2.1. Experimental site and soil The experimental site is located at 24.75ºN latitude and 90.5ºE longitude which falls under the AEZ of Old Brahmaputra Floodplain. The experiment was set up in typical rice growing silt loam soil at the Soil Science Field laboratory, Bangladesh Agricultural University, Mymensingh during aman season of 2015. The soil was silt loam in texture having pH 6.27, organic matter content 1.95%, total N 0.14%, available P 3.16 ppm, exchangeable K 0.09 me% and available S 10.5 ppm. The experimental area has sub-tropical climate which is characterized by adequate rainfall associated with moderately low temperature during Aman season.
2.2. Treatments The experiments composed of eight treatments which include: T1: Control, T2: PU, 104 kgN/ha, T3: USG (100%) = 104 kg N/ha (two 1.8 g briquette), T4: USG (75%) = 78 kg N/ha (one 2.7 g briquette). T5: USG (50%) = 52 kgN/ha (one 1.8g briquette), T6: NPK (100%) = 104 kgN/ha (two 3.4 briquette), T7: NPK (75%) = 78 kgN/ha (two 2.4 briquette), T8: NPK (50%) = 52 kgN/ha (one 3.4 briquette).
2.3. Lay out of the experiment: The experiment was laid out in a randomized complete block design (RCBD), where the experimental area was divided into 3 blocks representing the replications to reduce the heterogenic effects of soil. There were 8 different treatment combinations. Each block was subdivided into 8 plots and the treatments were randomly distributed to the unit plots in each block. Thus the total number of unit plot was 24. The size of each plot was 4m×3m and plots were separated from each other by ails 30cm. There was 1 m drain between the blocks that separated the blocks from each other.
2.4. Land preparation The land was prepared by ploughing and cross ploughing with power tiller followed by country plough and laddering at suitable times. The land was then cleaned by collecting and removing the weeds, stubble and previous crop residues. After puddling, the plots were made according to the design by making ails around each plot. 2.5. Application of fertilizers The fertilizers were applied as per treatment. All the treatments except T6, T7 and T8 received 20 kg P, 50 kg K and 18 kg S respectively. In T6, T7 and T8 treatments, P and K were supplied from NPK briquettes. Prilled urea was applied in three splits. The first dose of PU was applied at 10 days after transplanting (DAT), the second dose was added as top-dressed at 35 DAT (active tillering stage) and the third dose was top-dressed at 55 DAT (panicle initiation stage). USG and NPK briquettes were applied on 10 DAT and the briquettes were placed at 8- 10 cm depth between four hills at alternate rows. Before the application of N fertilizers, the water in the rice plots was drained out.
2.6. Transplanting of seedling Seedlings were carefully uprooted from nursery bed and transplanted in the plots on 3 August, 2015 maintaining spacing of 20 m×20 m. 2.7. Intercultural operations: Intercultural operations were done as and when necessary for ensuring and maintaining a favorable environment for normal growth and development of crop. Weeding and irrigation were performed. 2.8. Harvesting, threshing and weighing The crop was harvested at maturity on 17 December 2015. From each plot the area of 4m2was harvested and the crop bundled separately. The harvested crop was threshed plot wise. Grain and straw yields were recorded and moisture percentage was calculated after sun drying.
2.9. Determination of N from plant samples Total N was determined by micro-Kjeldahl method where1g of oven dry ground sample was taken into microKjeldahl flask to which 1.1 g catalyst mixture (K2SO4: CuSO4.5H2O: Se = 100:10:1), 2 mL 30% H2O2 and 3 mL H2SO4 were added. After swirling the flask, it was allowed to stand for about 30 minutes. Then the flask were heated (380 ºC) until the digest became clear and colorless. After cooling, the content was taken into 100 mL volumetric flask and the volume was made up to the mark with distilled water. A reagent blank was prepared in a similar manner. This digest was used for nitrogen volumetric flask and the volume was made up to the mark with distilled water. A reagent blank was prepared in a similar manner. This digest was used for nitrogen determination. For this, 40% NaOH was added with the digest for distillation. The evolved ammonia was trapped into 4% H3BO3 solution and 5 drops of mixed indicator of bromocresol green (C21H14O5Br4S) and methyl red solution and the distillate was titrated with standard 0.01 N H2SO4 until the color changed from green to pink (Bremner and Mulvaney, 1982). The amount of N was calculated using the following formula: %N = ((T-B) × N × 0.014 × 100)/S Where, T = Sample titration value (mL) of standard H2SO4, B = Blank titration value (mL) of standard H2SO4, N = Strength of H2SO4and S = Weight of soil sample in g.
2.10. Apparent recovery of applied N (ANR) ANR is defined as kg of N taken up per kg of fertilizer applied. ANR (kg /ha) = (UN+N – UN0N) /FN Where, UN+N is total N uptake (kg with grain and straw), UN0N is the N uptake (kg /ha) in control and FN is the amount of fertilizer N applied (kg /ha). 2.11. Nitrogen use efficiency: Nitrogen use efficiency is defined as kg grain yield increase kg-1N applied. As N fertilizers were applied in different plots at different doses, the use efficiency N was calculated by the following formula: NUE = (GY+N – GY0N) /FN Where, GY+N = grain yield in treatment with N application, GY0N = grain yield in treatment without N application, FN = amount of fertilizer N applied (kg /ha).