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LITTER DYNAMICSAND CUMULATIVE SOILFERTILITY CHANGES INSILVOPASTORAL SYSTEMSOF A HUMID TROPICALREGION IN CENTRALKERALA, INDIASUMAN JACOB GEORGE a & B. MOHAN KUMARa

a College of Forestry, Kerala AgriculturalUniversity, Vellanikkara, Thrissur, 680654,IndiaPublished online: 05 Apr 2012.

To cite this article: SUMAN JACOB GEORGE & B. MOHAN KUMAR (1998): LITTERDYNAMICS AND CUMULATIVE SOIL FERTILITY CHANGES IN SILVOPASTORALSYSTEMS OF A HUMID TROPICAL REGION IN CENTRAL KERALA, INDIA,International Tree Crops Journal, 9:4, 267-282

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International Tree Crops Journal, 1998, Vol. 9, pp. 267-282 0143-5698$10 © 1998 A B Academic Publishers-Printed in Great Britain

LITTER DYNAMICS AND CUMULATIVE SOIL FERTILITY CHANGES IN SILVOPASTORAL SYSTEMS OF A HUMID TROPICAL REGION IN CENTRAL KERALA, INDIA

SUMAN JACOB GEORGE AND B. MOHAN KUMAR*

College of Forestry, Kerala Agricultural University, Vellanikkara, Thrissur 680654, India

SUMMARY

Litter dynamics and associated nutrient turnover were studied in 4-5 yr-old silvopastoral systems involving four fast growing multipurpose tree species (Leucaena leucocepha/a, Casuarina equisetifo/ia, Acacia auriculiformis and Ailanthus triphysa). Our objectives were to characterise the variations in amount and quality of litter, decay rates and release of nutrients through litter decomposition, and to evaluate the possible cumulative influence on soil physico-chemical properties. Annual addition of litter ranged from 1.92---{).25 Mg ha- 1• Leucaena showed the highest NPK levels in leaf litter and Ailanthus the least. Results of a litter bag study revealed that residual litter mass declined either exponentially or linearly with time. Casuarina and Leucaena litter decomposed completely within 6-7 months. Regardless of species, K remaining in the decomposing litter mass showed an exponential decline over time while both N and P had brief accumulation phases during the course of decomposition. Five years of tree growth has apparently improved the soil organic C, N, P and K content.

Key words: decomposition, litterfall, litter decay, nutrients, silvopasture

INTRODUCTION

Micro-site enrichment through improvement in the soil organic matter and mineral nutrient pools is an important attribute of trees and shrubs in agroforestry (Altieri et al. 1987). Litter on the forest floor acts as an input-output system for nutrients (Das and Ramakrishnan 1985) and litter dynamics of the humid tropical forest ecosystems (Kumar and Deepu 1992, Khiewtam and Ramakrishnan 1993) and plantation forestry systems (Lugo et al. 1990, Lisanework and Michelsen 1994) have received considerable research attention.

Although many descriptions on litter dynamics and nutrient turnover in tropical forest ecosystems are available, such reports characterising litterfall, decay and the resulting changes in soil nutrient pool are scarce in tropical agroforestry. A few workers, nonetheless, have addressed problems such as decomposition of applied leaf mulches (Budleman 1988) and nutrient cycling in systems based on crops including coffee (Coffea spp) and cacao (Theobroma cacao L.) (Alpizar et a! 1986, Glover and Beer 1986, Imbach et al. 1989), large

*Corresponding Author. Received: 17 June 1997; revised version accepted: II May 1998.

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268 GEORGE & KUMAR

cardamom (Amomum subulatum Roxb.) and mandarin (Citrus reticulata Blanco) (Sharma et al 1997). The present paper attempts to quantify litter production and associated nutrient turnover in tropical silvopastoral systems involving four multipurpose tree species.

Specific objectives included evaluating the amount, quality, decay rates and release of nutrients in the litter of four multipurpose tree species. We analysed the phenology of litterfall, seasonal variations in litter nutrient concentrations and their in situ rate of decomposition. In addition, an attempt was made to compare the effects of four multipurpose trees on soil physico-chemical properties and inter alia to relate litterfall nutrient input to cumulative changes in soil nutrient availability.

