elsevier Science B.V. Keywords: Litter decay; Nitrogen; Phosphorus; Humid subtropics
1. Introduction Litter decomposition plays a crucial role in the nutrient budget of the tropical forest ecosystems where vegetation depends mainly on the recycling of nutrients contained in the plant detritus (Vogt et al., 1986). The decomposition rate is strongly in¯uenced by climatic conditions and initial chemical composition of the litter (Couteaux et al., 1995). *Corresponding author. 0378-1127/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0378-1127(98)00240-0
Among the climatic variables, rainfall and air temperature determine the rate of decomposition in areas subjected to unfavourable weather conditions. Nitrogen (N) and lignin have been reported to have a better control over the litter decay rate in the humid tropics than in the temperate zones (Swift et al., 1979). A strong negative linear relationship between initial lignin/N ratio and disappearance rate of leaf litter was reported by Melillo et al. (1982). Lee (1974) has emphasized the relative importance of soil microand macrofauna in litter decomposition. Horward and
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Table 1 Dominant tree species and characteristics of the top soil (0±10 cm) layer in the three forest regrowths Age of forest regrowth 7-year 13-year 16-year
Dominant tree species P. kesiya Q. dealbata Q. dealbata
Soil characteristics SMC a (%)
pH
SOM b (%)
TKN c (%)
Available P (mg gÿ1)
30.33 6.10 43.86
5.15 4.72 4.44
6.56 8.67 10.88
0.35 0.49 0.61
7.64 11.94 11.23
a
Soil moisture content. Soil organic matter. c Total Kjeldahl nitrogen. b
Horward (1980) and Van Vuuren et al. (1993) have emphasized that tree and shrub leaf litter decomposition varies depending on species and type of soil. The role of N availability on litter decomposition in forests has been discussed by Prescott (1995). This paper presents the ®ndings of a study on the effects of initial litter chemistry and soil conditions on the litter decomposition and nutrient mineralization patterns of major tree species in three different-aged forest stands recovering after selective tree cutting in a subtropical humid forest of northeastern India.
The corresponding basal areas were 3.1, 19.9, 44.2 m2 haÿ1 in the 7-, 13- and 16-year old stands, respectively. The soil (latosol±oxisol) is derived from Precambrian igneous rocks. The mineral soil (0± 10 cm depth) was sandy loam in the 7-year old stand, sandy clay to clay loam in the 13-year old stand and clay loam in the 16-year old stand. The water holding capacity (WHC), soil organic matter (SOM), total Kjeldahl nitrogen (TKN) and available P in the soil increased gradually from the 7- to 16-year old stands (Table 1).
2. Study area
3. Methods
Disturbed forest stands of three different ages (7-, 13- and 16-year old) located near Shillong (latitude 258340 N, longitude 918560 E, altitude 1900 m a.s.l.) were selected for the present study. As the stands were selectively cut, a few older trees (DBH 10± 40 cm) were found interspersed in the stands. All stands were located on a gentle hill slope (6±138S) and covered an area of ca. 1 ha each. The area is highly monsoonal (average annual rainfall 1547 mm) with winter, spring, rainy and autumn seasons. A total of 80% of the annual rain was con®ned to the rainy season. Mean annual air temperature during the study period was 258C. Detailed account of climate, soil and vegetation of the study sites has been given in Arunachalam (1997), Arunachalam et al. (1996a, b). The 7-year old stand is dominated by young trees of Pinus kesiya with resprouting from stumps of broadleaved trees of Quercus dealbata and Schima khasiana. The density of woody vegetation was 680 stems haÿ1 in the 7-year old stand, 1260 stems haÿ1 in 13-year old stand and 1440 stems haÿ1 in the 16-year old stand.
3.1. Leaf-litter chemistry Freshly fallen needles/leaves of dominant tree species recorded in 1993 vegetation survey in the three forest stand were collected during peak litterfall period (March): P. kesiya (7-year old stand); Q. dealbata (13-year old stand); Q. grif®thii, Rhododendron arboreum and S. khasiana (16-year old stand). Leaves of Q. dealbata collected from the 13-year old stand were also used in the 16-year old stand. The litter samples were air-dried in the laboratory and sub-samples were kept at 808C for 48 h for determining the dry mass. The oven-dried materials were powdered in a Cyclotec (TECATOR) and analyzed for their chemical composition. The ash content was determined by igniting 1 g of ground litter sample at 5508C for 6 h in a muf¯e furnace. A total of 50% of the ash-free mass was calculated as the carbon (C) content. Nitrogen (N) was estimated following semimicro-Kjeldahl method in a Kjeltec Auto 1030 Analyzer (TECATOR). Total phosphorus (P) was analyzed colorimetrically while lignin
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and cellulose contents were measured gravimetrically (Anderson and Ingram, 1993).
and 99% (t99) decay were calculated as t500.693/k and t995/k.
3.2. Litter decomposition
3.4. Statistical analysis
Decomposition of leaf litter of dominant tree species was studied in their respective stands using the nylon mesh (2020 cm) bag technique (Gilbert and Bocock, 1960). The mesh size was 2 mm, small enough to prevent major losses of needles or leaves, yet large enough to permit aerobic microbial activity and free entry of small soil animals. However, it excludes larger arthropods and earthworms, which are important primary accessors of litter and, presumably, it becomes a litter system dominated by termites, bacteria, fungi and actinomycetes. Consequently, there could be an underestimation of litter decomposition rates. For the experiment, 10 g of the air-dried material was kept in each bag which was then stitched with nylon threads. For each species, 60 bags were prepared and randomly dispersed on the forest ¯oor in the respective stands in the month of May 1993. After 60, 120, 180, 240, 300, 360, 420, 480, 540 and 600 days, ®ve litter bags for each species were brought to the laboratory carefully avoiding loss of material from the litter bags. The litter was washed in a bucket full of tap water by swirling brie¯y and carefully decanting through 2-mm mesh sieve to remove extraneous matter. According to Anderson and Ingram (1993), such brief washing permits little leaching. The litter was then dried at 808C for 48 h and weighed. The samples were powdered and used for the analysis of N and P contents according to semimicro-Kjeldahl and colorimetric procedures given in Anderson and Ingram (1993).
In order to distinguish between different phases of weight-loss pattern during decomposition, multiple regressions were developed using dummy factors (0 or 1) as the indicator variables (Zar, 1974). The composite linear-regression model (Arunachalam et al., 1996b) used for this purpose was YabX1 cX2dX3. . ., where Y is the percentage of initial mass remaining, a the Y intercept, b the rate of change in Y with respect to time, c the shift parameter for adjustment of the Y intercept in phase-II and d the shift parameter for adjustment of the Y intercept in phase-III. The values of c and d were taken as zero, if decay was slow, and/or equal to one, if the decay was rapid. The effect of climatic variables, initial litter chemistry and a few soil characteristics on the decomposition rate of leaf litter was assessed using simple linearregression function, YabX. As the type and number of species used for decomposition study in the three stands were different, there were chances of pseudoreplication in the application of analysis of variance (ANOVA, ®xed-effects model). To overcome the problem, the ANOVA was used to test the signi®cance of differences in the decomposition rate due to sampling time for each species in the three stands and among the species in the 16-year old stand only. However, comparison between plots were done by regression.
