Rainstorms peak governs nitrate export pathways and patterns: Insights from high-frequency sampling and stable...

Rainstorm events can flush large amounts of nitrate from agricultural lands into aquatic environments, posing significant threats to drinking water quality and ecosystem health. However, how intra-event rainstorm structure governed hydrological responses and nitrate transport remain poorly understood. Here, we conducted high-frequency monitoring and sampling to determine nitrate export pathways and patterns during 20 rainstorm events. Intra-event rainstorm regime was classified as early-, center-, and late-peak regimes based on the co-occurrence position of the fluctuation index of rainstorm peak and the maximum 30 min rainstorm intensity. Stable water isotopes technique was used to partition nitrate export pathways (soil water, groundwater, rainwater), concentration-discharge relationships further revealed nitrate export patterns under different rainstorm regimes. The results indicated that rainwater (16.7%) and shallow soil water (21.6%) served as the primary pathway of nitrate export for early-peak rainstorm events, with comparable contributions from dilution (46%) and chemostatic behavior (36%). In center-peak rainstorm events, deep soil water was the dominant export pathway of nitrate (42.1%), with chemostatic behavior dominating (75%) and resulting in the highest nitrate concentrations (4.97 mg L^-1). In contrast, late-peak rainstorms exhibited comparable contributions from different soil layers, promoting dilution behavior (60%), corresponding to lower nitrate concentrations (3.00 mg L^-1). These results indicate that the intra-event rainstorm structure can alter nitrate export pathways and patterns, involving more complex activation patterns of hydrological pathways than previously recognized. Understanding these dynamics provides a scientific foundation for developing watershed management strategies aimed at controlling nitrate pollution in aquatic ecosystems.