Cultivar, nitrogen, and water effects on productivity, and nitrogen-use efficiency and balance for rice–wheat sequences of Bangladesh

Rice ( Oryza sativa L.) and wheat ( Triticum aestivum L.) are often grown in sequences under a range of nitrogen (N), water (W), and planting date in South Asia. Field experiments were conducted from 1994–1995 to 1996–1997 to define the effects of two W and three N regimes on growth and productivity, N uptake and N-use efficiencies, and N balance for rice–wheat systems of northern Bangladesh. Mean grain yields of rice and wheat were greatest (4.9 and 3.1 t ha −1, respectively) during the first and smallest (2.2 and 2.4 t ha −1, respectively) during the third year. The cultivars of rice and wheat responded to irrigation and to N, with greater response to irrigation in rice, but to N in wheat. Delayed wheat seeding reduced wheat yields in all years. Agronomic N-use efficiency (kg grain yield per kg N applied), physiological efficiency (kg grain yield per kg N absorbed), and fertilizer N-recovery efficiency (kg N absorbed per kg N applied, expressed as %) for rice across treatments ranged from 2.8 to 10.8, 5.2 to 27.5, and 33 to 61, respectively, and all were greater for N application at 90 compared with 135 kg N ha −1. For wheat, those values ranged from 15 to 27, 33 to 51, and 45 to 63, respectively, and were greater at 120 compared with 180 kg N ha −1, and under irrigation than rainfed. All those parameters had greater values under irrigation than rainfed. Total soil N increased slightly after 3 years of cropping, while organic carbon and pH decreased slightly in all treatments. There was a net increase of soil ammonium N (80 kg ha −1) and a zero balance of N after the first year of cropping under irrigation with high N (135 and 180 kg ha −1 for rice and wheat, respectively), but without N there was a decrease of soil mineral N (70 kg ha −1) with a balance of +16 kg ha −1. Biological N fixation accounted for N balance in N-omitted as well as N-applied treatments. The results emphasize the need for regular monitoring of weather, crop performance, irrigation water, and soil and plant mineral N for further understanding the growth, productivity, N-use efficiencies, and balance in rice–wheat systems.