Nitrogen use efficiency and yield of rice under different nitrogen and water stress conditions at grain-filling stage

"Gangyou 527" (hybrid indica rice) and "Nongken 57" (conventional japonica) were used to study nitrogen utilization and yield under three nitrogen fertilizers (ammonium sulfate with ammonium/nitrate ratio of 100∶0, ammonium nitrate with ammonium/nitrate ratio of 50∶50 and sodium nitrate with ammonium/nitrate ratio of 0∶100) and four water stress (soil water potential of 0 kPa, 25 kPa, 50 kPa and 75 kPa) treatments for 14 consecutive days at the grain filling stage. The results showed a significantly higher increase in rice grain yield for ammonium/nitrate ratio of 50∶50 than 100∶0 under soil water potential of 25 kPa. When the proportion of ammonium nitrogen was ≥50%, further increase in the proportion of nitrate eased the adverse effects of severe soil moisture shortage on yield formation. Increase in the proportion of nitrate under soil water potential range of 0~ 25 kPa promoted nitrogen accumulation in the rice plant; which significantly different from that of pure nitrate treatment, but not significantly from that of pure ammonium treatment. The advantage of enhanced nitrate was not so obvious when soil water potential ≤ 50 kPa. On the contrary, increased ratio of ammonium greatly enhanced production. Nitrogen absorption by the rice plant before tillering increased with increasing proportion of ammonium nitrogen. However, appropriate increase in nitrate ratio to a certain proportion promoted growth at the mid-late stage of rice. It also enhanced the absorption rate of nitrogen and nitrogen accumulation in seed, which in turn increased nitrogen use efficiency. Moderate water stress promoted seed nitrogen absorption, promoted seed dry matter accumulation, improved grain nutrient content and enhanced grain harvest index. Yield response of hybrid indica rice "Gangyou 527" and conventional japonica rice "Nongken 57" to nitrogen utilization was basically the same under different nitrogen and water stress treatments at the grain-filling stage.