MATERIALS AND METHODS

A field experiment was used involving sixteen tree-grass combinations (four grass and four tree species) and four grass monoculture plots, initiated in June 1988, at the Livestock Research Station, Thiruvazhamkunnu, Palakkad district, Kerala, India (between 11°21 '30" and 11°21 '50"N latitude, 76°21 '50"E longitude and at 60-70 m above mean sea level). The soil at the experimental site is an Oxisol (pH 5.5 prior to experiment starting; Mathew et al. 1992). The area experiences a warm humid climate having a mean annual rainfall of 2570 mm, most of which is received during the southwest monsoon season (June to August). The second peak in rainfall distribution corresponds to the northeast monsoon season (September to October). These two monsoon seasons together constitute the wet season, with more than 200 mm of rainfall every month (June to October). Mean maximum temperature ranges from 28.4°C (October) to 38.0°C (April) and mean minimum temperature varies from 19.5°C (January) to 25.9°C (November). The dry season usually corresponds to the period from February to May (scanty rainfall and a mean maximum temperature >32°C). Details of the layout of the field experiment presented elsewhere (Mathew et al. 1992, George et al. 1996).

The four fast growing multipurpose tree species used in the study were: Leucaena leucocephala (Lam.) de Wit., Casuarina equisetifolia JR. & G. Forst., Acacia auriculiformis A. Cunn. ex Benth. and Ailanthus triphysa (Dennst.) Alston. All except Ailanthus are N2 fixers. The four grass species were: Pennisetum purpureum Schumach. (hybrid napier), Brachiaria ruziziensis Germain & Everad. (Congo signal), Panicum maximum Jacq. (Guinea grass) and Zea mexicana (Schrad.) Reeves & Mangelsd. (teosinte). Monocultures of these four grass species (tree-less control plots) were included for comparative purposes. Each grass tree (binary) mixture/treeless control formed a treatment and was replicated thrice. The treatments were arranged in a randomised block design.

Trees were planted in plots of 6 m x 6 m in two rows, 4 m apart. Each row consisted of six trees spaced at 1-m distance, giving a population of 12 trees per plot. Mean crown diameters at five years of age were: Acacia-4.3 m, Casuarina-3.0 m, Leucaena-3.0 m and Ailanthus-1.7 m. Trees in the experimental field were routinely pruned at the beginning of every fodder planting season (June) and

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SILVOPASTORAL LITTER DYNAMICS AND SOIL FERTILITY 269

the foliage fraction of the pruned materials incorporated in the respective plots (mean dry weight for 1992 lopping: 1970, 237, 549 and 1125 kg ha-1 respectively for Acacia, Casuarina, Leucaena and Ailanthus). Twigs (on dry weight basis) recorded about 1587, 861, 1451 and 536 kg ha-1 and branches (>2 mm dia.), 2844, 0, 1396 and 576 kg ha-1 respectively for Acacia, Casuarina, Leucaena and Ailanthus.

Litter collection

Litter collections were made for a one-year period from I February I992 (tree age, 44 mo) to 3I January I993, using specially designed circular traps (collection area: 0.24 m2 , 0.5 m above the ground, Hughes et al. 1987). Nineteen such traps were randomly placed in a stratified random design among trees (five each in Acacia, Leucaena and Ailanthus plots and four in Casuarina). This arrangement facilitated sampling a range of conditions from under open sky to below crown positions. Border areas were, however, avoided. Although more traps might have improved the experimental design, it is likely that, in plantation plots (cf. natural forests) with regularly spaced trees of a single species, variation (spatial and temporal) in litterfall might be fairly low.

The contents of the traps were collected at monthly intervals and the catch sorted into litter from target species (species allocated to the plot) and litter from neighbourhood trees (adjoining plots). Litter was further separated into leaves, twigs, branches and reproductive parts. The samples were oven-dried at 80°C until weights were constant, the litterfall computed on unit-area basis for each month and species. Monthly litterfall values were summed to obtain the total annual litter yield which included both target tree litter as well as neighbourhood tree litter (1.7-I9.8% of the total).

Litter decomposition

Freshly fallen/senescent leaves of Leucaena, Casuarina, Acacia, and Ailanthus were collected from the experimental area during January I992 and air dried under shade for approximately 48 h. Three sub-samples from each species were analysed for initial lignin (AOAC I980) and initial nitrogen content (micro­Kjeldhal method).

