Environmental nitrogen losses in winter wheat across yield zones under long-term precipitation variability

Nitrogen (N) loss dynamics from winter wheat in humid agroecosystems remain uncertain due to variability in soil characteristics across yield zones and high interannual precipitation variability. We hypothesized that dominant N loss pathways vary significantly across yield zones, driven primarily by the interaction of soil hydrology and rainfall patterns. The objective of this study was to integrate field experimentation with process-based modeling to identify and quantify environmental N loss (N_envloss) pathways across contrasting yield zones under long-term climatic variability. In this study, a commercial winter wheat field in the Southeastern U.S. was delineated into high-yielding (HYZ) and low-yielding (LYZ) zones based on the historic yield data to explicitly account for within-field heterogeneity. The DSSAT CSM-CERES-Wheat model was calibrated and evaluated using two growing seasons (2022-23 and 2023-24) and subsequently utilized to simulate soil N balance and long-term N losses over 32 years classified into drought, normal, and wet years using the Standardized Precipitation Index (SPI). The model evaluation showed robust performance in simulating crop phenology, biomass accumulation, grain yield, soil water dynamics, and crop N uptake across yield zones, with normalized RMSE values of 6-7% for aboveground biomass, 17% for aboveground N, and 7-25% for soil moisture content. The nitrate (NO₃^-) leaching was identified as the dominant N_envloss pathway across both seasons, accounting for 94-98% of total simulated N losses, whereas volatilization (1-2 kg[N] ha^-1) and denitrification (0.4-1.0 kg[N] ha^-1) contributed small fractions. The HYZ consistently showed higher leaching losses (39-98 kg[N] ha^-1) than LYZ (69-70 kg[N] ha^-1), driven by higher initial soil mineral N (N_smn) at planting and higher soil permeability that facilitated rapid drainage, rather than by variations in crop N uptake, which were statistically insignificant between zones. The long-term simulations revealed that interannual precipitation variability substantially regulated N losses, with average leaching increasing from 65 ± 12 kg[N] ha^-1 in drought years to 96 ± 21 kg[N] ha^-1 in wet years. In contrast, volatilization losses (6.1 ± 10.1 kg[N] ha^-1) increased during drought conditions. These findings demonstrated that soil hydrological properties and climate variability primarily govern N_envloss in winter wheat systems. This highlights the need for weather-responsive, site-specific N management strategies in humid regions.