Quantifying natural and anthropogenic impacts on riverine total nitrogen concentration and load in the Yellow River Basin.

Understanding the spatiotemporal dynamics of riverine nitrogen and its responses to socioeconomic and hydroclimatic variability is crucial for sustainable water management, particularly in water-scarce basins facing increasing anthropogenic pressures. This study investigated multiple time-scale fluctuations in surface total nitrogen (TN) concentration and load in the Yellow River Basin (YRB) from 2019 to 2022, utilizing high-frequency water monitoring data. We compiled various anthropogenic pollution sources and natural factors, including climate, topography, and hydrology across sub-basins. Random forest models were employed to identify drivers of TN variations, and a spatiotemporal statistical model quantified anthropogenic contributions to TN loads. Our results indicated that approximately 70 % of daily TN concentrations exceeded China's Class V water quality standard. Riverine TN concentrations and loads exhibited distinct seasonal patterns, with peak concentrations during the dry season and maximum loads during the wet season. Notably, 45-85 % of annual TN loads were exported under high-flow and wet conditions. Precipitation emerged as the dominant driver of monthly concentration changes, while variations in upstream loads primarily influenced monthly load changes. Atmospheric nitrogen deposition, often overlooked in water pollution assessments, was an important contributor to annual TN loads in populated areas. These findings emphasize the need for strengthened monitoring and pollution control measures in the YRB to address challenges posed by human activities and climate change.