The standard litter bag technique was employed for characterising litter decomposition dynamics (Bocock and Gilbert I957, Anderson and Ingram I989). Samples of 20 g each were transferred to nylon mesh bags (20 em x 20 em, 4 mm mesh size). Three sub-samples of each species were also simultaneously collected for moisture estimation. Initial oven-dry weights in the litter bags were calculated on the basis of this moisture content. The bags (total 480, four species with 10 replicates each for 12 mo) were placed in the litter layer (Luizao and Schubart I987) of the plot adjoining the experimental area, on 3I January 1992.

At monthly intervals, from I March I992 to I February 1993, litter samples were carefully retrieved from the soil (ten litter bags per species) and returned to the laboratory (in paper bags to avoid possible spillage). The bags were gently rinsed to remove soil and other extraneous material (Anderson and Ingram 1989).

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270 GEORGE & KUMAR

The residual litter mass was removed from the bags, oven-dried at 80°C and weighed after excluding the fine roots, and any macro fauna.

Chemistry of leaf litter

Triplicate samples of leaf litter, drawn from the pooled (species-wise) monthly litter collections from different traps, were analysed as it represented the most important component by mass (71-99% ), as follows. Total N was assayed following the micro-Kjeldhal method, P by vanado-molybdo phosphoric yellow colour method (using Spectronic 20 colorimeter, Bausch & Lomb, USA) and K by flame photometry (Ellico Flame Photometer model CL 22D, ELICO, Hyderabad, India) following the methods of Jackson (1958). Contents of the litter bags (residual biomass; triplicate samples) were also analysed at monthly intervals for N, P and K to monitor nutrient release from the decomposing litter. Nutrient concentrations were calculated as percentage of the original dry weight.

Statistical analyses and calculations

Litterfall data were analysed for differences between species and time intervals using ANOV A with repeated measures (MANOV A) employing the statistical package SPSS/PC+ (Advanced Statistics V2.0). Hierarchical cluster analysis was performed, as the multivariate tests for species-by-month and month effects were significant. Clustering was done using average linkage between groups (Everitt 1974). The distance measure used was squared Euclidean distance. Data on litter chemistry parameters were analysed using ANOV A with MSTATC (version 1.2). All data sets exhibited homogeneity of variances, a condition considered necessary for parametric tests (Zar 1984). The model for constant potential weight loss (Olson 1963) was fitted using the data on mass disappearance in LOTUS 2.0. This model is represented by the equation:

(Eq. 1)

where x is the weight remaining at time t, X0 is the original mass, e is the base

of the natural logarithm, k is the decay rate coefficient and t is time. Half lives (t0.5) of decomposing litter samples were estimated from the k-values using the equation:

ln(0.5) 0.693 to5=--=--

. -k -k (Eq. 2)

To evaluate the nutrient release pattern, nutrients remaining in the decomposing leaf were estimated by the equation (Bockhelm et al. 1991 ):

%Nutrient remaining= (.!2_)x( DM )x102

co DMO (Eq. 3)

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SILVOPASTORAL LITTER DYNAMICS AND SOIL FERTILITY 271

where C is the concentration of nutrient element in leaf litter at the time of sampling, C

0 is the concentration at the beginning of the study, DM is the mass

of dry matter at time of sampling and DM0

is the initial dry matter of the litter kept for decomposition. Three mathematical models (linear, quadratic and cubic functions) were fitted to the data on nutrient contents of the decomposing leaf litter, following the non-linear least squares method, to illustrate the changes in nutrient status of decomposing litter over time.

Soil chemical analyses

Soil samples from beneath the crowns of trees and the control plots (three randomly chosen points in each plot; 0-15 em layer) were collected during February 1993. The samples were air dried and ground to pass through a 2-mm sieve. Each soil sample consisted of a mixture of humus and mineral soil. Two sub-samples per plot were drawn from the composite samples for chemical analyses: soil pH in soil and water in the ratio I :2 (Elico Digital pH meter, ELICO, India), organic C by the Walkley and Black method, total N by the micro-Kjeldhal method, available P by the chloromolybdic acid blue colour method (using Spectronic 20 colorimeter) , and available K by flame photometry (Elico Flame Photometer) using one N neutral ammonium acetate solution as extractant (Jackson I 958). Means (corrected for dry weight) of the duplicate samples were analysed using ANOV A for RBD to compare treatment differences.

