This study was conducted at the experimental farm of Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU), Gazipur, Bangladesh, as part of an ongoing research project entitled “Carbon sequestration in soils of Bangladesh,” which ran from April 2010 to July 2012. This period covered five consecutive seasons of transplanted aman (T. aman) and boro rice, starting with T. aman in 2010. Aman rice is grown as a rainfed crop during the monsoon season, when maximum rainfall occurs (300 - 650 mm; supplemental irrigation is occasionally required when there is insufficient rainfall), while boro rice is grown after T. aman had been harvested under fully irrigated conditions during the dry season (0 - 55 mm rainfall). This report covers the last of the five consecutive growing seasons which was T. aman season rice. 3.1. Experimental Site and Climate The study site belongs to the Salna series and has been classified as Shallow Red-Brown Terrace soil (Inceptisol) under the US Department of Agriculture classification system. The area is part of the Madhupur Tract agro-ecological zone (AEZ No. 28), in which the soils are acidic in nature and characterized by clay loam within 50 cm of the surface. The site is located at 24.090N latitude and 90.250E longitude, with an elevation of 8.2 meters above sea level. The experimental area experiences a subtropical climate, and is characterized by comparatively high levels of rainfall, high humidities, high temperatures, and relatively long days from April to September, and minimum levels of rainfall, low humidities, low temperatures, and short days from October to March. 3.2. Experimental Treatments The experimental crop was rice and variety BRRI dhan39 was procured from Bangladesh Rice Research Institute (BRRI), Gazipur, Bangladesh and used as the experimental crop. This variety is non-photosensitive, short-duration. The grains are long slender and kernels are white. This variety takes about 130 to 145 days to mature. It attains a plant height of 95 - 100 cm and at maturity the flag leaf remains green and erect. The experiment comprised five treatments (T1 = Control [no fertilizer]; T2 = cow dung; T3 = poultry manure; T4 = rice straw; and T5 = soil test based chemical fertilizer [STB]), which were laid out in a randomized complete block design with four replications per treatment. IPNS-based chemical fertilizers were applied in combination with treatments T2, T3, and T4.The organic materials were applied 7 days before transplanting at 2 t C ha−1 during the Aman seasons. On August 5, 2012, 30-day-old rice seedlings were transplanted into the experimental plots at 20 cm × 20 cm spacing. The recommended doses of chemical fertilizers were calculated based on soil test results. Urea was applied in three equal parts, whereby one-third of the urea and the entire amount of triple superphosphate (TSP), murate of potash (MOP), and gypsum were applied during final land preparation; one-third of the urea was applied at the maximum vegetative growth stage (30 days after transplanting [DAT]); and one-third of the urea was applied before the panicle initiation stage of boro season rice (45 DAT). Irrigation was used to keep the paddy field well flooded until the rice plants reached maturity. This was applied at 1, 5, 10, 20, 30, 40, 48, 54, 63, and 78 DAT, with a total of 45 cm of irrigation water being applied during rice growth. Weeding was performed at 25 DAT. Precautionary measures were taken at every stage of crop production using natural methods of pest and disease management, such as light traps, the placement of sticks for birds to stand on, and the removal of disease-infected plants at an early stage of infection, removing any need for chemical and herbicide use. 3.3. Soil Sample Collection and Processing Soil samples were collected from depths of 0 - 5, 5 - 10 and 10 - 15 cm separately at 0, 30, 60, 90, and 120 DAT, and were immediately placed in separate labeled ice bags to minimize microbial activity. The samples were then taken to the BSMRAU Soil Science Laboratory for analysis as quickly as possible to preserve their field properties. In the laboratory, they were spread out for partial air drying following removal of the roots, insects, worms, stones, and some small pieces of organic matter. This procedure was necessary to reduce the excess soil moisture and to allow the samples to be passed through a 2-mm mesh sieve. After sieving, part of each sample was kept in a polythene bag and incubated aerobically for 3 weeks at 25?C to remove the effect of handling on soil respiration. The remainder of each sample was fully air-dried, ground, passed through a 2-mm sieve and then stored in a clean polythene bag for physical and chemical analysis. The soil samples were analyzed for bulk density, pH, SOC, total N, available N (mineral N), available P, exchangeable K, MBC, and MBN. The maximum water holding capacity was also determined. Initial soil samples were also collected at 0 - 15 cm depth and analyzed for pH, SOC, total N, available P, and exchangeable K.
3.6. Statistical Analysis SPSS statistical software was used to analyze the data using one-way analysis of variance (ANOVA) and univariate analysis. Means were separated using the least significant difference (LSD) test at the 95% confidence level.