Using a nitrogen and oxygen isotopic approach to identify nitrate sources and cycling in the Wei River of northwestern China

The Wei River is the largest tributary of the Yellow River in China. To understand the sources and cycling of nitrate in the Wei River, we determined the concentrations and nitrogen and oxygen isotopic values of nitrate from water samples. Our results revealed that NO3−-N dominated the inorganic N and ranged from 0.1 to 8.8 mg/L (averaging 3.3 mg/L). Although this NO3−-N concentration does not exceed the World Health Organization's drinking water standard of 10 mg/L, the NO3−-N content of most water samples exceeded 3 mg/L, indicating poor water quality. The NO3−-N concentrations and δ15N-NO3− values demonstrate that there are significant differences in the spatial distribution of nitrogen between the tributaries and the main stream of the Wei River. In addition, a negative linear relationship (r2 = 0.63) between NO3−-N concentrations and δ18O-NO3− values suggests mixing between two distinct sources (fertilizer and manure or sewage). Furthermore, we infer that the main source of nitrate is not manure or sewage itself, but rather the nitrification of NH4+ in manure and sewage. Finally, no obvious denitrification processes were observed. These results expand our understanding of sewage as a major source of nitrate to the Wei River, emphasizing the role of nitrification.