Nitrogen supply under mulched drip irrigation increases the rice yield by improving the photosynthetic nitrogen distribution strategy and promoting biomass accumulation

Nitrogen (N) application can significantly regulate the N distribution strategy of crop leaf photosynthesis to optimize photosynthetic N use efficiency (PNUE) and yield, but its effects on PNUE in drip-irrigated rice and its relationship with yield formation remain unclear. We hypothesize that water–fertilizer integration can improve the leaf photosynthetic N distribution strategy and whole-plant biomass accumulation through precise N fertilizer supply to improve water use efficiency while ensuring yield in drip-irrigated rice. A two-year field experiment was conducted in 2021–2022 using two rice varieties with different degrees of drought resistance (T-43 and Liangxiang 3) with two irrigation methods (FI, traditional flood irrigation; DI, mulched drip irrigation) and four N application levels (0, 150, 300, and 450 kg·ha⁻¹). Flag leaf traits, N distribution, PNUE and rice yield were assessed. The N distribution ratios in the carboxylation system (P_c), light-harvesting components (P_L), PNUE and spike biomass in T-43 and LX-3 rice were 32.3–109.1 %, 22.7–152.7 %, 18.9–69.6 % and 35.1–93.7 % lower under DI than FI; the two-year mean leaf area, specific leaf weight, N content per unit mass and N content per unit area were 1.3 % and 1.6 %, 17.3 % and 9.2 %, 7.6 % and 24.5 %, 32.7 % and 45.1 % higher under DI than FI. Finally, the maximum net photosynthetic rate (P_max) and yield of T-43 did not significantly differ between FI and DI. Under both irrigation methods, the mean chlorophyll content, P_L, P_max, maximum electron transport rate (J_max), PNUE, spike biomass and yield of T-43 were highest under 300 kg·ha⁻¹ N. Under DI (10,200 m³·ha⁻¹), selecting drought-resistant rice (T-43) and applying 300 kg·ha⁻¹ N fertilizer increased the specific leaf weight, leaf chlorophyll content, P_c and P_L to increase PNUE and material accumulation, thereby increasing yield, representing an effective management strategy for improving productivity and water use in drip-irrigated rice in arid areas.