Soil Hydraulic Properties Control Nitrate Leaching Under Continuous Rice Cultivation: Insights from Local Hydro-Pedotransfer Modeling and HYDRUS-1D

Nitrogen fertilization in flooded rice systems can promote nitrate leaching and groundwater contamination, particularly where groundwater is used for human consumption. This study aimed to assess soil susceptibility to nitrate leaching toward groundwater in rice-growing areas of Rioja, San Martín, Peru. Sampling was based on a semidetailed soil map of the Rioja sector, from which 14 soil series were identified; one georeferenced point was established per series, with soil collected at two depths (0–20 and 20–40 cm). From these points, four representative soil profiles were selected through a Delphi consensus process and classified as Typic Endoaquepts (P1 and P2), a Typic Endoaquoll (P3), and a Fluventic Endoaquept (P4). The 28 saturated hydraulic conductivity (Ks) measurements were used to develop a local pedotransfer function, in which the inverse of bulk density was the main predictor of Ks. Profile-specific Ks estimates were incorporated into HYDRUS-1D to simulate nitrate transport under an identical fertilization regime over 10 years. Ks averaged 0.21 cm·h−1 with high variability (CV = 84.12%), and the pedotransfer function explained 42.9% of this variability (R2adj = 0.383, RMSE = 0.127 cm·h−1). Despite receiving identical fertilizer inputs, the four profiles showed simulated nitrate transport differing by up to 19.9%, driven by differences in soil hydraulic properties and profile structure rather than by management practices. Flow-weighted mean nitrate concentrations at the lower model boundary ranged from 48.33 to 57.96 mg·L−1; two profiles (P4 and P1) exceeded the 50 mg·L−1 WHO guideline, reflecting a greater leaching risk in soils that are more stratified and have higher hydraulic conductivity, while the profile with the highest organic carbon content (P3) showed lower susceptibility, likely due to greater denitrification. These findings indicate that local soil properties are a key control on nitrate leaching susceptibility, supporting the need for soil-type-specific nitrogen management to protect groundwater resources.