Sources and transformations of nitrogen in an agricultural watershed on the Jianghan Plain, China: an integration of d15N-NH4+, d15N-NO3-, d???????18O-NO3- and a Bayesian isotope mixing model

Due to the complexity of nitrate sources and transformation processes under extensive human activities and climate change, there are a great number of uncertainties associated with ascertaining nitrogen sources and transformations in agricultural watersheds. In this study, multiple isotopes of ammonium (delta N-15-NH4+), nitrate (delta N-15-NO3-, delta O-18-NO3-), water (delta O-18-H2O) and hydrogeochemical data were integrated to identify nitrogen sources and elucidate the mechanisms of nitrogen transformation in the Quanshui River watershed, a typical agricultural watershed in central China. The characteristics of nitrate nitrogen and oxygen isotopes, lower delta N-15-NO3- values (< 9.73 parts per thousand), and the results from the Bayesian isotope mixing model showed that the main sources of nitrogen in the watershed were mostly ammonium fertilizers (AF), nitrate fertilizer (NF) and soil organic nitrogen (SON). In the fertilization season, the enrichment of delta N-15-NH4+ and the negative correlation between delta N-15-NH4+ and delta N-15-NO3- indicated that the volatilization and partial nitrification of nitrate in chemical fertilizers were the main factors affecting nitrogen isotope changes in the water. Seasonal changes in delta N-15-NH4+ and delta N-15-NO3- values can provide more direct isotopic evidence for the occurrence of nitrification, which may provide more useful insights into nitrogen transformation in future studies. Seasonal variations in delta O-18-NO3- can effectively indicate the impact of rainfall events on the nitrogen cycle.