3.3. Computation
4.1. Initial chemistry of leaf litter
Organic matter decay, and N and P mineralization constants for leaf litter were computed using negative exponential decay model of Olson (1963): X/X0 exp(ÿkt), where X is the weight remaining at time t, X0 the initial weight, exp the base of natural logarithm, k the decay rate coef®cient and t the time (year). N and P mineralization constants (kN and kP) were calculated by substituting dry weight with N and P contents in the foregoing formula (Singh and Shekhar, 1989). Further, the time required for 50% (t50)
Mean N concentration in the leaf litter of ®ve dominant species varied from a minimum of 0.59% in R. arboreum to a maximum of 1.03% in S. khasiana. Phosphorus concentration was relatively low, varying between 0.03±0.06% in different species. Lignin concentration was maximum (43.2%) in the coniferous species, P. kesiya. Among the broadleaved species, R. arboreum had the highest concentration of lignin (37.3%). In other species lignin concentration varied from 23.7±25.1%. Cellulose concentration was much
4. Results
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Table 2 Chemical composition of leaf litter of dominant tree species used for decomposition study by the litter-bag method (SEM (n5)) Species
C (%)
N (%)
P (%)
Lignin (%)
Cellulose (%)
C/N
Lignin/N
P. kesiya Q. dealbata Q. griffithii R. arboreum S. khasiana
47.31.2 47.60.3 46.42.1 45.63.6 47.31.0
0.980.01 0.890.01 0.730.05 0.590.06 1.030.03
0.050.004 0.060.01 0.050.001 0.030.001 0.040.001
43.21.1 24.41.4 23.70.9 37.3 25.13.1
7.40.9 7.11.9 11.90.3 9.30.7 5.10.1
48.27 53.48 63.56 77.29 45.92
44.08 27.42 32.47 63.223.1 24.37
higher (11.9%) in Q. grif®thii than in R. arboreum (9.3%) and other species (5.1±7.4%). Lignin and cellulose concentrations were minimum in Q. grif®thii and S. khasiana, respectively (Table 2).
4.2. Weight-loss pattern Needles of P. kesiya decomposed in a three-phased manner (Fig. 1(a)). The ®rst phase lasted for ca. 60
Fig. 1. Decay pattern of leaf litter of different species in the regrowing forest communities. (&), Observed values, and the curve has been fitted using expected values calculated by the composite linear-regression equation. (a), P. kesiya (7-year old stand), (b), Q. dealbata (13-year old stand), (c), Q. dealbata (16-year-old stand), (d), Q. griffithii (16-year old stand), (e), R. arboreum (16-year old stand) and (f), S. khasiana (16-year old stand).
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days and was characterized by a slow rate (0.05% wtloss dayÿ1) of decay. This was followed by a period of rapid weight loss (0.75% wt-loss dayÿ1) for up to 120 days (Table 3). During the third phase (i.e. 120±600 days), decay continued more-or-less at a constant rate (0.08% wt-loss dayÿ1). Q. dealbata, Q. grif®thii and S. khasiana followed a two-phased decay pattern (Fig. 1(b±d) and (f)). The ®rst phase lasting for about 120 days, was characterized by a rapid weight loss, followed by a slow rate of decay until 600 days (Table 3). Leaves of R. arboreum followed a uniform pattern of weight loss from the beginning to the end of the experiment (Fig. 1(e)). A composite linear regression model, YabX1cX2dX3 showed a good ®t for the weight-loss pattern in P. kesiya. The multiple regression equation, YabX1cX2 ®tted well for the observed decay pattern of all broadleaved species except for R. arboreum, where a simple linear-regression function, YabX was the best ®t. The correlation coef®cients describing the decay rates over time were highly signi®cant (P<0.001). The variations in the decay coef®cients (k) were statistically signi®cant between different species (P<0.01). The dry masses remaining (% of initial) at the end of the experiment were, P. kesiya (11.9), Q. dealbata (23.8 in the 13-year and 13.3 in 16-year-old regrowths), Q. grif®thii (10.3), R. arboreum (28.9) and S. khasiana (17.1). The annual decay constant (k) was a minimum (0.77) for R. arboreum and maximum (1.39) for Q. grif®thii (Table 4). Simple linear-regression analysis between mean daily rate of weight loss and ®fteen independent variables of climate, soil and initial litter chemistry yielded signi®cant correlations with only eight of them, of which, initial lignin and N concentrations and lignin/N ratio in the litter showed negative correlations, whereas seasonal means of soil moisture content (SMC), pH, total Kjeldahl nitrogen (TKN), and climatic variables such as mean daily rainfall and mean air temperature showed signi®cant positive correlations (Table 5). 4.3. N and P release N and P concentrations in the decomposing leaf litter of different tree species broadly followed a similar temporal pattern. Following an initial drop
155
Table 3 Relative decay/nutrient mineralization rates (mean wt-loss/nutrient release (mg and % per day) during different phases of leaf litter decomposition a Species
Dry wt loss k k
mg (7-year-old regrowth) P. kesiya I phase 4.63 II phase 67.20 III phase 3.82 (13-year-old regrowth) Q. dealbata I phase 26.20 II phase 6.54 (16-year-old regrowth) Q. dealbata I phase 29.54 II phase 6.73 Q. griffithii I phase 30.45 II phase 8.14 R. arboreum I phase 9.75 S. khasiana I phase 29.11 II phase 6.89
Nutrient mineralization kN kP kN c
b
kP d
%
mg
%
mg
%
0.05 0.75 0.08
0.58 0.15 0.03
0.64 0.27 0.07
0.008 0.022 0.003
0.17 0.51 0.09
0.31 0.08
0.20 0.05
0.26 0.09
0.026 0.001
0.54 0.04
0.35 0.09
0.17 0.06
0.23 0.12
0.012 0.004
0.24 0.12
0.35 0.10
0.17 0.05
0.27 0.11
0.014 0.002
0.35 0.10
0.11
0.08
0.16
0.002
0.07
0.33 0.09
0.31 0.04
0.35 0.08
0.018 0.001
0.45 0.05
a Note: Phase I in P. kesiya is 0±60 days, Phase II is 60±120 and Phase III is 120±600 days. Phase I for other species except R. arboreum (Phase I - 0±600 days) is 0±120 days and Phase II is 120± 600 days). b Decay constant. c N mineralization constant. d P mineralization constant.
after 60 days, the nutrient concentrations in the decaying leaves gradually increased up to 240 days and then declined until 420 days to rise again until 600 days (Fig. 2). Initial drop in nutrient concentration coinciding with the rainy season indicated the peak period of N and P mineralization rates in all species. Subsequent gradual increase in N and P concentration during autumn and winter seasons indicated nutrient immobilization (Figs. 3 and 4). The N mineralization constant (kN) for P. kesiya, Q. dealbata (in the 16-year-old regrowth), Q. grif®thii and S. khasiana ranged from 1.02±1.28. Compared to these species, Q. dealbata in the 13-year-old regrowth and R. arboreum had much lower kN (0.62±0.69).
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Table 4 Annual decay and mineralization constants of leaf litter of the studied species in three forest regrowths Decay/mineralization Dry mass kd t50 e t99 f Nitrogen kN t50 t99 Phosphorus kP t50 t99
P. kesiya a
Q. dealbata b
Q. dealbata c
Q. griffithii c
R. arboreum c
S. khasiana c
1.28 0.50 3.90
0.88 0.80 5.70
1.24 0.60 4.00
1.39 0.50 3.60
0.77 0.90 6.50
1.06 0.70 4.70
1.24 0.60 4.00
0.69 1.00 7.20
1.24 0.60 4.00
1.28 0.54 3.90
0.62 1.12 8.10
1.02 0.71 4.90
1.20 0.56 4.20
0.80 0.90 6.23
1.13 0.61 4.42
1.28 0.54 3.91
0.37 1.90 13.70
0.92 0.76 5.48
a
In 7-year-old regrowth. In 13-year-old regrowth. c In 16-year-old regrowth. d The decay constant. e N mineralization constant. f P mineralization constant. b
Phosphorus mineralization constant for R. arboreum was minimum (kP0.37). In other species, it varied between 0.80 and 1.28 (Table 4). Despite variations in
kN and kP, N and P stocks in decomposing leaf litter were positively correlated (P<0.001) with the corresponding dry weight (Fig. 5).
