The flux of dissolved inorganic nitrogen (DIN), predominantly nitrate (NO 3 - ) and ammonium (NH 4 + ), from land to coastal waters via rivers is commonly estimated simply by multiplying water flux with nitrogen concentration. Understanding DIN fluxes in gated estuaries is critical as these systems often serve as hotspots for nutrient transformations, influencing coastal water quality and ecosystem health. However, the subsequent interactions involving NO 3 - and NH 4 + adsorption or desorption on suspended sediments are often overlooked. To better understand the impact of these interactions on the overall NO 3 - and NH 4 + sorption or desorption and subsequently, the mobility and transport to the coastal zone, we conducted a series of NO 3 - and NH 4 + adsorption and desorption experiments. These experiments involved varying suspended sediment concentrations, particle sizes, salinities, and sea-salt ions to assess their potential effects. Results indicate that desorption of NO 3 - and NH 4 + from suspended sediments is more prominent than adsorption, with NH 4 + desorption being particularly significant. Notably, at low suspended particle concentrations and high salinity, NH 4 + desorption from sediments increased markedly, which further amplified in polyhaline conditions. This effect could result from ion pairing between NH 4 + and seawater anions, along with competition from seawater cations for sediment cation exchange sites, enhancing NH 4 + diffusion from estuarine sediments, and the elevated NH 4 + release could promote DIN transport to nearshore waters, especially in gated estuaries where sediment resuspension occurs. Given the critical role of NH 4 + in estuarine nitrogen cycling, ignoring these dynamics could lead to underestimations of DIN transport in river-estuary systems. Therefore, incorporating sediment dynamics into DIN flux estimations is crucial for accurately assessing nitrogen transport in gated estuaries.
