Decadal-scale nitrate legacy in loess soils: Storage, residence time, and groundwater coupling.

Intensive agriculture can transform the vadose zone into a long-term reservoir of legacy nitrate (NO3-). However, the magnitude, residence time, and release dynamics of this legacy remain poorly quantified in thick loess agricultural systems. Here, we developed a high-resolution, coupled modeling framework that integrates a multiple-column vadose-zone model with MODFLOW-MT3D to reconstruct 41-year (1982-2022) NO3- dynamics across the Guanzhong Plain, where widespread cropland-to-orchard conversion has occurred since the 1990s. The model was constrained by extensive soil hydraulic measurements, profile NO3- observations, and groundwater records. Vadose-zone NO3- storage increased nearly 11-fold during agricultural intensification, from 670 kt N in 1982 to 7631 kt N in 2022. In 2022, annual NO3- leaching accounted for only 0.6% of the stored pool, revealing a strong storage-leaching imbalance and indicating that most accumulated NO3- remains retained within the deep loess profile. Simulated NO3- residence times ranged from 10 to >100 years and were governed primarily by vadose-zone thickness. Groundwater NO3- storage exhibited a delayed U-shaped response, declining to 3190 kt N in 1998 before increasing to 3328 kt N by 2022, even though fertilizer application had been substantially reduced after 2011. This delayed response demonstrates that the loess vadose zone regulates the timing and magnitude of groundwater contamination over multi-decadal to centennial timescales. These findings highlight the central role of soil profile properties and thickness in controlling NO3- legacy behavior, and suggest that effective NO3- mitigation in loess agricultural regions must account for vadose-zone storage and residence time as integral components of soil system management.