Multi-isotope (15N, 18O and 13C) indicators of sources and fate of nitrate in the upper stream of Chaobai River, Beijing, China
Dual isotopes of nitrate (15N and 18O) and carbon isotopes of dissolved inorganic carbon (13C) together with water chemistry were used to identify the sources and fate of nitrate in the upper stream of Chaobai River, north China. The results show that NO3− concentrations ranges from 0.03 mmol L−1 to 0.80 mmol L−1. Sampling sites from watershed with dominant forest land had higher NO3− concentrations and lower δ15N–NO3− (<10‰) in the wet season than in the dry season, while those from watershed with more anthropogenic activities had lower NO3− concentrations and higher δ15N–NO3− (>10‰) in the wet season. Compositions of isotopes and chemistry indicated that NO3− originated mainly from soil N, sewage and livestock wastes and atmospheric nitrogen. Furthermore, the mixing model suggested that soil N was the major NO3− source in the wet season, while the sewage and livestock wastes contributed the most in the dry season. Compared to rivers, the Miyun Reservoir had a higher contribution of atmospheric N and the N input from the upper rivers exerted significant influence over the reservoir. Mineralization and nitrification played an important role in N biogeochemistry based on the isotopes (15N and 18O and 13C) and chemical data. There appeared to be no significant denitrification in the watershed according to the three isotopes and chemical ions. The combined use of 15N, 18O and 13C proved to be useful for further identification of the sources and fate of nitrate in watersheds with dominant forest land in the wet season.