RESULTS

Litterfall

Annual production of tree litter showed variations between different silvopastoral systems (Table 1 ). Hierarchical cluster analysis (Pillais multivariate test of

TABLE I

Annual litterfall in four fast growing multipurpose tree species grown in silvopastoral systems in a humid region of central Kerala, India

Species Annual Iitterfall (Mg ha- 1 yr- 1)

Mean min max SD

Acacia auriculiformis 6.27" 3.72 (Aug) 10.26 (Feb) 2.76 Casuarina equisetifolia 2.31 b 1.16 (Oct) 4.83 (Feb) 1.08 Leucaena leucocephala 2.30b 0.98 (Jun) 3.91 (Feb) 1.08 Ailanthus triphysa i.92C 0.70 (Aug) 3.42 (Mar) 0.84

n

60 48 60 60

Values with the same superscript are not significantly different (p <0.001). Min and max Iitterfall corresponds to the least and maximum monthly litter yields for a given species, multiplied by 12. Months in parenthesis indicate the corresponding period of min and max Iitterfall values. SD = standard deviation n = number of samples

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272 GEORGE & KUMAR

significance: F significant at 0.026 for the effect of species-by-month and F significance at 0.002 with respect to the effect of month) using average linkage between species showed that both Acacia and Ailanthus formed two distinct groups while Casuarina and Leucaena together formed a third cluster. Target species and foliage biomass (including phyllodes) accounted for most of the litterfall in all four stands. Relative proportion of target species litter in total litterfall was Acacia-92.9%, Casuarina-88.0%, Leucaena-80.2% and Ailanthus-83.0%. Contributions of foliar biomass to total litterfall were as follows: Acacia-10.8%, Casuarina-96.3%, Leucaena-83.4% and Ailanthus-99.6%. Monthly litterfall values differed significantly between species (F test ratio for comparing species = 93.47 and P < 0.001). Timing of the large pulse of deposition generally coincided with the dry period (February to May) and the period of reduced litterfall with that of the wet season (June to October, Table 1 ).

TABLE 2

Mean NPK concentrations in freshly fallen leaf litter samples (n = 36, triplicate samples for 12 mo) of four fast growing multipurpose

tree species grown in silvopastoral systems in a humid region of central Kerala.

Species Mean Min Max so

N Acacia auriculiformis 1.09 0.82 (Mar) 1.53 (Jun) 0.25 Casuarina equisetifolia 1.41 1.10 (Nov) 1.82 (Jun) 0.23 Leucaena leucocephala 2.67 2.04 (Dec) 3.53 (Sep) 0.43 Ailanthus triphysa 0.82 0.33 (Feb) 1.53 (Aug) 0.44 p <0.01 SEM (±) 0.014 CD (0.05) 0.028

p

Acacia auriculiformis 0.03 0.02 (Oct) 0.06 (Jul) 0.02 Casuarina equisetifolia 0.07 0.05 (Feb) 0.11 (Aug) 0.02 Leucaena leucocephala 0.12 0.06 (Dec) 0.21 (Aug) 0.05 Ailanthus triphysa 0.09 0.06 (Sep) 0.12 (Jul) 0.22 p <0.01 SEM (±) 0.005 CD (0.05) 0.011

K Acacia auriculiformis 0.18 0.08 (Aug) 0.25 (Mar) 0.07 C,asuarina equisetifolia 0.20 0.05 (Aug) 0.35 (Mar) 0.11 Leucaena leucocephala 0.24 0.09 (Aug) 0.34 (Mar) 0.10 Ailanthus triphysa 0.13 0.04 (Jun) 0.24 (Dec) 0.54 p <0.01 SEM (±) 0.006 CD (0.05) 0.012

n = number of samples, p = probabilty level of significance, CD = critical difference, SEM = standard error of means. Months in parenthesis indicate the corresponding period of min and max concentrations. SO = standard deviation.

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SILVOPASTORAL LITTER DYNAMICS AND SOIL FERTILITY 273

Chemical composition of litter

Macronutrient composition of litter for N, P and K concentration varied considerably (Table 2). N,P and K concentration of Leucaena litter was significantly greater than other tree species. Overall, N and K concentration of the litter samples followed the order: Leucaena > Casuarina > Acacia > Ailanthus, while P concentration declined in the order: Leucaena > Ailanthus > Casuarina > Acacia. Litter N and P concentrations were higher during the wet season (June to August southwest monsoon period; Table 2), although K concentration was lower. Peak K concentration for all species, except Ailanthus, was in March (dry period).

