Analysis of nitrogen flow in the Yellow River Basin over a long time series

Since the Industrial Revolution, human activities have led to increased nitrogen (N) inputs, causing excess N in the Yellow River Basin (YRB) and leading to various environmental issues. Based on the basic statistical data and related parameters of the YRB from 2000 to 2019, an N flow model was characterized using the full nitrogen flow analysis (FNFA) and the emission factor method to analyze the characteristics of N inputs and outputs in the YRB. The results revealed that over the past 20 years, both the total N inputs and outputs in the YRB have shown a significant increasing trend. Specifically, the total N input rose from 12,806.69 to 18,553.42 Gg, while the total output increased from 9250.93 to 12,955.0 Gg. Among the subsystems, the industrial and cropland subsystems were the largest contributors to the N balance, accounting for 28.30% and 26.22% of total N input and 40.48% and 26.23% of total N output, respectively. Overall nitrogen utilization efficiency (NUE) across the subsystems requires improvement, particularly within the cropland subsystem, which exhibited an NUE ranging from 25.67% to 36.10%. In contrast, the livestock subsystem had only half the NUE of the cropland subsystem. High emissions and inefficient N utilization have led to a continuous increase in environmental N loads, particularly in atmospheric N loads. Additionally, the life cycle assessment (LCA) of industrial N revealed that a substantial amount of N was enriched in the atmosphere. These findings provide a scientific basis for regulating N inflow and outflow within watershed areas and for formulating more rational integrated management strategies for N.