Transformation and source identification of N in the upper reaches of the Han River basin, China: evaluated by a stable isotope approach

Given the spatial and temporal variability in hydrological conditions and nitrogen (N) processes, it is of great uncertainty to identify the N sources and evaluate N transformation processes in the upper Han River. Investigations were conducted in November 2015 and January, April, and July 2016, using an isotopic method and water quality monitoring. The significant and positive correlation between NO3− concentrations and Cl− (p < 0.01) in most sampling months suggested that the great influence of human activities and sewage or manure was the dominant NO3− source. The δ15NO3− values and NO3−/Cl− variations indicated that riverine N mainly came from soil organic N and sewage in November. Fertilizer and sewage were the major N sources in January and April, respectively. In July, water was influenced by various N inputs. The nitrification process played an important role in the low δ15NO3− values in January, while both nitrification and plant uptake resulted in the increase in δ15NH4+ values in April. The simultaneous effect of N fixation and plant uptake maintained the stabilization of δ15NH4+ concentrations. Our study provides theoretical basis on N sources and transformations for controlling N pollution and improving water quality in the upper Han River in the near future.