Coupled Hydrologic and Biogeochemical Responses of Nitrate Export in a Tile‐Drained Agricultural Watershed Revealed by SAS Functions and Nitrate Isotopes

Abstract The combination of high nitrogen (N) inputs on tile‐drained agricultural watersheds contributes to excessive nitrate (NO 3 − ) loss to surface‐ and groundwater systems. This study combined water age modeling based on StorAge Selection functions and NO 3 − isotopic analysis to examine the underlying mechanisms driving NO 3 − export in an intensively tile‐drained mesoscale watershed typical of the U.S. Upper Midwest. The water age modeling revealed a pronounced inverse storage effect and strong young water preference under high‐flow conditions, emphasizing evolving water mixing behavior driven by groundwater fluctuation and tile drain activation. Integrating NO 3 − concentration‐isotope‐discharge relationships with water age dynamics disentangled the interactions between flow path variations and subsurface N cycling in shaping seasonally variable NO 3 − export regimes at the watershed scale. Based on these results, a simple transit time‐based and isotope‐aided NO 3 − transport model was developed to estimate the timescales of watershed‐scale NO 3 − reactive transport. Model results demonstrated variable NO 3 − source availability and a wetness dependence for denitrification, indicating that interannual NO 3 − chemostasis is driven by coupled and proportional responses of soil NO 3 − production, denitrification, and flow path activation to varying antecedent wetness conditions. These findings suggest that intensively tile‐drained Midwestern agricultural watersheds function as both N transporters and transformers and may respond to large‐scale mitigation efforts within a relatively short timeframe. Collectively, the results of this study demonstrate the potential of integrated water age modeling and NO 3 − isotopic analysis to advance the understanding of macroscale principles governing coupled watershed hydrologic and N biogeochemical functions.