Surface water and groundwater interactions contributed the seasonal NO3− and NH4+ sources and transformations in a hilly region, China

Comprehensive understanding of nitrogen (N) sources and transformations under the interactions of variable water cycle processes were of great significance for formulating nitrogen mitigation strategies in aquatic systems. Multiple stable isotopes (δ¹⁵N-NO₃^-, δ¹⁸O-NO₃^-, δ¹⁵N-NH₄⁺, δ²H-H₂O, and δ¹⁸O-H₂O) and hydrochemical characteristics in surface water (SW) and groundwater (GW) were to reveal seasonal NO₃⁻ and NH₄⁺ sources and transformations in a hilly region with frequent vertical movements in this study. The δ²H-H₂O, δ¹⁸O-H₂O, and hydrochemical analysis highlighted surface water and groundwater interactions. The correlation between δ¹⁵N-NO₃^- and d-excess demonstrated soil leaching. Nitrification was the predominant nitrogen transformation process in the watershed during both seasons although denitrification occurred in certain points. Primary source of NO₃⁻ in SW was soil nitrogen (SN; 34.5 ± 14.9 % in the dry season and 76.4 ± 8.1 % in the wet season). The contributions of various sources to NH₄⁺ in SW were relatively equal during the dry season, while GW (30.5 ± 19.3 %) and manure and sewage (M&S) (29.0 ± 15.2 %) were the primary sources during the wet season. This study proved that groundwater was a significant source for NO₃⁻ and NH₄⁺ input when utilizing source-track models, especially in hilly regions. These findings improve input sources for mixture models and identify the sources and transformations of water cycling and N biogeochemical processes.