Nitrate in the Changjiang diluted water: an isotopic evaluation on sources and reaction pathways

cruise covering two transects in the Changjiang (Yangtze) estuary in July 2017 was conducted, aiming to explore the sources for riverine NO_3^- and identify reactions involved in the NO_3^- transformations along the transport of the Changjiang diluted water (CDW). In the river water, NO_3^- was fundamentally contributed by chemical fertilizer leakage in the watershed according to isotope signals. Sewage discharge may also be significant on riverine NO_3^- inventory, while the isotope signal was masked by nitrification. Together with the transport of the CDW, NO_3^- production was observed in waters with low salinities (<20) and high turbidities. Nitrification resulted from the mineralization of riverine organic nitrogen; therefore, the high turbidity was linked to active production. In the outer plume, coupled with stratification, a significant decrease in NO_3^- concentration was observed in the surface water. In parallel, enrichment in δ^15N-NO_3^- and δ^18N-NO_3^- was found, indicating biological consumption by phytoplankton. The difference in the stratification intensity between two transects led to variations in NO_3^- concentrations and isotope compositions. In the benthic water, denitrification (sediment-water interface) and nitrification (bottom water) coexisted. Furthermore, accumulations of NH_4^+ and dissolved organic nitrogen in the bottom water were observed, indicating that nitrification was constrained by oxidant (mainly dissolved oxygen) supplies.