Removal of High Concentrations of Ammonium in the Subterranean Estuary of the Yangtze River

Excessive NH4+ is detrimental to human health and the ecological environment. Although a large amount of NH4+ is discharged from the subterranean estuary (STE) of the Yangtze River into the ocean, the sources and removal processes of high concentrations of NH4+ in the STE remain underexplored. We analyzed multiple isotopes (δ15N-NH4+, δ15N-NO3−, δ18O-NO3−, δ18O−H2O, δ2H−H2O, and δ13C-DIC), geochemical data, and microorganisms to investigate the sources and removal processes of high concentrations of NH4+ and the crucial role of low concentrations of NO3− in the STE of the Yangtze River. Our results indicated that terrestrial ammonium transport and anammox are the primary source and removal process, respectively, of high concentrations of NH4+ in the STE. Moreover, the real removal of NH4+ exceeds 55%, with a removal rate greater than 300 μmol/L/m. Additionally, NH4+ removal involves two mechanisms (SO42−-driven and carbon-driven processes), but carbon-driven removal may be the main NH4+ removal process in the STE. We also revealed that a low concentration of NO3− plays a crucial role in driving the complete nitrogen cycle in the STE. Our study contributes to an improved understanding of the fate of high concentrations of ammonium in the STE and is also advantageous for coastal ecological management and harm prevention.