Which factors and how they determine nitrogen runoff loss in rice agroecosystems across China

Clarifying the characteristics and mechanisms of nitrogen (N) runoff loss in rice cropping systems is crucial for optimizing agricultural N management strategies and controlling non-point source pollution in China. However, the key factors and how they determine N runoff loss in rice cropping systems, remain unclear at a national scale. This study collected 560 data sets from 86 published papers, using the Random Forest Ranking and Piecewise Structural Equation Modeling (SEM) to determine key factors and their linkage to N runoff loss in the Chinese rice cropping systems. The results showed that the average N runoff loss was 9.52 kg N ha−1 season−1 in the Chinese rice cropping systems, and the average N runoff loss was significantly higher in the paddy rice system (PRS) compared to the paddy rice-upland rotation system (PRURS) (10.40 vs. 8.60 kg N ha−1 season−1). The Random Forest Ranking determined that precipitation, N fertilization rate, and total soil N (TSN) were identified as key influencing factors for N runoff loss, while soil elemental stoichiometric ratios (e.g., C:N:P) were negatively correlated with N runoff loss, and the later factors highlighted the importance of paddy soil biogeochemical cycling in regulating N runoff loss in the Chinese rice cropping systems. Soil C:N:P and soil nutrient properties were more influential in the PRS, whereas N fertilization rate and precipitation were more critical in the PRURS. The Piecewise SEM suggested that the pathway effects of the composite variables of fertilization rate, soil nutrient properties, and soil C:N:P on N runoff loss were ranked as follows: Fertilization (0.52) > Soil nutrient properties (0.29) > Soil C:N:P (-0.14), in the Chinese rice cropping systems. The pathway effects of the three composite variables were ranked as Fertilization (0.52) > Soil nutrient properties (0.38) > Soil C:N:P ratio (−0.19) in the PRS, and Fertilization (0.52) > Soil nutrient properties (0.13) > Soil C:N:P ratio (−0.10) in the PRURS. This indicate that soil nutrient properties and the soil C:N:P ratio exert stronger effects on N runoff loss in the PRS than in the PRURS. These results provide deeper insights for mitigating N runoff loss and improving N use efficiency, and they highlight the critical role of soil elemental stoichiometry in regulating N migration and transformation within paddy water in rice cropping systems in China.