Litter decomposition

Mass loss of litter, decay rate coefficients (k) and half-lives varied among multipurpose trees (Figure 1; Table 3). On the whole, residual litter mass decreased exponentially with time, except for Ailanthus which exhibited a linear

TABLE 3

Monthly decay rate coefficients (k) and half lives of decomposing litter of four multipurpose trees grown in silvopastoral systems of a

humid region in central Kerala, India

Species

Acacia auriculiformis Casuarina equisetifolia Leucaena leucocephala Ailanthus triphysa

k

0.416 0.666 0.510 0.143

0.874 0.908 0.502 0.806

n

124 61 75

124

SE = standard error of estimates, n = number of samples

TABLE 4

SE

0.727 0.476 1.566 0.325

Initial concentration of lignin and nitrogen and the ratio of lignin/nitrogen in the litter of four multipurpose tree species grown in silvopastoral systems of a humid region in central

Kerala, India (n = 3).

Species

Acacia auriculiformis Casuarina equisetifolia Leucaena leucocephala Ailanthus triphysa

n = number of samples

Initial lignin (%)

22.2 15.7 15.7 9.6

Initial Lignin: N (%) nitrogen

ratio

1.4 16.3 0.8 19.1 2.3 6.9 1.7 3.4

Half life (t05)

(mo)

1.6 1.0 1.3 4.8

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274 GEORGE & KUMAR

20~------------------------------------------~ --A- Acacia ---- Casuarina ----.- Leucaena -s 15 __.__ Ailanthus

ICJ')

.5 c ·; E 10 e en en CIS E .2 5 m

oL--L~~~~~~~~~_z~~~~~~

0 1 2 3 4 5 6 7 8 9 10 11 12

Time (mo)

Figure I. Biomass remammg in the litter bags at various time intervals for four multipurpose tree species grown in silvopastoral systems of a humid region in central Kerala, India. Refer to Table 3 for decay rate coefficients and half-lives.

pattern. Decomposition was characterised by an initial faster rate of dis­appearance followed by a subsequent slower rate (Figure 1). The negative expo­nential decay function fitted to the mass disappearance data gave reasonably high R2 values, except for Leucaena. Of the four tree species studied, only Casuarina and Leucaena litter lost mass completely during the experimental period. Rapid mass loss occurred in Casuarina needles, which had a half-life of one month. Table 4 contains data on initial concentration of lignin, N content and lignin/N ratio. Although species differences were apparent, no significant and direct relations between structural chemistry parameters and decay rate coefficients were discernible.

Nutrient turnover in decomposing litter mass

As litter decomposition proceeded, the concentration of N in the residual mass increased. Variations in absolute amounts of N, P and K in decomposing leaf litter as a function of both species and sampling period was apparent (data not shown). Relative proportion of nutrients remaining in the residual litter mass exhibited divergent patterns with respect to NPK concentrations and species (Figure 2). While K content of the residual mass showed an exponential decline, both N and P showed brief accumulation phases. Although N release from the decomposing Ailanthus and Acacia litter implies a three-phase pattern (an initial rapid release followed by a modest accumulation phase and a final release phase),

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SIL VOPASTORAL LITTER DYNAMICS AND SOIL FERTILITY 275

-c Cl) 0 :s ::::s

.: 100 ~ 80 0 :e 60 0 40 Q.

e 20 D.

0

0 1 2 3 4 5 6 7 8 9 Decomposition period (mo)

Nitrogen

Phosphorus

Potassium

---A- Acacia ------ Casuarlna ...........-. Leucaena ___._ Ailanthus

10 11 12

Figure 2. Changes in the relative proportion of macronutrients (as a percentage of the original) remaining in decomposing litter of four multipurpose tree species grown in silvopastoral systems in a humid region of central Kerala, India. For Casuarina and Leucaena, decay was completed in six and seven months, respectively.

species with fast decomposing litter (e.g. Casuarina and Leucaena) showed only a two-phase pattern. Furthermore, in all cases, the cubic model performed better than other models in describing the relative changes in the N, P and K concentrations of decomposing litter (Table 5).

Soil properties

An attempt was made to evaluate the cumulative changes in soil chemistry associated with tree plantations through litterfall by comparing the tree-grass plots with tree-less grass monoculture plots. Five years of tree growth has apparently been associated with significant changes in soil chemical properties. Species influence also was apparent in this respect. Table 6 contains mean values for different tree species. Acacia and Leucaena were associated with a significantly

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276 GEORGE & KUMAR

TABLE 5

Relations between N, P and K concentrations (% of the original; refer to Fig. 2) of the residual litter mass and time elapsed with their R2 values

(t stands for decomposition period (mo) for four multipurpose trees grown in silvopastoral systems of a humid region in central Kerala, India. Only

equations having R2 > 0.85 are presented.

