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 the nitrogen cycle and ecosystem in semi-enclosed inland seas. However, the difficulty in observational sampling hinders the acquisition of continuous data necessary to understand their seasonal variations and underlying mechanisms. In response to this issue, we have developed a one-dimensional vertical model of the nitrogen cycle within sediments and used it to reproduce the seasonal changes of observed nitrogen fluxes in a typical semi-enclosed inland sea and investigate their controlling factors through sensitivity experiments. Model results indicate that 40 % of particulate organic nitrogen (PON) settling into sediments is returned to the bottom water as dissolved inorganic nitrogen (DIN), while 30 % is removed via N-loss flux (dinitrogen gas and nitrous oxide). Although PON flux is controlled by PON concentration in the bottom water, DIN and N-loss fluxes show temperature-driven seasonal variations, suggesting a decoupling between nitrogen return and PON input. Additionally, seasonal variations in oxygen penetration depth (OPD), ranging from 1 to 3 mm, also affect nitrogen fluxes. In nitrate-depleted sediments of semi-enclosed seas, the denitrification rate is no longer significantly higher than the anammox rate in the nitrogen removal.