Climatic and anthropogenic driving forces of the nitrogen cycling in a subtropical river basin

To date, basin-scale understanding of nitrogen (N) cycling is lacking, which undermines riverine N pollution control efforts. Applying a multiple-isotopic approach, this study provided insights into the impacts of climate and anthropogenic activities on the N cycling at a basin scale. The isotopic compositions of the river water were regulated by a simple mixing process in winter, while unconservative processes (nitrification and denitrification) occurred in warm seasons. Denitrification dominated the N transformations in summer, while coupled nitrification-denitrification in soils after fertilization was responsible for the isotopic fractionations in spring and autumn. While at least 58.7% of the nitrate (NO₃^-) was removed from the basin, the NO₃^- loadings in the river remained high, suggesting that the ecosystem services could not balance the anthropogenic pollution. After correcting the isotopic fractionations, the sources of the riverine NO₃^- were quantified by a Markov chain Monte Carlo isotope mixing model. The contributions of point sources versus non-point sources changed dynamically with the precipitation and fertilization patterns. In summer and autumn, the soil organic N and chemical fertilizer dominated the riverine NO₃^-, with total contributions of 75.9% and 74.6%, respectively. The contributions from sewage and manure significantly increased during spring (47.9%) and winter (50.2%). Overall, the annual NO₃^- fluxes were from SON (28.7%), CF (28.1%), DS (18.2%), MA (23.9%), and AP (1.1%). In addition, we presented the large uncertainties in source apportionment that arose from the ignorance of isotope fractionations, highlighting the importance of considering the effect of isotopic fractionations in N source apportionment studies.