Acacia:

Ailanthus:

Casuarina:

Leucaena:

N = 93.52 - 1.55t + 3.32t2 - 0.2It3 (R2:0.97; n = 13); P = 93.47- 8.20t + 1.60t2 - 0.15t3 (R2:0.89; n = 13); and K = 96.78- 4.77t + 2.31t + 0.13t3 (R2:0.96; n = 13);

N = 101.45 - 1.58t + 0.53t2 - O.Oit3 (R2:0.98; n = 13); P = 97.42- 0.75t + 1.55t2 - 0.13t3 (R2:0.97; n = 13); and K = 100.35 - 9.35t + 0.75t2 + 0.08t3 (R2:0.98; n = 13);

N = 102.51 - 29.It + 5.IIt2 + 0.49t3 (R2:0.99; n = 7); P = 103.02 + 16.54t- 5.75t2 - 0.68t3 (R2:0.94; n = 7); and K = 97.50 + 9.96t- 9.58t2 + 0.99t3 (R2:0.96; n = 7)

N = 98.75 + 10.28t - 3.13t2 + 0.40t3 (R2:0.99; n = 8); P = 98.79 + 0.31t- 5.15t2 - 0.58t3 (R2:0.94; n = 8) and K = 97.87 + 4.66t- 8.15t2 + 0.8It3 (R2:0.99; n = 8)

TABLE 6

Soil properties (0-15 em layer) under different tree canopies of four multipurpose tree species grown in silvopastoral systems of a humid region

in central Kerala, India (n = 24 duplicate samples from each of the 12

Species

Acacia auriculiformis Casuarina equisetifolia Leucaena leucocephala Ailanthus triphysa Control

p2 SEM (±) CD (0.05)

n = number of samples 1available

plots under a tree species)

pH Organic C N (%) (%)

4.9 4.23 0.22 5.2 3.06 0.21 5.1 3.45 0.26 5.1 2.10 0.19 5.1 1.96 0.15

<0.01 <0.01 <0.01 0.03 0.10 0.01 0.08 0.27 0.04

pi KI (ppm) (ppm)

16.9 68.4 20.0 57.3 17.3 59.8 13.7 47.4 11.3 36.7

<0.01 <0.01 0.89 1.92 1.80 3.88

'for comparing species means; fodder means and tree-fodder interactions were not significant except in respect of pH and organic C.

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SILVOPASTORAL LITTER DYNAMICS AND SOIL FERTILITY 277

lower soil pH compared to tree-less control, Ailanthus and Casuarina. Soil organic matter pool in Acacia plots also was higher (Table 6). Nitrogen fixing tree species (Leucaena, Casuarina and Acacia) in general were associated with higher concentration of soil N, available P and K. Both the tree-less control and Ailanthus plots had relatively lower values in this respect.

DISCUSSION

Variations in litteifall

Our study of silvopastoral systems in the humid central Kerala depicts significant variations in the litterfall among four multipurpose tree species ( 1.92-6.25 Mg ha-1 yr1; Table I). Interspecific variations observed in the present study could be related to crown diameter of trees. Acacia, having the largest crown diameter (4.3 m), consistently gave the highest litterfall and Ailanthus, having a crown diameter of 1.7 m, the least. Others had intermediate values both in terms of litterfall and crown diameters.

Studies on litterfall rates in silvopastoral systems involving multipurpose trees are scarce. Total litterfall observed in the present study is lower than that of the reported values for nearby forested and plantation sites. In the nearby moist deciduous natural forests of Thrissur, Kumar and Deepu ( 1992) reported values ranging from 12.2 to 14.4 Mg ha-1 yr1 and for Acacia auriculiformis plantations (at Pullazhi, Thrissur), Kunhamu et al. (1994) reported an annual litter fall of 12.9 Mg ha-1• A vail able reports indicate that litterfall is generally lower in agroforestry than tropical (natural) forest ecosystems. For example, in a recent report, Sharma et al. ( 1997) cited values ranging from 1.5 to 2.9 Mg ha-1 yr1 for leaf and twig components in four agroforestry systems in the subtropical regions of Sikkim Himalayas, India (517-850 trees ha-1, compared to 2500 trees ha-1 in the present study).

