Installation of subsurface drainage systems has profoundly altered the nitrogen cycle in agricultural regions across the globe, facilitating substantial loss of nitrate (NO3-) to surface water systems. Lack of understanding of the sources and processes controlling NO3- loss from tile-drained agroecosystems hinders the development of management strategies aimed at reducing this loss. The natural abundance nitrogen and oxygen isotopes of NO3- provide a valuable tool for differentiating nitrogen sources and tracking the biogeochemical transformations acting on NO3-. This study combined multi-years of tile drainage measurements with NO3- isotopic analysis to examine NO3- source and transport mechanisms in a tile-drained corn-soybean field. The tile drainage NO3- isotope data were supplemented by characterization of the nitrogen isotopic composition of potential NO3- sources (fertilizer, soil nitrogen, and crop biomass) in the field and the oxygen isotopic composition of NO3- produced by nitrification in soil incubations. The results show that NO3- isotopes in tile drainage were highly responsive to tile discharge variation and fertilizer input. After accounting for isotopic fractionations during nitrification and denitrification, the isotopic signature of tile drainage NO3- was temporally stable and similar to those of fertilizer and soybean residue during unfertilized periods. This temporal invariance in NO3- isotopic signature indicates a nitrogen legacy effect, possibly resulting from N recycling at the soil microsite scale and a large water storage for NO3- mixing. Collectively, these results demonstrate how combining field NO3- isotope data with knowledge of isotopic fractionations can reveal mechanisms controlling NO3- cycling and transport under complex field conditions.

The combination of high nitrogen (N) inputs on tile-drained agricultural watersheds contributes to excessive nitrate (NO3-) loss to surface- and groundwater systems. This study combined water age modeling based on StorAge Selection functions and NO3- isotope analysis to examine the underlying mechanisms driving NO3- 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 a strong young water preference under high-flow conditions, emphasizing evolving water mixing behavior driven by groundwater fluctuation and tile drain activation. Integrating water age dynamics with isotope-discharge relationships observed at the watershed scale illuminated interactions between flow path variations and subsurface N cycling as key drivers of variable NO3- export regimes at the seasonal scale. Based on these results, a simple transit time-based and isotope-aided NO3- transport model was developed to estimate the timescales of watershed-scale NO3- reactive transport. Model results demonstrated a wetness dependence for denitrification at the watershed scale and further suggested that interannual NO3- export regimes are controlled by coupled and proportional responses of soil N cycling and hydrologic transport to varying wetness conditions. Although uncertainties remain, the estimated transport and reaction timescales suggest a relatively minor N legacy effect for intensively tile-drained agricultural watersheds in this region. Collectively, the results of this study demonstrate the potential of integrated water age modeling and NO3- isotope analysis to advance the understanding of macroscale principles governing coupled hydrologic and N biogeochemical functions in watershed systems.

Recent theoretical advances related to time-variant water age in hydrologic systems have opened the door to a new method that probes water mixing and selection behaviors using StorAge Selection (SAS) functions. In this study, SAS functions were applied to investigate storage, water mixing behaviors, and nitrate (NO3-) export regimes in a tile-drained corn-soybean rotation field in the Midwestern United States. The natural abundance stable nitrogen and oxygen isotopes of tile drainage NO3- were also measured to provide constraints on biogeochemical NO3- transformations. The SAS functions calibrated using chloride measurements at tile drain outlets revealed a strong young water preference during tile discharge generation. The use of a time-variant SAS function for tile discharge generated unique water age dynamics that reveals an inverse storage effect driven by activation of preferential flow paths and mechanically explains the observed variations in NO3- isotopes. Combining the water age estimates with NO3- isotope fingerprinting delineated NO3- export dynamics at the tile-drain scale, where a lack of strong contrast in NO3- concentration across the soil profile results in chemostatic NO3- export regimes. For the first time, NO3- isotopes were embedded into a water age-based transport model to model reactive NO3- transport under transient conditions. Results from this modeling study provided a proof-of-concept for the potential of coupled water age modeling and NO3- isotope analysis in elucidating complex mechanisms that control the coupled water and NO3- transport. Further integration of water age theory and NO3- isotope biogeochemistry is expected to significantly improve reactive NO3- transport modeling.