0464-B5

Leaf Litter Decomposition in Different Agroforestry Tree Species as Influenced by Climatic Variables and Substrate Quality

R. Kaushal ¹ and K.S. Verma ²

Abstract

Leaf litter decomposition of Grewia optiva, Morus alba, Toona ciliata and Populus deltoides was studied in north-western mountain region of Indian Himalaya. Decomposition rate was highest in Morus followed by Grewia, Toona and Populus. Decomposition pattern was biphasic for all the four species. Initially, a slower phase (October-February) was observed which was followed by a rapid phase (March onwards). Relationship between cumulative weight loss and substrate/litter quality attributes viz. Ca, Mg, acid detergent fiber (ADF), lignin and cellulose was significant but negative. The correlation coefficient values increased with lapse of time i.e. from winter through summer to rainy season. Nitrogen, phosphorus, ash and water soluble compounds (WSC) content showed positive and significant correlation with cumulative weight loss. Climatic variables - rainfall and temperature influenced the decomposition rate significantly except non-significant influence of temperature on Populus. Temporal order of decomposition among the species was as: Morus>Grewia > Toona > Populus.

Introduction

Climatic variables, chemical composition of the litter and its palatability to soil fauna are most important determinants in the decomposition process (Van Cleve, 1974). The dry matter is mostly consists of carbon, oxygen, hydrogen and inorganic elements. Celluloses and hemicelluloses constitute about 30-70 per cent of plant carbon, which are decomposed by microbes after soluble carbohydrates. Lignins - complex compounds in older plant tissues intervenes the cell wall physically and protect cellulose and other cell wall constituents from degradation (Chesson, 1997). The essential elements such as N, P, K, Ca, and Mg are also of vital importance in meeting microorganism's body requirement. Thus, climatic conditions and litter quality play an important role in determining decomposition. In India, attention has been given to techniques devised for studying decomposition of plant litter in various climatic zones (Gupta and Singh, 1977; Gupta and Singh, 1981; Pandey and Singh, 1982; Singh et al., 1993; Singh et al., 1994 and Pande, 1999). However, information defining the role of climate and various chemical constituents on the decomposition of indigenously grown trees vis-à-vis exotics leaf litter are scanty. Thus, in the present study temporal pattern of decomposition as well as impact of climatic parameters and substrate quality on the decomposition rate of AF tree species leaf litter in North-western mountain region of North-western Indian Himalaya were studied.

Materials and Methods

Site description

Experiment was conducted at the Department of Silviculture and Agroforestry farm in Dr Y.S. Parmar University of Horticulture and Forestry, Nauni-Solan, India. (30 ^o 51'N latitude and 76 ^o 11'E longitude, Survey of India Toposheet no 55F/1) situated at an altitude of 1250m asl. The mean annual rainfall and temperature were 1150mm and 18 ^o C, respectively.

Methods

Decomposition of Grewia optiva, Morus alba, Toona ciliata and Populus deltoides leaves was investigated by litter bag technique. Nylon netting bags measuring 25cm x 25 cm with 1 mm mesh size were used. Bags were stapled all along the sides at 5 cm intervals leaving enough gap between two staples to ease the entry of most of macro fauna. Newly senesced leaves from different tree species were brought to the laboratory and oven dried. 15 g oven dried leaf mass was filled in each bag. Two sets of these bags were prepared. One was placed on the surface whereas the second, in the sub-surface plough layer (10-20 cm) of the cultivated field randomly on 31 ^st October 1999. Four bags from each set were recovered at monthly interval till the complete decomposition. In laboratory, the residual litter was washed with water to remove the soil particles and then oven dried at 60+5 ^o C to constant weight. Initial leaf samples were analyzed for nitrogen content in Kjeltec auto 1030 analyzer. P, K, Ca and Mg in samples were digested in 4:1 nitro-perchloric acid mixture. P was then determined by Vanado-molybdo-phosphoric yellow colour method (Jackson, 1957) using Spectronic 20-D. K, Ca and Mg were determined using Atomic Absorption Spectrophotometer. For substrate/litter quality, initial samples were analyzed for acid detergent fiber (ADF), lignin, cellulose and ash content (Gupta et al.,. 1988). Cumulative seasonal weight loss was calculated for each of the species. The values were pooled and correlation with substrate quality attribute was established.

Results and discussions

Temporal pattern of weight loss and influence of species

Litter decomposition expressed as loss of dry matter (DM) at the end of each month (Table 1) showed that Grewia optiva total leaf litter disappeared in 9 months (October 1999-July 2000) period. In surface placement, only 30 per cent of the DM was lost during the first 5 months (October-March) and the remaining 70 per cent, in the last 4 months (April-July). Hence, a biphasic pattern was observed i.e. an initial slow phase (October-March) followed by a rapid phase (April-July). Sub-surface placement followed the similar trend, albeit, the quantity of actual DM lost was higher. A total of about 37 per cent DM was lost in the initial slow phase.

Morus and Toona leaf litter decomposition also followed the biphasic pattern. For Morus, 40 per cent of the DM under surface placement and about 50 per cent in sub-surface placement was lost in the initial slow phase. For Toona, the values were about 33 and 40 per cent under surface and sub-surface conditions, respectively. The complete loss of DM in surface placed litter occurred in 10 months, whereas under sub-surface in 9 months.

