Rice yields and nitrogen use efficiency with different fertilizers and water management under intensive lowland rice cropping systems in Bangladesh

Optimum nitrogen rates and methods of application increase crop productivity and farm income while reducing negative environmental effects. Field experiments were conducted during four consecutive rice growing seasons in 2012–2013 to determine the effects of different N rates and methods of fertilizer application on floodwater ammonium concentration, rice yields and N use efficiency under two water regimes: continuous standing water and alternate wetting and drying (AWD). Fertilizer treatments included the use of deep placed urea briquettes and NPK briquettes (NPK), broadcast prilled urea (PU) and a control (without N). Deep placed fertilizer treatments irrespective of N rates and water regimes reduced floodwater ammonium similar to the control treatment, while broadcast PU treatment caused floodwater ammonium to increase as N rates increased. Deep placement of fertilizer above 52 and 78 kg N ha−1 during the Aus–Aman seasons (wet seasons) and during the Boro season (dry season), respectively, had no significant effects on grain yields but reduced N recovery. Although the differences in grain yields among deep placed and PU treatments were not significant, deep placement of 30 % less N compared to broadcast PU significantly increased N recovery (30–35 % vs. 48–55 %). AWD irrigation increased grain yield by 16 % along with increased harvest index, particularly under deep placed treatments. However, the effects of AWD on yield varied with seasons suggesting the need for long-term studies across different rice growing seasons and sites to arrive at more definitive conclusions.