Table 5 Leaf litter decomposition rate (% wt-loss dayÿ1) as influenced by climatic variables, soil characteristics and initial leaf chemistry Variable Weight loss vs. climatic variables Rainfall a (mm) Air temperature b (8C) Weight loss vs. soil characteristics c Temperature (8C) Moisture content (%) pH Organic matter (%) TKN (%) Available P (mg gÿ1) Weight loss vs. initial chemistry Lignin (%) C (%) N (%) P (%) Cellulose (%) Lignin/N C/N a
Regression equation
df
Y44.641.15X Y11.280.33X
40 40
0.992 0.691
0.001 0.001
Y17.100.04X Y57.400.25X Y4.840.006X Y9.760.005X Y0.550.0003X Y10.870.05X
40 40 40 40 40 40
0.155 0.743 0.502 0.144 0.443 0.246
NS d 0.001 0.001 NS d 0.01 NS d
Y41.62ÿ0.17X Y47.48ÿ0.01X Y1.03ÿ0.003X Y0.05ÿ0.0001X Y6.740.02X Y45.16ÿ0.17X Y40.05ÿ0.05X
28 28 28 28 28 28 28
ÿ0.712 ÿ0.318 ÿ0.533 ÿ0.275 0.268 ÿ0.510 ÿ0.120
0.001 NS d 0.05 NS d NS d 0.05 NS d
Mean seasonal rainfall, but averaged to get daily values. Mean seasonal air temperature. c Seasonal data from 0±10 cm soil depth were used for correlation analysis. d Not significant. b
r
P
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5.2. Decay pattern
Fig. 2. N (A) and P (B) concentrations (%) in the decomposing leaf litter through time. (-*-), P. kesiya; (-*-), Q. dealbata (13-yearold regrowth); (-~-), Q. dealbata (16-year-old regrowth); (-~-), Q. griffithii; (-&-), R. arboreum; and (-&-), S. khasiana. Vertical bars indicate LSD at P<0.05.
5. Discussion 5.1. Resource quality Initial N and lignin concentrations of leaf litter of the ®ve tree species are well within the range (0.36± 3.90% and 4.5±46.4%, respectively) reported for various tropical tree species (Das and Ramakrishnan, 1985; Laishram and Yadava, 1988; Okeke and Omaliko, 1992; Sankaran, 1993; Bloom®eld et al., 1993). However, R. arboreum and P. kesiya, having more sclerophyllous cells, had high lignin and low nutrient concentrations than the other three species. The cellulose concentration in the fresh leaf litter was much less than the values (21.3±31.7%) reported by Bloom®eld et al. (1993) for several tropical tree species. Myers et al. (1994) reported that substrates with C/N<25 is of high quality and release mineral N at a faster rate compared to low quality residues (C/N>25).
P. kesiya needles showed a slightly different decay pattern than the broadleaved species by showing an initial time lag, probably due to the delay in colonization and establishment of the microbes on the litter (Anderson, 1973). Low moisture, and high lignin contents in the needles (Arunachalam et al., 1996b) might have also contributed to slow colonization process. The rapid rate of decay after an initial phase lag was the net effect of a large number of processes such as utilization of readily available energy sources by microbes, loss of water-soluble components and non-structural carbohydrates from the leaf litter (Bloom®eld et al., 1993), and removal of litter particles by soil microfauna (Swift et al., 1979) also operating on the freshly fallen litter. A decline in the decomposition rate after the rapid phase of decay may be attributed to higher percentage of recalcitrant fractions like cellulose, lignin and tannin during the advanced stage of leaf decay. These substances are known to control decay rate by showing resistance to enzymatic attack and by physically interfering with the degradation of other chemical fractions of the cell wall (Bloom®eld et al., 1993). Within the overall weight-loss pattern, a relatively higher decay rate (24±48% of initial weight) during the rainy season could be the effect of physical determinants such as warm and humid conditions during this season, while a relatively slow rate (12± 21% of initial weight) during post-rainy seasons are due to low moisture level and reduced microbial activity in the soil (Maithani et al., 1996). The greater is the initial N and/or lower the lignin concentration in the foliage, the faster would be the decomposition and vice-versa. Thus, several workers have established a positive correlation between initial N and decay rate, and a negative correlation between initial lignin and the decay rate (Singh and Gupta, 1977; Meentemeyer, 1978; Melillo et al., 1982; Vogt et al., 1991; Couteaux et al., 1995). Nevertheless, both lignin and N concentrations, and lignin/N ratio showed a signi®cant negative correlation with the decay rate in this study (Table 5). A similar observation was made earlier by Melillo et al. (1982) for hardwood litter. Such a differential trend could be explained based on the substrate quality, particularly by the levels of N and lignin. For example, in spite of
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Fig. 3. Net N mineralization rate (% release dayÿ1) in the decomposing leaf litter of the studied species during different seasons (R ± rainy, A ± autumn, W ± winter and S ± spring seasons).
greater lignin in the pine needles, the decay was faster due mainly to the N level (0.98%) which was comparable to the fast decomposing non-sclerophyllous S. khasiana leaves (N1.03%). Though the leaves of R. arboreum had greater lignin, yet it was less than the pine needles, because of low initial N level the decay rate was signi®cantly lower compared to the other species. This indicates that the decomposition of the subtropical tree species may be governed mostly by the initial N rather than lignin. R. arboreum leaves having high lignin and low N concentration decomposed at a slow rate, and hence revealed slow rate of N and P release. The initial faster decay rate of Q. grif®thii, S. khasiana and Q. dealbata leaves (Table 3) appears to be related to high initial N concentration. Berg (1984) and Taylor et al. (1989)
have, however, suggested that as the decomposition proceeds the in¯uence of N decreases while that of lignin increases. Hence, the reduction in decomposition rate with time may be due in part to the slow breakdown of residual refractory materials. A rapid decay of Q. dealbata leaves in the 16-year-old regrowth, as compared to that in the 13-year-old regrowth, may be ascribed simply to the microenvironmental variability, particularly high soil-moisture regime and accumulation of soil organic matter leading to better growth and activity of soil microbial population (Maithani et al., 1996). Q. dealbata, Q. grif®thii and R. arboreum showed faster decay rates (k0.77±1.39) in the present study than the rates reported by Laishram and Yadava (1988) for the same species (k0.18±0.55) in an undisturbed subtropical
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Fig. 4. Net P mineralization rate (% release dayÿ1) in the decomposing leaf litter of the study species during different seasons (R ± rainy, A ± autumn, W ± winter and S ± spring seasons).
