[Nitrogen Flow Characteristics of Agricultural Production and Consumption System in the Yangtze River Delta Region and Its Driving Factors]

To assess the impact of human activities on regional nitrogen （N） flow, based on the statistical data of 27 cities in the Yangtze River Delta Region （YRD）, N flow characteristics of the agricultural production and consumption system （APC） in the YRD from 2011 to 2020 were analyzed using substance flow analysis, and driving factors for N flow were analyzed using scenario analysis. The results showed that from 2011 to 2020, the mean N input intensity of the APC in the YRD was 194.6 kg·（hm²·a）^-1, which was more than five times the national average value； thus, the YRD was a hotspot of N input intensity in China. Chemical N fertilizer was the largest component of N input, and the YRD changed from a net export area of grain and animal products to a net import area due to the rapid growth of food consumption demand. The N output of the system was mainly N loss to the environment, accounting for 53.2% on average. The N use efficiency （NUE） of cropland and the N recycling ratio of the APC ranged from 38.7-42.2% and 15.8-21.5%, respectively, which were both at a low level. In addition, the total amount of N input and output of the APC both showed a parabolic decline trend, decreasing by 11.3% and 10.0%, respectively. Spatially, the overall N input intensity showed a pattern of "high in the north and low in the south," and the spatial heterogeneity of N input intensity among cities was significant. Cities with high input intensity were mainly located in the north and east of Jiangsu, Shanghai, and northeast of Zhejiang. A significant positive spatial autocorrelation of the distribution of mean N input intensity was observed. The uncertainty of N flows was estimated using the error propagation equation. The uncertainty interval of N input and output ranged from 4.5% to 34.6%, which was roughly equivalent to the results of related studies, indicating that the model results were reliable. Based on the scenario analysis method, the decrease of the livestock scale led to a decrease of -0.27%-7.53% in the N input, making it the main reason for the decrease of total N input in the APC. Improving the NUE of cropland and re-establishing the linkage between cropland and livestock will help reduce N loss to the environment.