Description of the study site Southwestern Bangladesh is a low (10 m above mean sea level) flat, and fertile deltaic plain which is predominated by calcareous to noncalcareous alluvium soils (BBS 2004). Three districts (administrative unit) i.e. Khulna, Jessore and Satkhira were selected from southwestern Bangladesh, that lies between 22o 44'-23o 14' N and 89o 01'-89o 36' E. A tropical to subtropical monsoon climate characterizes this region with three distinct seasons i.e. summer (March–May), rainy (June–October), and winter (November–February). The annual average rainfall is 1,800±268 mm, while the monthly average rainfall is 155 mm. The highest monthly rainfall (339 mm) occurs during the month of June to September and the lowest monthly rainfall (16 mm) occurs in the month of November to February in the study area. January is the coldest month and May is the warmest month of the years. The mean annual temperature is 26o C with a range of 22–31o C (Kabir and Webb 2008). The average relative humidity is the highest (86%-88%) during the month of July to August and the lowest (72%-74%) during February to April.
Leaf litter collection and leaching experiment Yellowish senescence leaves were picked from M. azadirachta, A. indica, E. camaldulensis, S. macrophylla, M. indica, Z. jujuba, L. chinensis, A. saman, A. heterophyllus, A. auriculiformis, D. sissoo and K. anthotheca during March and April, 2013. The collected leaves were air-dried at room temperature for one week. Leaf discs (r = 0.80 cm) were prepared for leaves with higher leaf area (?34 cm2 ). Leaves/leaf discs were thoroughly mixed and accurately weighted to 5 g of leaves as individual sample. A total of 90 samples were prepared of each species. Eighty five samples were placed into individual beaker (500 mL) and 200 mL of distilled water was added to each beaker and few drops of HgCl2 solution (50 mg/L) were added to each beaker to prevent fungal decay (Ibrahima et al. 2008; Mahmood et al. 2009). The beakers were kept at room temperature and covered with polyethylene sheets; and the experiment was conducted for 192 hours. Five samples for each species were kept into the oven at 80°C for four days to get air-dry to oven-dry weight conversion ratio.
Sample collection and measurements Five samples of each species were collected at 0.5, 1, 1.5, 2, 2.5, 3, 4, 8, 12, 24, 48, 72, 96, 120, 144, 168 and 192 hours, respectively. The collected samples were oven dried at 80°C for 4 days. Mass loss due to leaching was calculated from the difference between initial and final oven-dried mass of the sample and the rate of mass loss was also calculated from the mass loss and the respective leaching time. Electric conductivity (μS/cm) and total dissolve solid (TDS) (mg/l) of leaching water samples at different time intervals were measured at the same time of sample collection by an Electric conductivity meter (EC-470L, Istek, Inc., Korea).
Nutrients measurement in leaf litter Ammonium (NH4) and phosphate (PO4) concentration in leaching water were measured according to Weatherburm (1967) and Timothy et al. (1984), respectively using UV-Visible Recording Spectrophotometer (HITACHI U-2910, Japan). Potassium concentration in leaching water samples were measured by Flame photometer (PFP7, Jenway LTD, England).
Statistical analysis Mass loss (%) due to leaching was transformed to arcsine and analysis of variance (ANOVA) was calculated to compare the mass loss among the species using SAS 6.12 statistical software. Electric Conductivity (EC) and Total Dissolve Solid (TDS) of leached water samples and NH4, PO4 and K concentration in leaching water at different time interval were compared by ANOVA using SAS 6.12 statistical software. The relationship among mass loss, EC and TDS of leaching water and leaching time were calculated by using SPSS (17) statistical software.