Fig. 5. Relationships between dry matter and (A) N and (B) P stocks, and (C) N and P stocks in the decomposing leaf litter.
forest at Shiroy Hills in northeastern India. Since both sites have similar climate (annual rainfall 1617 mm, mean temperature 238C), the possible cause for the
relatively faster decay rate could be related to disturbance induced changes in forest microclimate and soil conditions. Pastor and Post (1986) have reported
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that the litter decay rates increase due to disturbances in forest ecosystems which brings about changes in both internal and external environmental conditions. Likewise, a faster rate of decay of P. kesiya needles (k1.28) in the 7-year-old regrowth than in the 22year-old pine stand (k0.38) of the same area (Das and Ramakrishnan, 1985) could be attributed to the confounding effect of stand age over resource quality of the litter. Earlier, we reported that the ®ne-root decay constant increased gradually from 1.62 in the 7year old stand to 1.74 in the 16-year old stand, which was attributed largely to the lignin concentration that showed a decreasing trend with stand age (Arunachalam et al., 1996b). A signi®cant negative correlation between initial lignin and litter decay rate (Table 5) conforms with that of the ®ne roots. 5.3. N and P mineralization A marked decline in N and P concentrations until 60 days of decomposition may be ascribed to leaching losses caused by heavy rainfall during that period. Subsequent increase in nutrient concentration could be the result of microbial immobilization (Anderson, 1973; Maithani et al., 1996), nutrient inputs from throughfall and atmospheric precipitation (Bocock, 1963), and/or atmospheric N2 ®xation (Wood, 1974). Despite variations in N and P concentrations in the decaying leaf litter due to mineralization and immobilization, N and P stocks in the decomposing leaf litter were positively correlated with its dry mass (Fig. 5). A similar trend has been reported by Prescott et al. (1993) for litter and by Arunachalam et al. (1996b) for ®ne roots. N and P release during decomposition was in¯uenced by seasonal cycle of nutrient immobilization and mineralization processes. A warm, humid season, being more favourable for mineralization, was characterized by rapid rate of N and P release, whereas a dry winter, being more favourable for immobilization, was the dominant process on the forest ¯oor. Interestingly, the periods of active mineralization and immobilization coincided with minimum and maximum microbial biomass in these stands (Maithani et al., 1996). On an average, a slower rate of leaf litter decomposition and nutrient mineralization in the 13- and 16year old regrowths compared to P. kesiya in the 7-yearold regrowth indicates the development of nutrient
conservation mechanism with species enrichment during the progression of vegetation recovery after tree cutting. The rapid N and P release in the pine-dominated stand indicates rapid recycling of these nutrients within the system and/or the potential loss of inorganic forms of the nutrients from the system, due to a low canopy density. This is true for the ®ne roots with reference to N only, whereas P release was greater in the 16-year old stand. It could, however, be considered that such a rapid rate of mineralization in the 7-year old regrowth is due to the interactive in¯uence of disturbance which, in turn, may be useful in meeting greater N and P requirements of the immediately regrowing forest vegetation. Acknowledgements The authors are thankful to the Council of Scienti®c and Industrial Research, Government of India for ®nancial assistance (Sanction No. 38(0840)/92 EMR-II, dated April 1992). The authors are grateful to Professor P.S. Ramakrishnan for having kindly provided us with a copy of the TSBF-Methodology Book. A. Arunachalam extends his thanks to the Director, NERIST, for providing essential communication facilities. The authors thank the two anonymous referees for their comments which helped in improving the earlier draft of the manuscript. References Anderson, J.M., 1973. The breakdown and decomposition of sweet chestnut (Castenea sativa Mill.) and beech (Fagus slavatica L.) leaf litter in two deciduous woodland soils. II. Changes in carbon, hydrogen, nitrogen and polyphenol content. Oecologia 12, 275±288. Anderson, J.M., Ingram, J.S.I., 1993. TSBF-Handbook of Methodology. CAB International, Wallingford, UK. Arunachalam, A., 1997. The role of litter and fine roots in organic matter and nutrient dynamics during the recovery of degraded subtropical forest ecosystems, Ph.D. Thesis, North-Eastern Hill University, Shillong, India. Arunachalam, A., Pandey, H.N., Tripathi, R.S., Kusum Maithani, 1996a. Biomass and production of fine and coarse roots during regrowth of a disturbed subtropical humid forest of northeast India, Vegetatio 123 73±80. Arunachalam, A., Pandey, H.N., Tripathi, R.S., Kusum Maithani, 1996b. Fine root decomposition and nutrient mineralization patterns in a subtropical humid forest following tree cutting, For. Ecol. Manage. 86 141±150.
A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161 Berg, B., 1984. Decomposition of root litter and some factors regulating the process: Long-term root litter decomposition in a Scots pine forest. Soil Biol. Biochem. 16, 609±617. Bloomfield, J., Vogt, K.A., Vogt, D.J., 1993. Decay rate and substrate quality of fine roots and foliage of two tropical tree species in the Luquillo experimental forest, Puerto Rico. Plant Soil. 150, 233±245. Bocock, K.L., 1963. Changes in the amount of nitrogen in decomposing leaf litter of sessile oak (Quercus petraca). J. Ecol. 51, 555±566. Couteaux, M.M., Bottner, P., Berg, B., 1995. Litter decomposition, climate and litter quality. Tree 10, 63±66. Das, A.K., Ramakrishnan, P.S., 1985. Litter dynamics in Khasi pine of north-east India. For. Ecol. Manage. 10, 135±154. Gilbert, O., Bocock, K.L., 1960. Changes in the leaf litter when placed on the surface of soils with contrasting humus types. II. Changes in the nitrogen content of oak and ash litter. J. Soil Sci. 11, 10±19. Horward, D.M., Horward, P.J.A., 1980. Effect of species, source of litter, type of soil and climate on litter decomposition. Oikos 34, 115±124. Laishram, I.D., Yadava, P.S., 1988. Lignin and nitrogen in the decomposition of leaf litter in a subtropical forest ecosystem at Shiroy Hills in north-eastern India. Plant Soil 106, 59±64. Lee, B.J., 1974. Effects of mirex on litter organisms and leaf decomposition in a mixed hardwood forest in Athens, Georgia. J. Environ. Quality 3, 305±311. Maithani, K., Tripathi, R.S., Arunachalam, A., Pandey, H.N., 1996. Seasonal dynamics of microbial C, N and P during regrowth of a disturbed subtropical humid forest in north-east India. Appl. Soil Ecol. 4, 31±37. Meentemeyer, V., 1978. Macroclimate and lignin control of litter decomposition rates. Ecol. 59, 465±472. Melillo, J.M., Aber, J.D., Muratore, J.E., 1982. Nitrogen and lignin control of hardwood leaf litter decomposition dynamics. Ecol. 63, 621±626. Myers, R.J.K., Palm, C.A., Cuevas, E., Gunatilleke, I.U.N., Brossard, M., 1994. The synchronisation of nutrient mineralization and plant nutrient demand. In: Woomer, P.L., Swift, M.J. (Eds.), The Biological Management of Tropical Soil Fertility, TSBF-John Wiley and Sons, pp. 81±116. Okeke, A.I., Omaliko, C.P.E., 1992. Leaf litter decomposition and carbon dioxide evolution of some agroforestry fallow species in southern Nigeria. For. Ecol. Manage. 50, 103±116.
