Experimental site:
The experiment was conducted at three different locations like Central Research Farm of BARI, Joydebpur; Regional Agricultural Research Station, Jessore and Wheat Research Centre, Nashipur, Dinajpur. The soils of Joydebpur location belongs to clay loam textural class while Jessore and Dinajpur are sandy loam. The characteristics of initial soil (0-15 cm depth) are given.
Collection and preparation of organic materials:
Eight types of organic materials comprising crop straw, manure and slurry were used in the experiment like Rice straw, Corn straw, Wheat straw, Cowdung, Cowdung slurry, Poultry manure, Poultry manure slurry and Vermicompost. Straws were collected from the experiment plots of Central Research Farm of Bangladesh Agricultural research Institute, Gazipur immediately after harvest and manure and slurry were from Pazulia, Gazipur. All the materials were air dried at room temperature. Plants materials were chopped into small pieces (2 cm) and organic manures and slurry were ground and passed through 5mm sieve for field incubation experiments. In addition, sub-samples of oven dried organic materials were ground into 0.5mm with a Foss Laboratory Mill for chemical analyses. Determination of total N was firstly digested with sulfuric acid (H2SO4) and then hydrogen peroxide (H2O2) according to the steam distillation method (Bremner and Mulvaney, 1982). Determination of organic C was accorded to the wet digestion method (Nelson and Smmers, 1982). The initial chemical composition of organic materials is presented.
Preparation of Nylon mesh bag for incubation experiment:
Fifty grams ground organic material (2 mm) was placed into 0.075 mm diameter nylon mesh bag (20 ´ 15 cm). The 0.075mm mesh size was small enough to minimize loss of fragments, but large enough to allow colonization by microorganisms and to reduce alteration of the microclimate (Tian, 1992). A total of 8 sampling period viz, 1 month, 3 month, 6 month, 9 month , 12 month, 18 month, 24 month, and 36 month were employed over a three-year period of field incubation at three different locations. A total of 576 litterbags (8 types’ organic materials, 3 replicates, 8 decomposition period and 3 locations) were prepared. A total of 192 litterbags were randomly horizontally buried to eight experimental plots at 10-12 cm soil depth at each location. Each sampling period contains one plot. The plot size was 2.5m ´ 1.5m and no plants were grown in the plot during the experimental period. The experiments were started on 5th November 2015 at Joydebpur, 11th November 2015 at Jessore and 28th November 2015 at Dinajpur. Here we collected the samples until 12 month period of decomposition and prepared the report based on 5 sampling time. Rest of the sampling will be done at schedule time.
Sampling and analysis:
The bags were collected from the field after their settlement at 1 month, 3 month, 6 month, 9 month and 12 month and cleaned through gentle flow of water and then air dried for some days. After air dry, the bags were oven dried for 2 days at 65°C and biomass losses of organic materials were then recorded. After that the samples were prepared for analyzing carbon and nitrogen using standard procedure.
Considering that mass loss or C decomposition over time followed the first order exponential single-pool decay model (Lekasi et al., 1999), the decomposition rate constant (k) was calculated using the following equation with a SigmaPlot10 software package.
Yt = Y0 +a*exp-kt (1)
Where, Yt is the % dry weight of biomass at t; Y0 is the % dry weight of biomass or C remaining when the time is approaching infinite; a is a constant and k is the decomposition rate (month-1).