Journal ArticleCoupled Hydrologic and Biogeochemical Responses of Nitrate Export in a Tile‐Drained Agricultural Watershed Revealed by SAS Functions and Nitrate IsotopesSha, Minghui [Department of Natural Resources and Environmental Sciences University of Illinois at Urbana‐Champaign Urbana IL USA]; Yu, Zhongjie [Department of Natural Resources and Environmental Sciences University of Illinois at Urbana‐Champaign Urbana IL USA] (ORCID:0000000249350154); Benettin, Paolo [Institute of Earth Surface Dynamics Université de Lausanne Lausanne Switzerland] (ORCID:0000000175561417); Gentry, Lowell E [Department of Natural Resources and Environmental Sciences University of Illinois at Urbana‐Champaign Urbana IL USA] (ORCID:0000000294873026); Mitchell, Corey A [Department of Natural Resources and Environmental Sciences University of Illinois at Urbana‐Champaign Urbana IL USA] (ORCID:0000000261846523)The combination of high nitrogen (N) inputs on tile‐drained agricultural watersheds contributes to excessive nitrate loss to surface‐ and groundwater systems. This study combined water age modeling based on StorAge Selection functions and nitrate isotopic analysis to examine the underlying mechanisms driving nitrate 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 nitrate concentration‐isotope‐discharge relationships with water age dynamics disentangled the interactions between flow path variations and subsurface N cycling in shaping seasonally variable nitrate export regimes at the watershed scale. Based on these results, a simple transit time‐based and isotope‐aided nitrate transport model was developed to estimate the timescales of watershed‐scale nitrate reactive transport. Model results demonstrated variable nitrate source availability and a wetness dependence for denitrification, indicating that interannual nitrate chemostasis is driven by coupled and proportional responses of soil nitrate 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 nitrate isotopic analysis to advance the understanding of macroscale principles governing coupled watershed hydrologic and N biogeochemical functions.AGU2025-10-0110662058Water Resources Research61100043-1397https://doi.org/10.1029/2024WR0397182442450National Science Foundation