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Olson, J.S., 1963. Energy storage and the balance of producers and decomposers in ecological systems. Ecol. 44, 322±331. Pastor, J., Post, W.M., 1986. Influence of climate, soil moisture on forest carbon and nitrogen cycles. Biogeochem. 2, 3±27. Prescott, C., Taylor, B.R., Parsons, W.F.J., Durall, D.M., Parkinson, D., 1993. Nutrient release from decomposing litter in rocky mountain coniferous forests: Influence of nutrient availability. Can. J. For. Res. 23, 1576±1586. Prescott, C.E., 1995. Does nitrogen availability control rates of litter decomposition in forests?. Plant Soil 168, 83±88. Sankaran, K.V., 1993. Decomposition of leaf litter of albizia (Paraserianthes falcataria), eucalyptus (Eucalyptus tereticornis) and teak (Tectona grandis) in Kerala, India. For. Ecol. Manage. 56, 225±242. Singh, J.S., Gupta, S.R., 1977. Plant decomposition and soil respiration in terrestrial ecosystems. Bot. Rev. 43, 449±528. Singh, K.P., Shekhar, C., 1989. Concentration and release pattern of nutrients (N, P, K) during decomposition of maize and wheat roots in a seasonally dry tropical region. Soil Biol. Biochem. 21, 81±85. Swift, M.J., Heal, O.W., Anderson, J.M., 1979. Decomposition in Terrestrial Ecosystems. 5 vols., University of California Press, Berkeley. Taylor, B.R., Parkinson, D., Parsons, W.E.J., 1989. Nitrogen and lignin content as predictors of litter decay rates. A microcosm test. Ecol. 70, 97±104. Van Vuuren, M.M.I., Berendse, F., de Visser, W., 1993. Species and site differences in the decomposition of litters and roots from wet heathlands. Can. J. Bot. 71, 167±173. Vogt, K.A., Grier, G.C., Vogt, D.J., 1986. Production, turnover and nutrient dynamics of above- and below-ground detritus of world forests. Adv. Ecol. Res. 15, 303±377. Vogt, K.A., Vogt, D.J., Bloomfield, J., 1991. Input of organic matter to the soil by tree roots. In: McMichael, B.L., Persson, H. (Eds.), Plant Roots and Their Environment. Proceedings ISSR Symposium on Developments in Agricultural and Managed Forest Ecology, Uppasala, Sweden; Elsevier, Amsterdam, pp. 171±190. Wood, T.G., 1974. Field investigations under decomposition of leaves of Eucalyptus delegatensis in relation to environmental factors. Pedo-biol. 14, 343±371. Zar, J.H., 1974. Biostatistical Analysis. Prentice-Hall Inc., Englewood Cliff, New Jersey.
Leaf litter decomposition and nutrient mineralization patterns in regrowing stands of a humid subtropical forest after tree cutting A. Arunachalama,*, Kusum Maithania, H.N. Pandeyb, R.S. Tripathib a
Restoration Ecology Laboratory, Department of Forestry, North Eastern Regional Institute of Science and Technology, Nirjuli-791109, India b Plant Ecology Laboratory, Department of Botany, North-Eastern Hill University, Shillong-793022, India Accepted 16 December 1997
Abstract Decomposition dynamics, and N and P mineralization patterns of leaf litter of Pinus kesiya, Quercus dealbata, Q. grif®thii, Rhododendron arboreum and Schima khasiana were studied in forest of three different ages in a humid subtropical region of India. The decay pattern varied from species to species. The decay pattern, characterized using a composite linear regression equations, exhibited two to three distinct phases during leaf litter decomposition. Initial lignin, nitrogen (N) and lignin/N showed signi®cant negative correlations with decay rate, whereas soil properties like pH, moisture and total Kjeldahl nitrogen (TKN) and climatic variables, e.g. rainfall and air temperature, showed positive correlations. The annual dry matter decay constants (k) varied from 0.77 in R. arboreum to 1.39 in Q. grif®thii. Nutrient release from the decomposing litter was in¯uenced by the seasonal cycle of mineralization and immobilization processes. Net mineralization was rapid during rainy season, as nitrogen (N) and phosphorus (P) concentrations in the decomposing leaf litter decreased by ca. 20±50% from the preceding season, while immobilization occurred during winter when nutrient concentration increased up to 60%. Annual dry matter decay, net N and P mineralization constants for Q. dealbata were higher in the 16-year old regrowth than in the 13-year old regrowth. # 1998 Elsevier Science B.V. Keywords: Litter decay; Nitrogen; Phosphorus; Humid subtropics
1. Introduction Litter decomposition plays a crucial role in the nutrient budget of the tropical forest ecosystems where vegetation depends mainly on the recycling of nutrients contained in the plant detritus (Vogt et al., 1986). The decomposition rate is strongly in¯uenced by climatic conditions and initial chemical composition of the litter (Couteaux et al., 1995). *Corresponding author. 0378-1127/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0378-1127(98)00240-0
Among the climatic variables, rainfall and air temperature determine the rate of decomposition in areas subjected to unfavourable weather conditions. Nitrogen (N) and lignin have been reported to have a better control over the litter decay rate in the humid tropics than in the temperate zones (Swift et al., 1979). A strong negative linear relationship between initial lignin/N ratio and disappearance rate of leaf litter was reported by Melillo et al. (1982). Lee (1974) has emphasized the relative importance of soil microand macrofauna in litter decomposition. Horward and
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Table 1 Dominant tree species and characteristics of the top soil (0±10 cm) layer in the three forest regrowths Age of forest regrowth 7-year 13-year 16-year
Dominant tree species P. kesiya Q. dealbata Q. dealbata
Soil characteristics SMC a (%)
pH
SOM b (%)
TKN c (%)
Available P (mg gÿ1)
30.33 6.10 43.86
5.15 4.72 4.44
6.56 8.67 10.88
0.35 0.49 0.61
7.64 11.94 11.23
a
Soil moisture content. Soil organic matter. c Total Kjeldahl nitrogen. b
Horward (1980) and Van Vuuren et al. (1993) have emphasized that tree and shrub leaf litter decomposition varies depending on species and type of soil. The role of N availability on litter decomposition in forests has been discussed by Prescott (1995). This paper presents the ®ndings of a study on the effects of initial litter chemistry and soil conditions on the litter decomposition and nutrient mineralization patterns of major tree species in three different-aged forest stands recovering after selective tree cutting in a subtropical humid forest of northeastern India.
The corresponding basal areas were 3.1, 19.9, 44.2 m2 haÿ1 in the 7-, 13- and 16-year old stands, respectively. The soil (latosol±oxisol) is derived from Precambrian igneous rocks. The mineral soil (0± 10 cm depth) was sandy loam in the 7-year old stand, sandy clay to clay loam in the 13-year old stand and clay loam in the 16-year old stand. The water holding capacity (WHC), soil organic matter (SOM), total Kjeldahl nitrogen (TKN) and available P in the soil increased gradually from the 7- to 16-year old stands (Table 1).
2. Study area
3. Methods
Disturbed forest stands of three different ages (7-, 13- and 16-year old) located near Shillong (latitude 258340 N, longitude 918560 E, altitude 1900 m a.s.l.) were selected for the present study. As the stands were selectively cut, a few older trees (DBH 10± 40 cm) were found interspersed in the stands. All stands were located on a gentle hill slope (6±138S) and covered an area of ca. 1 ha each. The area is highly monsoonal (average annual rainfall 1547 mm) with winter, spring, rainy and autumn seasons. A total of 80% of the annual rain was con®ned to the rainy season. Mean annual air temperature during the study period was 258C. Detailed account of climate, soil and vegetation of the study sites has been given in Arunachalam (1997), Arunachalam et al. (1996a, b). The 7-year old stand is dominated by young trees of Pinus kesiya with resprouting from stumps of broadleaved trees of Quercus dealbata and Schima khasiana. The density of woody vegetation was 680 stems haÿ1 in the 7-year old stand, 1260 stems haÿ1 in 13-year old stand and 1440 stems haÿ1 in the 16-year old stand.
