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

A cruise covering two transects in the Changjiang (Yangtze) estuary in July 2017 was conducted, aiming to explore the sources for riverine NO3- and identify reactions involved in the NO3- transformations along the transport of the Changjiang diluted water (CDW). In the river water, NO3- was fundamentally contributed by chemical fertilizer leakage in the watershed according to isotope signals. Sewage discharge may also be significant on riverine NO3- inventory, while the isotope signal was masked by nitrification. Together with the transport of the CDW, NO3- 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 NO3- concentration was observed in the surface water. In parallel, enrichment in δ15N-NO3- and δ18N-NO3- was found, indicating biological consumption by phytoplankton. The difference in the stratification intensity between two transects led to variations in NO3- concentrations and isotope compositions. In the benthic water, denitrification (sediment-water interface) and nitrification (bottom water) coexisted. Furthermore, accumulations of NH4+ and dissolved organic nitrogen in the bottom water were observed, indicating that nitrification was constrained by oxidant (mainly dissolved oxygen) supplies.