Spatial divergence of nitrogen fate in China's wheat systems: a meta-analysis and machine-learning roadmap for region-specific management

Optimizing crop productivity while mitigating pollution requires a system-wide understanding of nitrogen (N) fertilizer fate and its loss pathways. However, regional variability in N fate and its linkage to yield response in China's wheat systems remains poorly quantified. We collected 4077 observations to analyze the effects of N fertilizer management, climate, and soil properties on wheat yield, N fate, and reactive nitrogen (Nr) losses using meta-analysis and machine learning. At the national scale, wheat yield increased by 65.40 ​% with N fertilization, with applied N partitioned into 41.56 ​% uptake, 29.66 ​% residual, and 38.81 ​% losses. The Nr losses comprised NH3 (9.35 ​%), N2O (0.73 ​%), NO (0.38 ​%), leaching (7.38 ​%), and runoff (4.68 ​%). At the regional scale, N uptake exhibited an increasing trend from north to south, whereas N residual and N loss gradually decreased. NH3 volatilization accounted for 91.76 ​% of total N loss in northern China (NC). In central China (CC), NH3 constituted 53.45 ​% of the losses, with N leaching accounting for 41.38 ​%. By contrast, southern China (SC) showed a more even distribution of losses across pathways. N application rate was the key determinant of N fate, whereas pH, mean annual precipitation, mean annual temperature, and bulk density had the greatest influence on Nr losses. Nationally, N uptake was the dominant driver of yield response, accounting for 54.06 ​% of the variation. Regionally, uptake remained the key factor in CC (37.83 ​%), whereas NC (27.33 ​% uptake; 16.09 ​% loss) and SC (15.20 ​% uptake; 12.37 ​% loss) showed substantial sensitivity to N loss. N residual had minimal impact in most regions but was significant in CC (11.66 ​%). Enhancing nitrogen uptake is the top priority in increasing wheat yield across different regions of China, while the role of fertilizer N loss and residual regionally varied. Accordingly, N management should prioritize loss reduction in NC and SC, and residual management in CC.