3.1. Leaf-litter chemistry Freshly fallen needles/leaves of dominant tree species recorded in 1993 vegetation survey in the three forest stand were collected during peak litterfall period (March): P. kesiya (7-year old stand); Q. dealbata (13-year old stand); Q. grif®thii, Rhododendron arboreum and S. khasiana (16-year old stand). Leaves of Q. dealbata collected from the 13-year old stand were also used in the 16-year old stand. The litter samples were air-dried in the laboratory and sub-samples were kept at 808C for 48 h for determining the dry mass. The oven-dried materials were powdered in a Cyclotec (TECATOR) and analyzed for their chemical composition. The ash content was determined by igniting 1 g of ground litter sample at 5508C for 6 h in a muf¯e furnace. A total of 50% of the ash-free mass was calculated as the carbon (C) content. Nitrogen (N) was estimated following semimicro-Kjeldahl method in a Kjeltec Auto 1030 Analyzer (TECATOR). Total phosphorus (P) was analyzed colorimetrically while lignin
A. Arunachalam et al. / Forest Ecology and Management 109 (1998) 151±161
153
and cellulose contents were measured gravimetrically (Anderson and Ingram, 1993).
and 99% (t99) decay were calculated as t500.693/k and t995/k.
3.2. Litter decomposition
3.4. Statistical analysis
Decomposition of leaf litter of dominant tree species was studied in their respective stands using the nylon mesh (2020 cm) bag technique (Gilbert and Bocock, 1960). The mesh size was 2 mm, small enough to prevent major losses of needles or leaves, yet large enough to permit aerobic microbial activity and free entry of small soil animals. However, it excludes larger arthropods and earthworms, which are important primary accessors of litter and, presumably, it becomes a litter system dominated by termites, bacteria, fungi and actinomycetes. Consequently, there could be an underestimation of litter decomposition rates. For the experiment, 10 g of the air-dried material was kept in each bag which was then stitched with nylon threads. For each species, 60 bags were prepared and randomly dispersed on the forest ¯oor in the respective stands in the month of May 1993. After 60, 120, 180, 240, 300, 360, 420, 480, 540 and 600 days, ®ve litter bags for each species were brought to the laboratory carefully avoiding loss of material from the litter bags. The litter was washed in a bucket full of tap water by swirling brie¯y and carefully decanting through 2-mm mesh sieve to remove extraneous matter. According to Anderson and Ingram (1993), such brief washing permits little leaching. The litter was then dried at 808C for 48 h and weighed. The samples were powdered and used for the analysis of N and P contents according to semimicro-Kjeldahl and colorimetric procedures given in Anderson and Ingram (1993).
In order to distinguish between different phases of weight-loss pattern during decomposition, multiple regressions were developed using dummy factors (0 or 1) as the indicator variables (Zar, 1974). The composite linear-regression model (Arunachalam et al., 1996b) used for this purpose was YabX1 cX2dX3. . ., where Y is the percentage of initial mass remaining, a the Y intercept, b the rate of change in Y with respect to time, c the shift parameter for adjustment of the Y intercept in phase-II and d the shift parameter for adjustment of the Y intercept in phase-III. The values of c and d were taken as zero, if decay was slow, and/or equal to one, if the decay was rapid. The effect of climatic variables, initial litter chemistry and a few soil characteristics on the decomposition rate of leaf litter was assessed using simple linearregression function, YabX. As the type and number of species used for decomposition study in the three stands were different, there were chances of pseudoreplication in the application of analysis of variance (ANOVA, ®xed-effects model). To overcome the problem, the ANOVA was used to test the signi®cance of differences in the decomposition rate due to sampling time for each species in the three stands and among the species in the 16-year old stand only. However, comparison between plots were done by regression.
3.3. Computation
4.1. Initial chemistry of leaf litter
Organic matter decay, and N and P mineralization constants for leaf litter were computed using negative exponential decay model of Olson (1963): X/X0 exp(ÿkt), where X is the weight remaining at time t, X0 the initial weight, exp the base of natural logarithm, k the decay rate coef®cient and t the time (year). N and P mineralization constants (kN and kP) were calculated by substituting dry weight with N and P contents in the foregoing formula (Singh and Shekhar, 1989). Further, the time required for 50% (t50)
Mean N concentration in the leaf litter of ®ve dominant species varied from a minimum of 0.59% in R. arboreum to a maximum of 1.03% in S. khasiana. Phosphorus concentration was relatively low, varying between 0.03±0.06% in different species. Lignin concentration was maximum (43.2%) in the coniferous species, P. kesiya. Among the broadleaved species, R. arboreum had the highest concentration of lignin (37.3%). In other species lignin concentration varied from 23.7±25.1%. Cellulose concentration was much
4. Results
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Table 2 Chemical composition of leaf litter of dominant tree species used for decomposition study by the litter-bag method (SEM (n5)) Species
C (%)
N (%)
P (%)
Lignin (%)
Cellulose (%)
C/N
Lignin/N
P. kesiya Q. dealbata Q. griffithii R. arboreum S. khasiana
47.31.2 47.60.3 46.42.1 45.63.6 47.31.0
0.980.01 0.890.01 0.730.05 0.590.06 1.030.03
0.050.004 0.060.01 0.050.001 0.030.001 0.040.001
43.21.1 24.41.4 23.70.9 37.3 25.13.1
7.40.9 7.11.9 11.90.3 9.30.7 5.10.1
48.27 53.48 63.56 77.29 45.92
44.08 27.42 32.47 63.223.1 24.37
higher (11.9%) in Q. grif®thii than in R. arboreum (9.3%) and other species (5.1±7.4%). Lignin and cellulose concentrations were minimum in Q. grif®thii and S. khasiana, respectively (Table 2).
4.2. Weight-loss pattern Needles of P. kesiya decomposed in a three-phased manner (Fig. 1(a)). The ®rst phase lasted for ca. 60
Fig. 1. Decay pattern of leaf litter of different species in the regrowing forest communities. (&), Observed values, and the curve has been fitted using expected values calculated by the composite linear-regression equation. (a), P. kesiya (7-year old stand), (b), Q. dealbata (13-year old stand), (c), Q. dealbata (16-year-old stand), (d), Q. griffithii (16-year old stand), (e), R. arboreum (16-year old stand) and (f), S. khasiana (16-year old stand).
