Optimizing nitrogen application and planting density improves yield and resource use efficiency via regulating canopy light and nitrogen distribution in rice

Coordinating light and nitrogen (N) distribution within a canopy is essential for improving rice yield and resource use efficiency. However, limited research has examined light and N distribution in response to planting density and N rate, and their relationships with grain yield, radiation use efficiency (RUE), and N use efficiency for grain production (NUEg) in rice. A two-year field experiment was conducted with two hybrid varieties under three N levels, 0 kg ha^–1 (N1), 90 kg ha^–1 (N2) and 180 kg ha^–1 (N3), and two planting densities, 22.2 hills m^–2 (D1) and 33.3 hills m^–2 (D2). Results showed 3.4% higher yield and 4.4% higher NUEg under N2D2 compared with N3D1. The extinction coefficient for N (K_N) and light (K_L) and their ratio (K_N/K_L) at heading stage were significantly influenced by N rate, planting density, and their interaction. K_N decreased with the increase of N input or planting density. Compared to N1, K_N decreased by 43.5 and 58.8% under N2 and N3, respectively, while K_N under D2 decreased by 16.0% compared to D1. Higher K_L and K_N/K_L values occurred under low N rates, with opposite trends under high N rates. Increased planting density led to decreased K_L and K_N/K_L values. N2D2 demonstrated higher K_L and K_N, and thus comparable K_N/K_L, compared to N3D1. Correlation analysis revealed K_L negatively correlated with RUE, while K_N and K_N/K_L positively correlated with NUEg. These findings indicate that increasing planting density under reduced N input could maintain rice yield while enhancing resource use efficiency through regulation of canopy light and N distribution.