Ammonium and N₂O production pathways in quaternary hill-plain groundwater: Evidence from multi-isotope and isotopomer analysis

Ammonium (NH₄⁺) and nitrous oxide (N₂O) are key reactive nitrogen species in aquatic systems, with implications for water quality and greenhouse gas emissions. However, their sources and transformation pathways in shallow groundwater remain poorly constrained, especially for dynamic redox groundwater in hill-plain transition zones. To address this gap, we investigated nitrogen sources and cycling in the piedmont hilly-plain zone of the Jianghan Plain (central China) by integrating hydrochemistry, nitrogen isotopes (δ¹⁵NNH₄⁺, δ¹⁵NNO₃⁻, δ¹⁸ONO₃⁻), and N₂O isotopomers (δ¹⁵N, δ¹⁸O, and site preference). A total of 56 water samples were collected during pre- and post-monsoon periods from rivers, lakes, and shallow aquifers. Previously unrecognized spatial patterns and underlying mechanisms controlling nitrate, ammonium, and N₂O along the groundwater flow path from hilly recharge zones to plain discharge areas are identified. δ²H-δ¹⁸O signatures showed that groundwater primarily derives from precipitation, with stronger evaporation effects in surface water. Dissolved N₂O concentrations were significantly higher in groundwater than in surface water, suggesting active subsurface nitrogen cycling. N₂O site preference (SP) and δ¹⁸O-N₂O values revealed coexistence of nitrification and denitrification, with enhanced N₂O reduction in plain aquifers exhibiting more reducing conditions. We quantitatively characterized the distinct N₂O production pathways across the hilly-plain transition zone, revealing the spatial variability in the contributions of nitrification and denitrification to N₂O generation. These results demonstrate that coupling hydro-geochemistry with N₂O isotopomer analysis provides a powerful approach for resolving nitrogen sources and transformation pathways in hydrogeologically complex zones. The findings offer critical insights for managing nitrogen pollution in rapidly developing agricultural plains.