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days and was characterized by a slow rate (0.05% wtloss dayÿ1) of decay. This was followed by a period of rapid weight loss (0.75% wt-loss dayÿ1) for up to 120 days (Table 3). During the third phase (i.e. 120±600 days), decay continued more-or-less at a constant rate (0.08% wt-loss dayÿ1). Q. dealbata, Q. grif®thii and S. khasiana followed a two-phased decay pattern (Fig. 1(b±d) and (f)). The ®rst phase lasting for about 120 days, was characterized by a rapid weight loss, followed by a slow rate of decay until 600 days (Table 3). Leaves of R. arboreum followed a uniform pattern of weight loss from the beginning to the end of the experiment (Fig. 1(e)). A composite linear regression model, YabX1cX2dX3 showed a good ®t for the weight-loss pattern in P. kesiya. The multiple regression equation, YabX1cX2 ®tted well for the observed decay pattern of all broadleaved species except for R. arboreum, where a simple linear-regression function, YabX was the best ®t. The correlation coef®cients describing the decay rates over time were highly signi®cant (P<0.001). The variations in the decay coef®cients (k) were statistically signi®cant between different species (P<0.01). The dry masses remaining (% of initial) at the end of the experiment were, P. kesiya (11.9), Q. dealbata (23.8 in the 13-year and 13.3 in 16-year-old regrowths), Q. grif®thii (10.3), R. arboreum (28.9) and S. khasiana (17.1). The annual decay constant (k) was a minimum (0.77) for R. arboreum and maximum (1.39) for Q. grif®thii (Table 4). Simple linear-regression analysis between mean daily rate of weight loss and ®fteen independent variables of climate, soil and initial litter chemistry yielded signi®cant correlations with only eight of them, of which, initial lignin and N concentrations and lignin/N ratio in the litter showed negative correlations, whereas seasonal means of soil moisture content (SMC), pH, total Kjeldahl nitrogen (TKN), and climatic variables such as mean daily rainfall and mean air temperature showed signi®cant positive correlations (Table 5). 4.3. N and P release N and P concentrations in the decomposing leaf litter of different tree species broadly followed a similar temporal pattern. Following an initial drop
155
Table 3 Relative decay/nutrient mineralization rates (mean wt-loss/nutrient release (mg and % per day) during different phases of leaf litter decomposition a Species
Dry wt loss k k
mg (7-year-old regrowth) P. kesiya I phase 4.63 II phase 67.20 III phase 3.82 (13-year-old regrowth) Q. dealbata I phase 26.20 II phase 6.54 (16-year-old regrowth) Q. dealbata I phase 29.54 II phase 6.73 Q. griffithii I phase 30.45 II phase 8.14 R. arboreum I phase 9.75 S. khasiana I phase 29.11 II phase 6.89
Nutrient mineralization kN kP kN c
b
kP d
%
mg
%
mg
%
0.05 0.75 0.08
0.58 0.15 0.03
0.64 0.27 0.07
0.008 0.022 0.003
0.17 0.51 0.09
0.31 0.08
0.20 0.05
0.26 0.09
0.026 0.001
0.54 0.04
0.35 0.09
0.17 0.06
0.23 0.12
0.012 0.004
0.24 0.12
0.35 0.10
0.17 0.05
0.27 0.11
0.014 0.002
0.35 0.10
0.11
0.08
0.16
0.002
0.07
0.33 0.09
0.31 0.04
0.35 0.08
0.018 0.001
0.45 0.05
a Note: Phase I in P. kesiya is 0±60 days, Phase II is 60±120 and Phase III is 120±600 days. Phase I for other species except R. arboreum (Phase I - 0±600 days) is 0±120 days and Phase II is 120± 600 days). b Decay constant. c N mineralization constant. d P mineralization constant.
after 60 days, the nutrient concentrations in the decaying leaves gradually increased up to 240 days and then declined until 420 days to rise again until 600 days (Fig. 2). Initial drop in nutrient concentration coinciding with the rainy season indicated the peak period of N and P mineralization rates in all species. Subsequent gradual increase in N and P concentration during autumn and winter seasons indicated nutrient immobilization (Figs. 3 and 4). The N mineralization constant (kN) for P. kesiya, Q. dealbata (in the 16-year-old regrowth), Q. grif®thii and S. khasiana ranged from 1.02±1.28. Compared to these species, Q. dealbata in the 13-year-old regrowth and R. arboreum had much lower kN (0.62±0.69).
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Table 4 Annual decay and mineralization constants of leaf litter of the studied species in three forest regrowths Decay/mineralization Dry mass kd t50 e t99 f Nitrogen kN t50 t99 Phosphorus kP t50 t99
P. kesiya a
Q. dealbata b
Q. dealbata c
Q. griffithii c
R. arboreum c
S. khasiana c
1.28 0.50 3.90
0.88 0.80 5.70
1.24 0.60 4.00
1.39 0.50 3.60
0.77 0.90 6.50
1.06 0.70 4.70
1.24 0.60 4.00
0.69 1.00 7.20
1.24 0.60 4.00
1.28 0.54 3.90
0.62 1.12 8.10
1.02 0.71 4.90
1.20 0.56 4.20
0.80 0.90 6.23
1.13 0.61 4.42
1.28 0.54 3.91
0.37 1.90 13.70
0.92 0.76 5.48
a
In 7-year-old regrowth. In 13-year-old regrowth. c In 16-year-old regrowth. d The decay constant. e N mineralization constant. f P mineralization constant. b
Phosphorus mineralization constant for R. arboreum was minimum (kP0.37). In other species, it varied between 0.80 and 1.28 (Table 4). Despite variations in
kN and kP, N and P stocks in decomposing leaf litter were positively correlated (P<0.001) with the corresponding dry weight (Fig. 5).
Table 5 Leaf litter decomposition rate (% wt-loss dayÿ1) as influenced by climatic variables, soil characteristics and initial leaf chemistry Variable Weight loss vs. climatic variables Rainfall a (mm) Air temperature b (8C) Weight loss vs. soil characteristics c Temperature (8C) Moisture content (%) pH Organic matter (%) TKN (%) Available P (mg gÿ1) Weight loss vs. initial chemistry Lignin (%) C (%) N (%) P (%) Cellulose (%) Lignin/N C/N a
Regression equation
df
Y44.641.15X Y11.280.33X
40 40
0.992 0.691
0.001 0.001
Y17.100.04X Y57.400.25X Y4.840.006X Y9.760.005X Y0.550.0003X Y10.870.05X
40 40 40 40 40 40
0.155 0.743 0.502 0.144 0.443 0.246
NS d 0.001 0.001 NS d 0.01 NS d
Y41.62ÿ0.17X Y47.48ÿ0.01X Y1.03ÿ0.003X Y0.05ÿ0.0001X Y6.740.02X Y45.16ÿ0.17X Y40.05ÿ0.05X
28 28 28 28 28 28 28
ÿ0.712 ÿ0.318 ÿ0.533 ÿ0.275 0.268 ÿ0.510 ÿ0.120
0.001 NS d 0.05 NS d NS d 0.05 NS d
Mean seasonal rainfall, but averaged to get daily values. Mean seasonal air temperature. c Seasonal data from 0±10 cm soil depth were used for correlation analysis. d Not significant. b
r
P
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5.2. Decay pattern
Fig. 2. N (A) and P (B) concentrations (%) in the decomposing leaf litter through time. (-*-), P. kesiya; (-*-), Q. dealbata (13-yearold regrowth); (-~-), Q. dealbata (16-year-old regrowth); (-~-), Q. griffithii; (-&-), R. arboreum; and (-&-), S. khasiana. Vertical bars indicate LSD at P<0.05.
5. Discussion 5.1. Resource quality Initial N and lignin concentrations of leaf litter of the ®ve tree species are well within the range (0.36± 3.90% and 4.5±46.4%, respectively) reported for various tropical tree species (Das and Ramakrishnan, 1985; Laishram and Yadava, 1988; Okeke and Omaliko, 1992; Sankaran, 1993; Bloom®eld et al., 1993). However, R. arboreum and P. kesiya, having more sclerophyllous cells, had high lignin and low nutrient concentrations than the other three species. The cellulose concentration in the fresh leaf litter was much less than the values (21.3±31.7%) reported by Bloom®eld et al. (1993) for several tropical tree species. Myers et al. (1994) reported that substrates with C/N<25 is of high quality and release mineral N at a faster rate compared to low quality residues (C/N>25).
