Seasonal variations and controlling factors of nitrogen fluxes at the sediment-water interface in a semi-enclosed inland sea

Abstract. Nitrogen fluxes across the sediment-water interface and nitrogen removal from sediments are essential components of nitrogen cycle in semi-enclosed inland seas. However, the difficulty in observational sampling hinders continuous data availability that is necessary to understand their seasonal variations and underlying mechanisms. To address this issue, we developed a one-dimensional sediment nitrogen-cycle model and used sensitivity experiments to quantify the relative roles of environmental and biogeochemical drivers. Model results indicate that 48 % of particulate organic nitrogen (PON) settling into sediments is returned to the bottom water as dissolved inorganic nitrogen (DIN), while 6 % is removed via N-loss flux (dinitrogen gas and nitrous oxide). The seasonal variations of PON and DIN fluxes are controlled by fundamentally different mechanisms, with PON flux primarily regulated by bottom-water PON concentration and bottom stress, while DIN flux is mainly governed by temperature-dependent biogeochemical transformations and nitrogen availability. These contrasting responses reveal a decoupling between particulate and dissolved nitrogen fluxes, reflecting the buffering capacity of sediments. Denitrification controls nitrogen removal but its amount is not large because the oligotrophic conditions of the study site limits the nitrate availability.