Methodical Nitrogen–Water Distribution System Enhances Rice Yield While Reducing Environmental Losses: Evidence from 15N Isotope Tracing

Sustainable rice production necessitates innovative strategies optimizing productivity while minimizing environmental impacts. This study developed and evaluated a Methodical Nitrogen–Water Distribution (MNWD) system, employing 15N isotopic tracing to quantify the fate of nitrogen under three management regimes: Farmer’s Practice (FP), Nitrogen–Water Coupling (NWC), and MNWD. Among them, NWC is conventional N–water coupling management, while MNWD is optimized management with reduced N, saved water and synchronous N–W uniform application. Two-year field experiments (2019–2020) demonstrated that MNWD achieved yield increases of 9.01–15.60% over FP and 2.51–5.73% over NWC, while reducing nitrogen application by 20%. Based on 15N tracing, the nitrogen recovery efficiency of MNWD reached 52.9–56.6%, and leaching losses were reduced by 65.4% compared to FP. The modular design of MNWD requires only moderate increases in labor input and basic fertigation infrastructure, ensuring its applicability to smallholder systems. The trade-off between emissions and efficiency confirmed the environmental benefits of MNWD: it resulted in 34.0% lower N2O emissions than NWC while achieving a 5.45–5.49 percentage-point higher nitrogen recovery efficiency. Relative to FP, MNWD reduced total nitrogen losses by 48.5–61.4% with only a 3.4% increase in N2O emissions. This indicates that nitrogen conservation was predominantly achieved through enhanced plant uptake rather than conversion to alternative loss pathways. The MNWD system demonstrates a viable pathway for sustainable rice intensification by successfully decoupling productivity gains from nitrogen input intensity.