Experimental Site, Soil Amendments, and Rice Cultivation:
The experiment was carried out in an ideal paddy field, Bangladesh Agricultural University Farm, Mymensingh in 2010. The soil in the experimental site was silt loam, which belongs to Old Brahmaputra flood plains category. The organic matter content of the soil before experimentation was 39.6 ± 4.8 g kg-1 and other chemical properties were soil pH (1:5 with H2O) 6.2 ± 0.2, available P2O5: 68.9 ± 2.9 mg kg-1, available SiO2 82.6 ± 3.2 mg kg-1. Two irrigation water management systems such as continuous and intermittent irrigation practices were followed in this experiment. The experimental field was divided into three side by side blocks under each water management regime. Each block had five plots. The area of each unit plot was 100 m2. Five treatments such as urea alone (220 kg ha-1), urea plus calcium carbide (30 ppm), urea plus silicate fertilizer (500 kg ha-1), urea plus phosphogypsum (500 kg ha-1), and urea plus biochar (1 t ha-1) amendments were selected in this experiment. The experimental field was laid down in a randomized block design with triple replications. In total, there were 30 unit plots in our experiment; i.e., 2 water regimes * 5 treatments * 3 replications.
All the soil amendments except biochar were applied 2 days before rice transplanting in the field. The biochar was applied 1 week before final land preparation. The biochar used (\10 mm sized fraction) in this study was bagasse, a byproduct of the sugarcane industry (pyrolysis at 400–500 C for 2 h). For the field study, the biochar mass was ground to pass through a 2-mm sieve, and mixed thoroughly to obtain a powder consistency that would mix more uniformly with the soil. The contents of total organic carbon (TOC) and total nitrogen (TN) in biochar were 59.7 and 0.65 %, respectively, a total ash content of 20.8 %, and a pH (H2O) of 9.5. With respect to elemental analysis, the biochar contained 1.0 % Ca, 0.6 % Mg, 0.4 % Fe, and 2.6 % K. The contents of electron acceptors were 1.2, 3.0, 5.5, 4.9, and 4.0 % in urea, urea plus calcium carbide, urea plus calcium silicate, urea plus phosphogypsum, and urea plus biochar amendments, respectively (acid ammonium oxalate in darkness, citrate dithionate extraction method, Loeppert and Inskeep 1996; 2 M Na–acetate extraction method, Kumada and Asami 1958; Loeppert and Inskeep 1996). The basal chemical fertilizers were applied just before rice transplanting: 45 kg N ha-1 (urea), 90 kg P2O5 ha-1 (super phosphate), and 41 kg K2O ha-1 (potassium chloride). Second split of urea fertilizer (35 kg N ha-1) was applied at tiller initiation stage (around 3 weeks after rice transplanting) and the third split of fertilizer (30 kg N ha-1, 17 kg K2O ha-1) was applied at panicle initiation stage (around 6 weeks of rice transplanting).
Under the continuous irrigation system, water level in the rice field was kept at 5 cm depth. Under the intermittent irrigation system, rice field was irrigated during the final land preparation to rice transplanting time, active tillering stage, and flowering stage. The field was kept at moist conditions during the rest of the rice cultivation period. The rice cultivar used in this study was BRRI Dhan 29, indica type, semi dwarf herb with 10–14 tillers, duration 125–133 days. Twenty-one-day-old seedlings of rice cultivar BRRI Dhan 29 (indica type) were transplanted into field plots at a spacing 25 9 25 cm2 by the first week of January in 2010 and harvested in the middle of May in the same year.
Gas Sampling and Analysis A static chamber-GC method (Wang and Wang 2003; Zou et al. 2005a) was used to estimate CH4 and N2O emissions during rice cultivation. The air-gas samples from the transparent glass chamber (diameter 60 cm, and height 110 cm) were collected by using 60-ml gas-tight syringes at 0-, 15-, and 30-min interval after chamber placement over the rice-planted plots. Gas samplings were carried out once on weekly basis during the rice cultivation. Gas samples were collected three times (8.00–12.00–16.00) in a day to get the average CH4 and N2O emissions during the cropping season. The surface area of each chamber was 0.25 m2 (0.5 9 0.5 m2). While gas sampling, the chamber was placed over six hills of rice vegetation. There were four holes at the bottom of each chamber through which water movement was controlled. Soil and air temperature inside the chamber were recorded for each set of emission measurements. Gas samples in the syringes were stored for analysis by GC in the Laboratory within a few hours. The mixing ratios of CH4 and N2O were simultaneously analyzed with a modified Gas chromatograph (Agilent 7890) equipped with a flame ionization detector (FID) and an electron capture detector (ECD; Wang and Wang 2003). A purified gas of nitrogen was used as the carrier gas for CH4, and a gas mixture of argon and methane (Ar–CH4) was used as the carrier gas for N2O. To remove CO2 and water vapor in the air samples entering the ECD detector, a filter column filled with ascarite was connected at the beginning of the separation column for N2O (Zheng et al. 2008). Nitrous oxide was separated by two stainless steel columns (column 1 with 1 m length and 2.2 mm i.d., column 2 with 3 m length and 2.2 mm i.d.) that were packed with 80–100 mesh porapack Q, and detected by the ECD. CH4 was detected by the FID. The oven was operated at 100 C, the ECD at 300 C, and the FID at 200 C. Fluxes were determined from the slope of the mixing ratio change in three samples, taken at 0, 15, and 30 min after chamber closure. Sample sets were rejected unless they yielded a linear regression value of r 2 greater than 0.90. Average fluxes and standard deviations of N2O were calculated from triplicate plots. Seasonal amounts of CH4 and N2O emissions were sequentially accumulated from the emissions between every two adjacent intervals of the measurements.