The lower litterfall observed in the present study compared to the nearby forested (natural) and plantation sites may be due to several factors. Pruning of lateral branches at the beginning of the fodder-planting season is important in this respect. In general pruning delays canopy closure and/or reduces crown width. Pruned trees are thus likely to yield less litter. Pruning, although reduces litterfall, does provide substantial quantities of green leaf manure and/or firewood and facilitates light infiltration into the understorey that promotes production of annual crops. Furthermore, it is customary to carry out pruning operations during the pre-monsoon/monsoon period (June-July) in the traditional land use systems of this region, presumably to meet the green leaf manure requirement of field crops. Pruning, however, can alter the periodicity of litterfall. Species interactions with pruning time also may be significant. The present data are, however, insufficient to make generalisations in this respect.

Age, stand density and/or stage of stand development are also major determinants of litterfall. Litterfall increases with stand age and/or until canopy closure. It may then follow an asymptotic pattern similar to that of gross primary production (sensu Waring and Schlesinger 1985). While peak litterfall may be

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independent of stand density, the rate at which this equilibrium is approached is perhaps not. Denser stands may reach the equilibrium faster. In order to obtain conclusive information on this aspect, detailed investigations covering a wider range of tree densities and/or age classes are required.

Litter chemistry

Litter concentrations of mineral elements (Table 2) were substantially lower than that of fresh foliage (fresh foliage data not shown). The lower nutrient content of the foliage fraction of litter in comparison to fresh foliage may be due to retranslocation of mineral nutrients from ageing foliage to other tissues before senescence (Helmissari 1992). Furthermore, we observed higher leaf litter N and P concentrations in four multipurpose trees during the wet season compared with the dry season. In contrast, Khiewtam and Ramakrishnan (1993) reported uniformly low values of N, P, K and Ca for composite leaf litter samples during the wet season. Lower N and P status of litter during dry season observed in the present study could be explained in terms of a higher retranslocation efficiency during this season (Das and Ramakrishnan 1985).

Contrary to N and P, K concentration of the litter declined during the rainy season, presumably due to leaching. As a result, K availability from litterfall during the rainy season is possibly underestimated because some K leached before it could be measured. Das and Ramakrishnan ( 1985) also observed that surface wash from intact foliage and loss of K from the litter collected in traps was higher during the rainy season. This in tum may lower the K capital of the site, as K is probably lost from the site because of leaching.

Leaf litter decomposition

Species differences in litter decay rates are evident from Figure I and Table 3. Based on the decay rate coefficients, the four species could be broadly divided into three categories: quick (labile litter: Casuarina and Leucaena), medium (Acacia) and slow decomposers (more refractory litter: Ailanthus). The time required for complete disappearance of the original biomass ranged from 7-12 mo. Mass disappearance rates (k) in the present study (Table 3), except for Ailanthus, were similar to those reported for six tropical moist deciduous species by Kumar and Deepu (1992). For 12-mo decomposition periods these authors reported monthly decay rate coefficients ranging from 0.29-0.44. In a study on cardamom and mandarin based subtropical agroforestry systems from the Sikkim Himalayas, India, Sharma et al. (1997) calculated annual decay rate coefficients as 1.55-0.27. Compared to published reports relating to temperate ecosystems (e.g. Melillo et al. 1982, Stohlgren 1988), the decay rate coefficients observed in our study were greater.

Although several authors have reported that biochemical quality (lignin and nitrogen and their ratios) of litter exerts a profound influence on the decay rate (e.g. Melillo et al. 1982, Harmon et al. 1990), we did not observe any strong and direct relationship between structural chemistry attributes and decay rates of multipurpose tree litter. Several other workers (Stohlgren 1988, Taylor et al.

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1989, Kumar and Deepu 1992) have also failed to find strong dependence of either lignin or lignin/N ratio on decay rate coefficients. However, before confirming this position in respect of multipurpose trees, more species with a wider range of initial lignin and nitrogen contents need be analysed, as the lignin contents of the four litter types studied clearly do not cover the entire natural range of tropical multipurpose tree leaf litter types.

