Nitrogen release–demand synchrony governs the productivity–environment trade-offs of organic nitrogen substitution

Partial replacement of mineral nitrogen (N) with manure is widely promoted to improve nutrient circularity and reduce N losses, yet its agronomic and environmental outcomes vary widely among sites and seasons. Because manure N becomes plant-available only through microbial decomposition, its performance depends on how release kinetics align with crop N demand under prevailing soil and climatic conditions. Here, we calibrated APSIM Next Generation against a winter wheat–summer maize field experiment in eastern China, explicitly representing manure decomposition and N mineralisation. We then simulated 0–100% substitution of mineral N with sheep manure across contrasting initial soil N, growing-season rainfall, and historical (1991–2020) and future (2041–2060; 27 CMIP6 models) climates. The model reproduced crop phenology (RMSE = 6.45 d), grain yield (R² = 0.79), soil mineral N (R² = 0.70) and cumulative N₂O emissions (R² = 0.73). Under the baseline climate, 15–45% substitution maintained grain yield within 0.6% of mineral fertiliser while reducing greenhouse-gas intensity by 14–41% in wheat and 9–25% in maize. Complete substitution achieved the largest emission reductions but reduced yield by up to 7.3%, increased interannual yield variability and raised the energy footprint. Rainfall most clearly determined the direction of yield–GHG trade-offs and explained 39–51% of the variance in simulated yield, whereas initial soil N mainly shifted response magnitude; GHG variance was dominated by rainfall in wheat (52%) and by substitution rate and climate scenario in maize (40% and 31%). Under SSP5–8.5, net N mineralisation increased by 167% in wheat and 70% in maize, whereas crop N demand changed by −1% and −47%, respectively. This widened the seasonal gap between N release and demand through crop-specific pathways: off-peak mineralisation in wheat, and earlier, truncated demand in maize. By quantifying how N release and crop demand diverge under warming, these results explain why no single replacement ratio is likely to remain optimal across crops and climates. Substitution strategies should therefore be designed for release–demand synchrony, matching the timing of plant-available N supply to crop demand, rather than for a fixed replacement fraction.