Populus leaf litter decomposed in longer period than Morus and Toona. Surface placed litter disappreared completely in 20 months, whereas for sub-surface in 17 months (Table 2). 40 per cent of DM was lost during April-September 2000, 27 per cent during October, 1999 - March 2000; 17 per cent from April -June 2001 and 14 per cent from October, 2000 - March, 2001 under surface conditions. In sub-surface conditions, the period between October 2000 - March 2001 caused about two times more loss of DM over surface conditions. Maximum DM, however, was lost during April -September 2000. Overall, the four species showed the following temporal order of decomposition : Morus > Grewia > Toona > Populus

The above temporal pattern of leaf litter decomposition thus indicated an initial slow decline in actual weight loss which later on declined rapidly towards the beginning of rainy season. The change in weight loss, albeit, was statistically significant. Initial slow decomposition rate till March can be ascribed to the low temperature and rainfall resulting into low activity of decomposers. Wiegert and McGinnis (1975) and Bahuguna et al. (1990) also reported similar results. From March, decomposition increased gradually until May. This is attributed to rising temperature during the season.

Substrate quality and decomposition

Initial chemical composition of the leaves species (Table 2) was correlated with the seasonal cumulative weight loss (Table 3). In winter, magnesium, lignin and cellulose showed negative but significant relationship with the weight loss, while phosphorus, ash and water soluble compounds a positive relationship. N, K, Ca and acid detergent fiber (ADF) exhibited non-significant relationship. In summer exactly similar relationship was evident with a little higher values. In rainy season, these values further increased for ADF, lignin, cellulose and water soluble compounds, however, decreased for Mg and ash content.

The climatic parameters - temperature and rainfall correlated positively and significantly with the monthwise weight loss (Table 4). The sub-surface placed litter decomposed faster than surface placed litter. Grewia showed highly significant correlation values for temperature both at surface and sub-surface placements. Morus in sub-surface and Toona in surface placed litter alone exhibited highly significant correlation coefficient. Populus behaved differently. It showed non-significant correlation values with temperature. Rainfall, on the other hand evinced significant and positive relationship. A rapid decomposition in rainy season was due to the favorable effect of soil moisture and temperature on the decomposers on one hand and stimulatory effect of frequent rain showers on leaching of water soluble substances on the other.

During the decomposition process soluble carbon is used as energy source by the decomposers, while nitrogen is assimilated into cell proteins and other components. Faster decomposition of Grewia and Morus may also have been due to high N concentration in leaf litter compared to Toona and Populus as high initial nitrogen promotes decomposition. Anderson, 1973; Gupta and Singh, 1981; Pandey and Singh, 1982; Rawat et al., 1995 and Mafongya et al., 1997a) also reported similar results.

After the depletion of soluble carbohydrates, in the next stage, the decomposer microbes attack acid detergent fiber comprising of cellulose and hemicellulose. Thus, these constituents are more resistant to decomposition and their role is observed in later stages. A negative and significant correlation of cumulative weight loss with these parameters, which increased with the lapse of time from winter to rainy season is thus understandable. Calcium as calcium pectate is the structural component in plant tissue and protected by lignin till last stages of decomposition. Lignin tends to dominate the shape of the long term decay curve (Minderman, 1968) in the last stage of decomposition. It provides little or no energy to the decomposers until the last stage of decomposition. Thus species having more lignin content decomposes more slowly (Jamma and Nair, 1996; Mafongya et al. , 1997b; Mugendi and Nair, 1997; Arunanchalam et al., 1998).

Comparatively faster loss of DM in Morus may be ascribed to more water soluble compounds (WSC), ash, phosphorus and low lignin content. The lowest decomposition rate exhibited by Populus may be due to highest lignin, ADF, cellulose, calcium, magnesium and lowest water soluble compounds, nitrogen and ash content (Table 2). The findings have further been substantiated by the highly significant and positive correlation of WSC, ash and N content whereas highly significant but negative correlation of lignin, cellulose and ADF with the seasonal cumulative weight loss (Table 3). The results follow the findings of Gillon et al. (1994), Waring and Schlesenger (1985) and Smith (1994) also.

Conclusions

Studies established that decomposition rate vary with tree species and method of placements. Litter placed in sub-surface layer decomposed faster as compared to surface placed litter. Species having more nitrogen, phosphorus, ash and water-soluble compounds decomposes fast while those having more lignin, magnesium, cellulose and acid detergent fiber decompose slow. Overall, the studies indicated the following order of litter decomposition among the four species : Morus > Grewia > Toona > Populus. Hence, it is deduced that Morus and Grewia could be the preferred species of agroforestry under sub-tropical sub-temperate mountainous region of North-western Indian Himalayas.

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Table 1. Loss of dry matter in decomposing leaf litter (% of original mass)

Table 2. Initial chemical composition (%) of fresh leaf- litter material of different tree species

Table 3. Simple correlation between cumulative litter weight loss of species and their quality attributes during different seasons.

Table 4. Correlation co-efficients for weight loss (% per month) and climatic parameters for surface and sub-surface placed litter.

¹ Biodiversity Division, Institute of Himalayan Bioresource Technology (CSIR),
Palampur-176061, India
² Department of Silviculture and Agroforestry, Dr. Y.S. Parmar University of Horticulture and Forestry, Nauni-173230, India