Dynamic N transport and NO emission during rainfall events in the coastal river.

The coastal zone exhibited a high population density with highly impacted by anthropogenic activities, such as river impoundment to prevent saline intrusion, which resulted in weak hydrological conditions. Rainfall events can result in dramatic changes in hydrological and nutrient transportation conditions, especially in rivers with weak hydrological conditions. However, how the nitrogen transport and NO emissions or biogeochemistry responds to the different types of rainfall events in the weak hydrodynamics rivers is poorly understood. In this study, the hydrological, nitrogenous characteristic, as well as NO dynamics, were studied by high-frequency water sampling during two distinct rainfall events, high-intensity with short duration (E1) and low-intensity with long duration (E2). The results displayed that the hydrologic condition in E1 with a wider range of d-excess values (from -9.50 to 32.1 ‰), were more dynamic than those observed in E2. The NO concentrations (0.01-3.33 μmol/L) were higher during E1 compared to E2 (0.03-1.11 μmol/L), which indicated that high-intensity rainfall has a greater potential for NO emission. On the contrary, the concentrations of nitrogen (e.g., TN and NO-N) were lower during E1 compared to E2. Additionally, hysteresis was observed in both water and nitrogen components, resulting in a prolonged recovery time for pre-rainfall levels during the long-duration event. Moreover, the results showed that the higher average NO flux (78.3 μmol/m/h) in the rainfall event period was much larger than that in the non-rainfall period (1.63 μmol/m/h). The frequency dam regulation resulted in the water level fluctuation, which could enhance wet-dry alternation and simulated NO emissions. This study highlighted the characteristic of N dynamic and hydrological responses to diverse rainfall events occurrences in the coastal river. Rainfall could increase the NO emission, especially during high-intensity rainfall events, which cannot be ignored in the context of annual NO release.