Triple oxygen isotopic evidence for atmospheric nitrate and its application in source identification for river systems in the Qinghai-Tibetan Plateau

Nitrate source identification in river systems is important for water quality management. Recently, the oxygen isotopic anomaly of nitrate in atmospheric deposition (Δ¹⁷O_atm) is used to identify unprocessed atmospheric nitrate in river systems to reduce the uncertainty caused by the wide range of δ¹⁸O. In high-elevation regions, such as the Qinghai-Tibetan Plateau (QTP) featured with lower temperature and pressure as well as strong radiation, the Δ¹⁷O_atm might be different from that in low-elevation regions, but no relevant studies have been reported. In this work, Δ¹⁷O_atm in the QTP was studied, and the fingerprints of nitrate isotopes in synthetic fertilizer, livestock manure, domestic sewage, and soil organic nitrogen (SON) were identified and used to quantify various source contributions to riverine nitrate in the Yellow River and Changjiang River source regions located in the QTP during 2016–2017. The results showed that the average of Δ¹⁷O_atm in the QTP was 16.4‰, lower than the range (19–30‰) reported for the low-elevation regions. The possible mechanism is decreased O₃ as well as increased hydroxyl and peroxy radical levels in the troposphere caused by the climate condition and ozone valley in the QTP will affect the production pathways of atmospheric nitrate. By combining the sewage discharge data with the output results of the SIAR (stable isotope analysis in R) model based on the stable isotope data, manure was determined to be one of the major sources to riverine nitrate for both rivers. The contributions of various sources to riverine nitrate were 47 ± 10% for manure, 30 ± 5% for SON, 10 ± 4% for atmospheric precipitation, 9 ± 2% for synthetic fertilizer, and 4 ± 0% for sewage in the Yellow River source region. This study indicates that the unique atmospheric conditions in the QTP have led to a lower Δ¹⁷O_atm value, and atmospheric source makes a considerable contribution to riverine nitrate in the QTP.