Optimizing Seasonal Nitrogen Allocation Reduces Reliance on High Fertilizer Inputs While Maintaining Productivity in Intensive Rice–Wheat Rotations in the Upper Yangtze River Basin of China

Intensive rice–wheat (RW) rotations in the Yangtze River Basin often rely on excessive nitrogen (N) inputs, leading to low N use efficiency and environmental risks. To test whether high productivity can be sustained with lower N inputs, a two-year field experiment was conducted in the Upper Yangtze River Basin to evaluate seasonal N allocation strategies across the annual rotation. A moderately reduced annual N input of 315 kg ha−1 maintained annual grain yields of 18.2–19.3 Mg ha−1, statistically comparable to traditional high-input practices using 360 kg N ha−1. Wheat yield was mainly determined by current-season N supply, and no significant yield carry-over effect of rice-season N input on subsequent wheat yield was detected. Instead, higher rice-season N input tended to increase nitrate accumulation in deeper soil layers, particularly at 40–60 cm. Optimized seasonal N allocation significantly improved the partial factor productivity of N while maintaining a positive apparent system N balance under full straw return. These results indicate that strategically limiting and reallocating seasonal N inputs according to crop demand can sustain high annual productivity, improve N use efficiency, and reduce potential N-related environmental risks in intensive RW rotations.