Quantifying nitrate dynamics in deep vadose zone under irrigated farmland in the North China Plain: Insights from continuous in-situ monitoring

The intensive agricultural regions of the North China Plain (NCP) have experienced significant nitrogen accumulation in the thick vadose zone due to prolonged excessive fertilizer use, threatening groundwater quality. However, Current understanding of nitrate transport and transformation dynamics within deep vadose zone remains inadequate, constraining accurate groundwater risk assessments. This study investigated the nitrate accumulation, leaching, and transformation processes in the deep vadose zone of a typical irrigated crop field in the NCP through continuous in-situ monitoring and multi-year sampling based on a 48-meter-deep observing caisson. Results revealed substantial NO₃-N accumulation, reaching 6951.5 kg ha^-1, with an annual increase of 90 kg ha^-1. Nitrate leaching velocity was estimated at 0.70 m year^-1, corresponding to a nitrate flux of 118.6 kg ha^-1 year^-1. The increase of extreme precipitation events in recent years have accelerated solute transport, heightening the risk of groundwater contamination. Denitrification potential was identified based on nitrate isotopic signatures and denitrification gene abundance in the deep vadose zone, but its intensity was limited by low organic carbon availability. The Structural Equation Model (SEM) identified soil water content, nitrogen content, and soil permeability as key factors influencing nitrate leaching flux, with the microbial-mediated denitrification exhibiting a secondary but significant regulatory effect. These findings extended the research depth of the thick vadose zone and enhanced the understanding of nitrate dynamics, which provided a foundation for predicting and mitigating groundwater nitrate contamination risks in intensively managed agricultural regions.