Field-Based Quantification of Isotope Effects during Nitrogen Transformations Using High-Resolution Geochemical Monitoring and Numerical Modeling at an Agricultural Site

Nitrogen transformations in agricultural soils strongly influence nitrate leaching, greenhouse gas emissions, and water quality. However, quantifying these processes under field conditions remains challenging because microbial reactions and hydrological transport interact across variable redox, temperature, and pH conditions. Stable isotopes are powerful tracers of nitrogen cycling, yet the isotope effects that govern these processes are often derived from laboratory studies or inferred from aggregated field observations, neither of which fully captures natural subsurface complexity. This study presents a field-based framework that integrates high-resolution geochemical monitoring and physically based numerical modeling to constrain isotope effects of nitrogen transformations under natural conditions. Results indicate that denitrification isotope effects are systematically influenced by redox potential, temperature, and pH. Smaller effects occur under strongly reducing conditions, where denitrification proceeds rapidly, whereas larger effects emerge under more oxidizing conditions. Isotope effects derived from this study are generally lower than typical laboratory estimates, reflecting the influence of subsurface heterogeneity. In contrast, isotope effects related to nitrification could not be robustly constrained, highlighting the need for additional isotopic and process-level measurements. Overall, this approach helps improve the assessment of nitrate attenuation in heterogeneous subsurface environments by providing field-based isotope effect estimates that better reflect natural variability.