Unveiling the biogeochemical mechanism of nitrate in the vadose zone-groundwater system: Insights from integrated microbiology, isotope techniques, and hydrogeochemistry

Clarifying the biogeochemical mechanism of nitrate (NO3−) in the vadose zone-groundwater system, particularly in agricultural contexts, is crucial for mitigating groundwater NO3− pollution. However, comprehensive studies on the impacts of changes in chemical indicators and microbial communities on NO3− are still lacking. This paper aims to address this gap by employing hydrogeochemistry, stable isotopes, and microbial techniques to assess the NO3− biogeochemical processes in the vadose zone-groundwater system. The findings suggested that NO3− in upper soil layers was primarily influenced by plant root absorption, assimilation, and nitrification processes. The oxygen contents gradually decreased with the nitrification process, resulting in the occurrence of the denitrification. However, denitrification predominantly occurred in the 60–80 cm soil layer in the study area. The limited thickness of the denitrification layer results in less NO3− consumption, leading to increased NO3− leaching into groundwater. Hydrochemical and isotopic characteristics further indicated that groundwater NO3− concentrations were mainly controlled by nitrification, followed by denitrification and mixing processes. The 16S rRNA sequencing analysis revealed great influences of soil sampling depths and groundwater NO3− concentrations on the microbial community structure. Additionally, the PICRUSt2-based prediction results demonstrated a stronger potential for dissimilatory reduction of NO3− to ammonium (DNRA) in both soil and groundwater compared to the other processes, potentially due to the widespread presence of the nrfH functional genes. However, the chemical indicators and isotopes used in this study did not support the occurrence of DNRA process in the vadose zone-groundwater system. This finding highlights the importance of an integrated approach combining microbiological, isotopic, and hydrogeochemical data to comprehensive understanding biogeochemical processes. The study developed a conceptual model elucidating the NO3− biogeochemical processes in the vadose zone-groundwater system within an agricultural area, contributing to enhanced comprehension and advancement of sustainable management practices for groundwater nitrogen.