Grain Yield, Nitrogen Use Efficiency and Antioxidant Enzymes of Rice under Different Fertilizer N Inputs and Planting Density.

Nitrogen fertilizer and planting density are key factors affecting rice yield and nitrogen utilization efficiency. There is still a need to optimize nitrogen fertilizer and planting density management for high yield. We studied the effects of different quantities of nitrogen application (N0 0 kg ha−1, N1 120 kg ha−1, N2 180 kg ha−1) and planting density (low-density: 18.8 hills m−2; high-density: 37.5 hills m−2) on rice yield, photosynthetic characteristics, antioxidant system, and nitrogen use efficiency. ANOVA results indicated that most tested traits were affected by environment, planting density, N application, and their interactions. Comparing the results of low-density planting, high-density planting increased the panicle number by 21.12% but decreased the grain number per panicle and yield by 3.97% and 22.48%, respectively. Similarly, the superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX) activities and PFPn (partial factor productivity of nitrogen) decreased by 8.20%, 6.99%, 16.41%, 14.28%, and 5.73%, respectively, while HIn (N harvest index) increased by 1.32%. Compared to no N application, N application treatments increased the panicle number, grain number per panicle, and yield by 9.92%, 17.64%, and 37.83% in the N1 treatment; and increased by 17.15%, 29.09%, and 128.94% in the N2 treatment, respectively. N application significantly increased net photosynthetic rates and enzyme activities of antioxidant antioxidases. Compared with N1, N2 decreased REn (apparent recovery efficiency of N), AEn (agronomic N use efficiency), and PFPn by 8.98%, 11.80%, and 25.14%, respectively, while, compared with N0, N1 increased HIn by 8.50%. It was observed that nitrogen fertilizer and planting density had an interaction effect on the net photosynthetic rate, antioxidant enzyme activities, and PFPn and HIn. Given a comprehensive consideration, it is best to apply nitrogen at a rate of 120 kg ha−1 at the planting density of 37.5 hills m−2 for high grain yield and high N use efficiency in our experimental site. [ABSTRACT FROM AUTHOR]