Integrating stable isotopes and hydrological models to track nitrogen sources and transport pathways in plateau watersheds: a case study in Southwest China

Exogenous nitrogen inputs from agriculture and anthropogenic activities have dramatically altered the material cycling processes in the Plateau watershed, leading to a range of water pollution issues. Effective management of nitrogen pollution in water bodies is predicated on clarifying N export loads under different pathways in the watershed, as well as the contributions of different sources. Here, this study proposes an integrated framework that introduces multiple stable isotope techniques (δD-H₂O, δ¹⁸O-H₂O, δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) based on the coupled Eckhardt's digital baseflow filter (ECK) and load estimation model (LOADEST). The integrated approach was applied for the first time in a typical plateau watershed in southwest China. Results showed that baseflow as the Fengyu River watershed (FRW) major hydrologic pathway, provides 69.6 % of the mean annual stream flow and 59.1 % of the mean annual NO₃⁻-N load. Furthermore, the FRW average annual TN and NO₃⁻-N export is 94.0 t and 55.1 t, respectively. The NO₃⁻-N was the primary form of N pollutant, with its average concentration in groundwater being 7 times that in river water. In groundwater, manure and sewage (M&S) and soil nitrogen (SN) contribution rates to NO₃⁻-N 53.6 %, and 37.2 %, respectively. While the river water shows low M&S (26.8 %) and high SN (61.5 %) characteristics. It can be seen that the baseflow is a key pathway for coupled water-nitrogen export from plateau agricultural watersheds. Blocking the migration of nitrogen-containing pollutants to groundwater is an important measure to control the degradation of the water environment in plateau watersheds from the root cause.