Abstract：
The storm-induced export of nitrogen (N) from agricultural watersheds significantly impacts aquatic ecosystems, yet the mechanisms of source supply and transport behind N species remain unclear. Here, we investigated the hydrological factors influencing the timing and magnitude of river N species export in a Chinese pomelo agricultural watershed. We conducted continuous observations of watershed hydrology, N species, and their isotopic ratios along a soil-groundwater-river continuum during two storm events in 2018-2019. We found the export flux of river NO3-N covers ∼80% of the total N flux during storms, and the rest for other N species. Our results further revealed distinct pathways and timing of N transport among different N species, especially between ammonium N (NH4-N) and nitrate N (NO3-N). NH4-N in stormflow predominantly originates from sewage and soil leachate, rapidly transported via surface runoff and interflow. Orchard fertilization (contributed 41-56% based on SIAR analysis) was the major source of river NO3-N, which underwent initial dilution via surface runoff and subsequently became enriched through delayed discharge of soil leachate and groundwater. The variations in timing and magnitude of N transport between storms can be explained by antecedent conditions such as precipitation, soil N pools, and storm size. These findings emphasize the hydrological controls on N export from agricultural watersheds, and highlight the variations in source supply and transport pathways among different N species. The insights gained from this study hold significance for managing agricultural pollution and restoring impaired aquatic systems.
摘要：
风暴引起的农业流域氮（N）的输出显着影响水生生态系统，然而，氮源供应和运输背后的机制仍不清楚。在这里，我们调查了影响中国柚子农业流域河流氮物种出口时间和大小的水文因素。我们对流域水文进行了连续观测，N种,在2018-2019年的两次风暴事件中，它们沿着土壤-地下水-河流连续体的同位素比。我们发现，在暴风雨期间，NO3-N河的出口通量占总N通量的80%，并为其他N物种休息。我们的结果进一步揭示了不同氮物种之间不同的氮运输途径和时间。特别是在铵N（NH4-N）和硝酸盐N（NO3-N）之间。暴雨中的NH4-N主要来源于污水和土壤渗滤液，通过地表径流和汇流排快速运输。果园施肥（根据SIAR分析贡献了41-56％）是NO3-N河的主要来源，最初通过地表径流稀释，随后通过土壤渗滤液和地下水的延迟排放而富集。风暴之间N传输的时间和大小的变化可以通过降水等先决条件来解释，土壤氮库，风暴的大小。这些发现强调了对农业流域氮素出口的水文控制，并强调了不同氮物种之间来源供应和运输途径的变化。从这项研究中获得的见解对于管理农业污染和恢复受损的水生系统具有重要意义。
