Leaf litter decomposition, Nutrient, Dynamics, Horticultural cropland, Agroforest, Tree species, Bangladesh
Description of the Study Area Bangladesh is bordered by the Bay of Bengal on the south and India on all the other sides except for a small section of Myanmar. Southwestern Bangladesh is low (<10 m above mean sea level), flat, and located on a fertile deltaic plain which is predominated by calcareous to noncalcareous alluvium soils. This study was conducted in a selected cropland without trees (paddy field) of Khulna district in southwestern Bangladesh lying between 22°46′ and 22°47′ N and 89°29′ and 89°30′ E. During the period of experiment no agricultural cultivation was carried out in that cropland. A tropical to subtropical monsoon climate characterizes this region with three distinct seasons, that is, summer (March–May), rainy (June–October), and winter (November–February). The monthly average rainfall is 155 mm. The highest average rainfall (339 mm) occurs from June to September and the lowest average rainfall (16 mm) occurs in the months of November to February. January is the coldest month and May is the warmest month. The mean annual temperature is 26°C with a range of 22–31°C [28]. The average relative humidity is the highest (86%–88%) from July to August and the lowest (72%–74%) from February to April. Collection and Processing of Leaf Samples Yellowish senescent leaves of A. heterophyllus, M. indica, Z. jujuba, and L. chinensis were picked from the trees of selected cropland agroforests. Leaves of individual species were thoroughly mixed. Experimental Design Decomposition of leaf litter was determined using the litter bag technique [6]. Five g of air dried leaf litter was placed in each 300 × 150 mm nylon bag with 1 mm2 mesh size. A total of 90 litter bags were prepared in the Nutrient Dynamics Laboratory, Khulna University, for each species in each season and 80 litter bags were placed randomly in the cropland and the remaining 10 bags were brought back to the laboratory for calculating oven-dry weights by drying at 80°C to constant weight. The litter bags were placed on December 1, 2012 for dry season and June 1, 2013 for wet season in the cropland. Sample Collection and Processing Ten bags were collected for each species at every 10-day interval for the initial 30 days and subsequently at 60-, 90-, 120-, 150-, and 180-day interval for the remaining periods. The collected leaf litter samples were gently washed and all sediments and dirt particles were then removed by using a soft brush with running tap water followed by final rinsing in distilled water. Each sample was then oven-dried at 80°C to constant weight in the said laboratory. Mass Loss and Decay Constant The loss in dry mass of leaf samples was calculated from the initial converted oven-dry mass and remaining mass. The rate of decomposition was calculated from the percentage of mass loss divided by respective days of sample collection. Decay constants for leaf litter were calculated using the negative exponential decay model. Nutrients Concentration in Leaf Litter The oven-dried leaf samples of individual species were ground and processed using acid digestion [30]. The digested sample extracts were processed according to [31, 32] to determine N and P concentrations using a UV-Visible recording spectrophotometer (U-2910, HITACHI, Japan) and K concentrations in sample extracts were determined by flame photometry (PFP7, Jenway LTD, England) in the said laboratory. The amount of nutrient released from leaf litters was calculated as differences between initial and final absolute mass also expressed as percentage of initial amount. Statistical Analysis The rate of mass loss and nutrients concentration (N, P, and K) in leaf litter of different tree species at different collection time intervals and also with the tree species were compared by ANOVA analysis followed by Duncan multiple range test using SAS 6.12 statistical software. A value of <0.05 was used for significance. The relationship among the rate of leaf litter decomposition and site factors (monthly rainfall and temperature) was evaluated by correlation analysis using SAS 6.12 statistical software. Furthermore, the relationship among mass loss, nutrient concentrations (N, P, and K), and time was evaluated by regression analysis using SAS 6.12 statistical software. The differences between mass loss, N, P, and K concentrations in dry season and wet season were evaluated by unpaired -test using SPSS (17) statistical software.