2.1. Location and soil The field experiment was conducted at the experimental farm of the Bangladesh Rice Research Institute, Gazipur, Bangladesh, located at 23o85'90"N latitude, 90o82'40"E longitude. The site is about 35 m above the mean sea level and has a subtropical climate, which is strongly influenced by the southwestern monsoon. The average annual rainfall is 2000 mm with more than 80% of it occurring from mid-June to the end of September. Mean temperature is lowest (15 oC) in January and highest (30 oC) in May. The soil of the experimental field is a Chhiata clay loam, a member of the fine, hyperthermic Vertic Endoaquept (Evangelista et al., 1999). Initial properties of the surface soil (0–15 cm depth) were as follows: soil texture clay loam, pH 6.4, organic C content 11.3 g kg-1, total N 0.80 g kg-1, available P (Olsen) 9.0 mg kg-1, exchangeable K 0.18 cmolc kg-1 soil and available S 14 mg kg-1.
2.2. Crop and cropping pattern, experimental design and treatments Two rice crops were grown in the experimental field under fully irrigated conditions. The dry season (DS) rice variety BR3 was transplanted in first week of January and was harvested in May; the wet season (WS) rice variety BR11 was transplanted in the first week of August and harvested in third week of November. Two/three rice seedlings (45–50 days old in dry season and 30–35 days old in wet season) were transplanted at 20 cm x 20 cm spacing. At maturity the crop was harvested manually at 15 cm above ground level, however, 16 hills from each plot were harvested at the ground level for total straw yield data. After harvest, the crop residue was incorporated into the soil by digging. Natural weed was allowed to grow during the fallow period between two crops and the weed biomass was incorporated into to the soil by digging during land preparation for the next crop. Weed biomass was not recorded. The experiment was conducted in a randomized complete block design with four replications. Unit plot size was 5 m x 4 m. The six nutrient management treatments are presented in Table 1.
2.3. Fertilizer, cow dung and ash applications Nitrogen was applied as urea in three equal splits: during final land preparation, at active tillering stage and about 1 week before panicle initiation stage. Phosphorus, K and S were applied as triple super phosphate, muriate of potash and gypsum, respectively, during final land preparation. The CD and ash were applied on a dry weight basis and mixed well with the soil, by manual digging, about 2 weeks before transplanting of dry season rice. The CD was collected from the BRRI cattle shed each year and ash was collected from a nearby rice-mill. Nutrient concentrations in CD and ash were determined in 1990, 1994 and 1997. The mean nutrient concentration in CD was 12:9 1:1 g kg-1 N, 5:8 0:31 g kg-1 P, 17:6 1:5 g kg-1 K, 2:6 0:32 g kg-1 S and 152 12 mg kg-1 Zn, while rice husk ash contained 0:2 0:03 g kg-1 N, 1:0 0:08 g kg-1 P and 10:0 1:1 g kg-1 K on a dry weight basis.
2.4. Data collection Straw yield data was also collected from the 16-hill sample which was recorded after oven drying. Grain yield was recorded from a harvest area of 5 m2 and the grain yield data was adjusted to 14% moisture. Soil samples were collected at the end of 10 years. Soil data will be presented in following papers. Straw and grain samples (200 mg each) were digested in 5:2 conc. HNO3 and HClO4 mixture (modified from Yoshida et al., 1976) and analyzed for K by atomic absorption spectrophotometer and P by colorimeter (Yoshida et al., 1976). Further 200 mg samples were digested in conc. H2SO4 and analyzed for N by micro-Kjeldahl distillation (Yoshida et al., 1976). The N, P and K uptake was calculated by from the nutrient concentration in straw, grain and yield data. Nutrient content in straw and grain were analyzed for each season.