Influence of long-term anthropogenic nitrogen input and its legacy on riverine output.

The accelerated accumulation of anthropogenic nitrogen input and nitrogen legacies (NL) is a primary driver for persistently high nitrogen loads in rivers. Understanding the impact of nitrogen inputs and NL on river nitrogen export (RNE) is critical for improving water quality. This study employs the net anthropogenic nitrogen input (NANI) approach and export coefficient model (ECM), coupled with remote sensing data, to analyze nitrogen input dynamics, total nitrogen export (TNE), and NL from 1949 to 2021 in China's Dongjiang River watershed. Results indicate that NANI peaked in 2017 while TNE and NL continued to grow. Over the 73 years, NANI grew rapidly from 1949 to 1992, exhibited fluctuating growth from 1993 to 2021, and reached its maximum value (6743 kg·km·y) in 2017. Meanwhile, Total nitrogen output and NL maintained a continuous growth trend, with NL contributing approximately 46.5% to RNE from 1986 to 2021. Despite reduced NANI, recent nitrogen inputs remain a major factor influencing TNE. Spatial analysis reveals that nitrogen management should prioritize agricultural practices in upstream and urban pollution control in downstream.