Nitrogen Surplus in Croplands and Its Key Driving Factors in Intensive Agricultural Areas

Intensive agricultural areas across China suffer severe nitrogen surplus, yet its spatiotemporal drivers remain unclear. Using the nutrient balance method, this study quantified cropland nitrogen surplus and its intensity in 47 prefecture-level cities in Beijing, Tianjin, Hebei Province, Shandong Province, and Henan Province from 2010 to 2022. The Geographically and Temporally Weighted Regression (GTWR) model was applied to analyze six driving factors. Nitrogen surplus decreased from 6.71 million t to 4.11 million t, representing a reduction of 38.7%, with a marked decline occurring after 2016. Surplus intensity dropped by 30.7% from 29.13 t/km2 to 20.20 t/km2, and nitrogen use efficiency rose from 46% to 60%. The GTWR model proved to perform well (R2 = 0.749). Both the multiple cropping index and the agricultural industrial structure shifted from positive to negative effects. Specifically, the effect of the multiple cropping index declined from 0.325 to −0.183, showing the largest absolute average annual change. Urbanization rate, agricultural mechanization level, and per capita cropland area exerted negative effects, whereas the agricultural planting structure varied among regions. The multiple cropping index was the most critical factor. Therefore, optimizing cropping systems is an effective pathway to reduce nitrogen surplus. Future research should focus on incorporating climate factors and exploring the interactive effects among driving factors at finer spatial scales (e.g., county level), to achieve more precise nitrogen management.