Sources and fate of nitrate in the unsaturated zone in an alluvial-lacustrine plain

Nitrate pollution in terrestrial and aquatic ecosystems in global agricultural areas poses an environmental concern. However, there is limited understanding of hydrogeological controls on the behavior of nitrogen compounds in unsaturated zones. Here, Self-Organizing Map and multiple isotopes approaches (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, and δ¹⁵N-NH₄⁺) were used to investigate the sources, transport and transformation of N-species in the unsaturated zone in an alluvial-lacustrine plain, southeast China. The results revealed significant spatial heterogeneity in soil texture and physicochemical properties with vertically four soil geochemical and N-species zones (high NO₃⁻, high Fe(Ⅲ) and Mn, low ionic, and high NH₄⁺ contents), dominated by agricultural input, soil minerals and redox conditions. Nitrate in the unsaturated zone primarily originated from fertilizers and soil nitrogen. Excess nitrogen fertilizers infiltrated into the soil, where mineralization, nitrification, and dissimilatory nitrate reduction to ammonium (DNRA) acted as key mechanisms for nitrogen transformation. The change in the depositional environment from the plain to the lakeshore area led to nitrification gradual decrease and DNRA significant increase. Consequently, a conceptual model of reactive transport of N-species, influenced by hydrogeologic conditions and biogeochemical processes, was proposed. This study provides a new insight into the nitrate behaviors in unsaturated zone and contributes to groundwater nitrogen management strategies.