Nutrient dynamics and soil fertility improvement

Litter dynamics frequently account for substantial nutrient inputs into the soil system. Higher soil N levels observed in the tree-plots compared with non-tree plots may underscore this (Table 6). Data on litter input, its chemical composition, decay rates and nutrients remaining in the residual litter mass indicate the possibility of soil fertility improvements under tree canopies through litter dynamics. However, in the absence of data on soil chemical attributes prior to the experiment starting, this cannot be conclusively stated. The present data set nevertheless provides an indication of the cumulative soil improvement potential of four multipurpose tree species vis a vis tree-less control plots under forage grasses. However, re-measurements of the soil nutrient content at periodic intervals are perhaps necessary to investigate the continuing effects on soils.

Chemical composition is an intrinsic property of litter that determines decay rates and thus the rate of turnover of organically bound nutrients. Species adapted to higher nitrogen availabilities (Table 4) will generally have a faster rate of organic matter turnover. Therefore, integration of nitrogen fixing trees (e.g. Leucaena, Casuarina, Acacia) in silvopastoral systems may lead to quicker release of nutrients to the associated crops, compared with non-N2 fixers. Aber and Melillo ( 1982) earlier suggested that N2 fixing species exhibit low mineral immobilisation rates. Further, Sharma et al. (1997) observed rapid decomposition and release of nutrients from nutrient rich litter of N2 fixers. Pruning generally favours a large pulse of nutrients through the return of nutrient rich foliage. Consequently the pruned materials may exhibit a faster turnover rate than detrital matter.

Our data also show that all three N2 fixing species had significantly higher N status than treeless control and Ailanthus plots (Table 6) in the surface (0-15 em) horizon of the soil profile. A plausible explanation in this respect is the release of biologically fixed N to the rhizosphere (LaRue and Patterson 1981 ). N2 fixing species, quite apart from the faster litter turnover mechanisms outlined earlier, may also release N into the soil through the nodule sloughing of and other related mechanisms (LaRue and Patterson 1981 ). Perhaps this is the main reason why N2 fixing trees are often preferred in agroforestry. However, our data on soil N levels can provide only indirect evidence in this respect.

The amount of P returned to the soil through litterfall was modest compared to N and K. Species-related differences in P content of decomposing leaf litter (Figure 2) was, however, apparent. The enhanced soil P and K status might be associated with litter dynamics and possibly abstraction of these elements

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released by rock weathering particularly from the B and C soil horizons, as proposed by Young ( 1991 ).

Potassium, unlike N and P, is not structurally bound (Attiwill et al. 1978). Therefore, leaching is an important mechanism of K loss. This would possibly explain the rapid decline in K levels of the residual litter mass (Figure 2). Species-related differences were also apparent in this respect. Quick decomposers such as Leucaena and Casuarina released almost the entire quantity of K contained in them over a short period. Leaching thus leads to a faster nutrient loss. June-pruned materials also are likely to undergo a similar fate with respect to K leaching.

Higher soil organic matter content in tree plots than controls may be due to the high rate of litter production (Acacia; Table 1 ). Root decay may be another mechanism in this respect (Young 1991). Litterfall, decomposition and nutrient cycling in agroecosystems involve complex and long-term processes that cannot be quantified by short-term studies such as those presented here. These results do, however, provide a basis for comparing litterfall and decomposition processes in silvopastoral systems involving different multipurpose tree species.

In conclusion, trees in integrated land use systems in the tropics may yield a substantial quantity of litter, albeit with variations in quantity and quality depending on the species involved. Litter decay rates were also correspondingly variable. Nutrient deposition in the surface layer of soil through litterfall is substantial, with Leucaena, Casuarina and Acacia having accreted the highest amounts of N, P and K respectively. Nevertheless, there is poor correspondence between litter nutrient input and soil nutrient content. Ailanthus, with the lowest litter input, has the lowest soil content of these nutrients in the tree plots, but there is no similar correspondence at the high end of nutrient input.

ACKNOWLEDGEMENTS

Associate Dean, College of Forestry, Vellanikkara and the Professor, Livestock Research Station, Thiruvazhamkunnu provided the necessary facilities. The work was a part of the All India Co-ordinated Research Project on Agroforestry funded by the Indian Council of Agricultural Research, New Delhi and the MSc (Forestry) thesis submitted to the Kerala Agricultural University, by the first author. Mr. Thomas Mathew, Dr. K.V.S. Babu and Mr. K. Umamaheswaran were involved in the initial layout of the field trial. Dr. K. Jayaraman provided statistical advice.

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