P. kesiya needles showed a slightly different decay pattern than the broadleaved species by showing an initial time lag, probably due to the delay in colonization and establishment of the microbes on the litter (Anderson, 1973). Low moisture, and high lignin contents in the needles (Arunachalam et al., 1996b) might have also contributed to slow colonization process. The rapid rate of decay after an initial phase lag was the net effect of a large number of processes such as utilization of readily available energy sources by microbes, loss of water-soluble components and non-structural carbohydrates from the leaf litter (Bloom®eld et al., 1993), and removal of litter particles by soil microfauna (Swift et al., 1979) also operating on the freshly fallen litter. A decline in the decomposition rate after the rapid phase of decay may be attributed to higher percentage of recalcitrant fractions like cellulose, lignin and tannin during the advanced stage of leaf decay. These substances are known to control decay rate by showing resistance to enzymatic attack and by physically interfering with the degradation of other chemical fractions of the cell wall (Bloom®eld et al., 1993). Within the overall weight-loss pattern, a relatively higher decay rate (24±48% of initial weight) during the rainy season could be the effect of physical determinants such as warm and humid conditions during this season, while a relatively slow rate (12± 21% of initial weight) during post-rainy seasons are due to low moisture level and reduced microbial activity in the soil (Maithani et al., 1996). The greater is the initial N and/or lower the lignin concentration in the foliage, the faster would be the decomposition and vice-versa. Thus, several workers have established a positive correlation between initial N and decay rate, and a negative correlation between initial lignin and the decay rate (Singh and Gupta, 1977; Meentemeyer, 1978; Melillo et al., 1982; Vogt et al., 1991; Couteaux et al., 1995). Nevertheless, both lignin and N concentrations, and lignin/N ratio showed a signi®cant negative correlation with the decay rate in this study (Table 5). A similar observation was made earlier by Melillo et al. (1982) for hardwood litter. Such a differential trend could be explained based on the substrate quality, particularly by the levels of N and lignin. For example, in spite of
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Fig. 3. Net N mineralization rate (% release dayÿ1) in the decomposing leaf litter of the studied species during different seasons (R ± rainy, A ± autumn, W ± winter and S ± spring seasons).
greater lignin in the pine needles, the decay was faster due mainly to the N level (0.98%) which was comparable to the fast decomposing non-sclerophyllous S. khasiana leaves (N1.03%). Though the leaves of R. arboreum had greater lignin, yet it was less than the pine needles, because of low initial N level the decay rate was signi®cantly lower compared to the other species. This indicates that the decomposition of the subtropical tree species may be governed mostly by the initial N rather than lignin. R. arboreum leaves having high lignin and low N concentration decomposed at a slow rate, and hence revealed slow rate of N and P release. The initial faster decay rate of Q. grif®thii, S. khasiana and Q. dealbata leaves (Table 3) appears to be related to high initial N concentration. Berg (1984) and Taylor et al. (1989)
have, however, suggested that as the decomposition proceeds the in¯uence of N decreases while that of lignin increases. Hence, the reduction in decomposition rate with time may be due in part to the slow breakdown of residual refractory materials. A rapid decay of Q. dealbata leaves in the 16-year-old regrowth, as compared to that in the 13-year-old regrowth, may be ascribed simply to the microenvironmental variability, particularly high soil-moisture regime and accumulation of soil organic matter leading to better growth and activity of soil microbial population (Maithani et al., 1996). Q. dealbata, Q. grif®thii and R. arboreum showed faster decay rates (k0.77±1.39) in the present study than the rates reported by Laishram and Yadava (1988) for the same species (k0.18±0.55) in an undisturbed subtropical
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Fig. 4. Net P mineralization rate (% release dayÿ1) in the decomposing leaf litter of the study species during different seasons (R ± rainy, A ± autumn, W ± winter and S ± spring seasons).
Fig. 5. Relationships between dry matter and (A) N and (B) P stocks, and (C) N and P stocks in the decomposing leaf litter.
forest at Shiroy Hills in northeastern India. Since both sites have similar climate (annual rainfall 1617 mm, mean temperature 238C), the possible cause for the
relatively faster decay rate could be related to disturbance induced changes in forest microclimate and soil conditions. Pastor and Post (1986) have reported
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that the litter decay rates increase due to disturbances in forest ecosystems which brings about changes in both internal and external environmental conditions. Likewise, a faster rate of decay of P. kesiya needles (k1.28) in the 7-year-old regrowth than in the 22year-old pine stand (k0.38) of the same area (Das and Ramakrishnan, 1985) could be attributed to the confounding effect of stand age over resource quality of the litter. Earlier, we reported that the ®ne-root decay constant increased gradually from 1.62 in the 7year old stand to 1.74 in the 16-year old stand, which was attributed largely to the lignin concentration that showed a decreasing trend with stand age (Arunachalam et al., 1996b). A signi®cant negative correlation between initial lignin and litter decay rate (Table 5) conforms with that of the ®ne roots. 5.3. N and P mineralization A marked decline in N and P concentrations until 60 days of decomposition may be ascribed to leaching losses caused by heavy rainfall during that period. Subsequent increase in nutrient concentration could be the result of microbial immobilization (Anderson, 1973; Maithani et al., 1996), nutrient inputs from throughfall and atmospheric precipitation (Bocock, 1963), and/or atmospheric N2 ®xation (Wood, 1974). Despite variations in N and P concentrations in the decaying leaf litter due to mineralization and immobilization, N and P stocks in the decomposing leaf litter were positively correlated with its dry mass (Fig. 5). A similar trend has been reported by Prescott et al. (1993) for litter and by Arunachalam et al. (1996b) for ®ne roots. N and P release during decomposition was in¯uenced by seasonal cycle of nutrient immobilization and mineralization processes. A warm, humid season, being more favourable for mineralization, was characterized by rapid rate of N and P release, whereas a dry winter, being more favourable for immobilization, was the dominant process on the forest ¯oor. Interestingly, the periods of active mineralization and immobilization coincided with minimum and maximum microbial biomass in these stands (Maithani et al., 1996). On an average, a slower rate of leaf litter decomposition and nutrient mineralization in the 13- and 16year old regrowths compared to P. kesiya in the 7-yearold regrowth indicates the development of nutrient
conservation mechanism with species enrichment during the progression of vegetation recovery after tree cutting. The rapid N and P release in the pine-dominated stand indicates rapid recycling of these nutrients within the system and/or the potential loss of inorganic forms of the nutrients from the system, due to a low canopy density. This is true for the ®ne roots with reference to N only, whereas P release was greater in the 16-year old stand. It could, however, be considered that such a rapid rate of mineralization in the 7-year old regrowth is due to the interactive in¯uence of disturbance which, in turn, may be useful in meeting greater N and P requirements of the immediately regrowing forest vegetation. Acknowledgements The authors are thankful to the Council of Scienti®c and Industrial Research, Government of India for ®nancial assistance (Sanction No. 38(0840)/92 EMR-II, dated April 1992). The authors are grateful to Professor P.S. Ramakrishnan for having kindly provided us with a copy of the TSBF-Methodology Book. A. Arunachalam extends his thanks to the Director, NERIST, for providing essential communication facilities. The authors thank the two anonymous referees for their comments which helped in improving the earlier draft of the manuscript. References Anderson, J.M., 1973. The breakdown and decomposition of sweet chestnut (Castenea sativa Mill.) and beech (Fagus slavatica L.) leaf litter in two deciduous woodland soils. II. Changes in carbon, hydrogen, nitrogen and polyphenol content. Oecologia 12, 275±288. Anderson, J.M., Ingram, J.S.I., 1993. TSBF-Handbook of Methodology. CAB International, Wallingford, UK. Arunachalam, A., 1997. The role of litter and fine roots in organic matter and nutrient dynamics during the recovery of degraded subtropical forest ecosystems, Ph.D. Thesis, North-Eastern Hill University, Shillong, India. Arunachalam, A., Pandey, H.N., Tripathi, R.S., Kusum Maithani, 1996a. Biomass and production of fine and coarse roots during regrowth of a disturbed subtropical humid forest of northeast India, Vegetatio 123 73±80. Arunachalam, A., Pandey, H.N., Tripathi, R.S., Kusum Maithani, 1996b. Fine root decomposition and nutrient mineralization patterns in a subtropical humid forest following tree cutting, For. Ecol. Manage. 86 141±150.
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