Orchard Expansion May Diminish Groundwater Replenishment While Amplifying Nitrate Pollution in a Cultivated Loess Critical Zone

Abstract Agricultural land use and management changes significantly alter water and nitrate (NO 3 − ) transport in the vadose zone (VZ) of the Earth's Critical Zone (CZ), thereby affecting groundwater recharge and quality. Here, we developed a multi‐column modeling approach to estimate recharge and NO 3 − transport in the cultivated loess CZ of China's Guanzhong Plain (CGP), with a specific focus on the cropland‐to‐orchard transition. The model also quantified uncertainties in water and NO 3 − fluxes caused by variability in soil hydraulic parameters (SHPs). Evaluation against observations from 12 sites demonstrate good model performance. Relative to measured SHPs, uncertainties in groundwater recharge and NO 3 − leaching fluxes ranged from 3% to 86% when using SHPs derived from Rosetta and global data sets, with higher uncertainties in orchards than in croplands. Simulations based on measured SHPs identified the central and eastern CGP as hotpots of groundwater NO 3 − contamination. The shift from corn‐wheat rotation to apple orchards increased NO 3 − leaching fluxes by 38 times while reducing groundwater recharge by 10%. Under both land‐use scenarios, NO 3 − travel times through the VZ spanned decades to centuries, and the cropland‐to‐orchard transition extended it by 23 years for NO 3 − to reach the aquifer. Although this conversion delays NO 3 − transport to the aquifer, the elevated leaching flux increases the risk of groundwater NO 3 − pollution, especially in regions with shallow VZs and coarse soil texture. This study highlights the critical need for caution when implementing large‐scale cropland‐to‐orchard conversions in the CGP and provides important insights for groundwater vulnerability assessments in regions with comparable hydrogeological and agricultural conditions.