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<p>Ammonia nitrogen (NH<sub>3</sub>-N) and total nitrogen (TN) are key components of freshwater nitrogen cycling, yet national-scale assessments of NH<sub>3</sub>-N/TN ratios, their seasonal variability, and environmental controls remain limited. Using observations from 307 river and 106 lake monitoring stations across China, we combined time series linear models, stepwise regression, and Random Forest with SHapley Additive exPlanations to quantify spatial patterns, seasonal dynamics, and NH<sub>3</sub>-N-TN coupling. Rivers consistently exhibited higher NH<sub>3</sub>-N, TN, and NH<sub>3</sub>-N/TN ratios than lakes and reservoirs. Although medium-ratio conditions dominated overall, rivers showed a greater proportion of high-ratio stations, particularly in the Southwestern River Basin, whereas the Songliao River Basin was characterized by predominantly low ratios. TN peaked in winter and during the non-flood season, whereas NH<sub>3</sub>-N/TN peaked in summer in rivers but in winter in lakes, revealing contrasting hydrological and thermal controls in lotic and lentic systems. NH<sub>3</sub>-N and TN were positively correlated nationally, especially in rivers under medium- and high-ratio conditions during flood and warm seasons, but this coupling weakened in lakes, low-ratio scenarios, and cold or dry periods. Temperature, runoff, and dissolved oxygen emerged as key drivers. These results show that NH<sub>3</sub>-N serves as a TN proxy only under limited basin-specific and hydrological-seasonal conditions.</p>

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National-scale nitrogen stoichiometry in China’s surface waters: spatiotemporal dynamics, proxy utility, and threshold-driven controls

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年6月3日

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  • Yi He,
  • Xizhi Nong,
  • Lihua Chen,
  • Jiahua Wei

摘要

Ammonia nitrogen (NH3-N) and total nitrogen (TN) are key components of freshwater nitrogen cycling, yet national-scale assessments of NH3-N/TN ratios, their seasonal variability, and environmental controls remain limited. Using observations from 307 river and 106 lake monitoring stations across China, we combined time series linear models, stepwise regression, and Random Forest with SHapley Additive exPlanations to quantify spatial patterns, seasonal dynamics, and NH3-N-TN coupling. Rivers consistently exhibited higher NH3-N, TN, and NH3-N/TN ratios than lakes and reservoirs. Although medium-ratio conditions dominated overall, rivers showed a greater proportion of high-ratio stations, particularly in the Southwestern River Basin, whereas the Songliao River Basin was characterized by predominantly low ratios. TN peaked in winter and during the non-flood season, whereas NH3-N/TN peaked in summer in rivers but in winter in lakes, revealing contrasting hydrological and thermal controls in lotic and lentic systems. NH3-N and TN were positively correlated nationally, especially in rivers under medium- and high-ratio conditions during flood and warm seasons, but this coupling weakened in lakes, low-ratio scenarios, and cold or dry periods. Temperature, runoff, and dissolved oxygen emerged as key drivers. These results show that NH3-N serves as a TN proxy only under limited basin-specific and hydrological-seasonal conditions.