Sedimentary nitrogen dynamics in a coastal reef area with relatively high nitrogen concentration

The migration and transformation of nitrogen (N) in sediments play an important role in regulating the N concentration and nutrient structures in shallow seas. However, studies of sedimentary N dynamics are rarely focused on carbonate sediments, although these account for about 40% of the continental shelf area. Thus, the regulation mechanisms of the N dynamics in the carbonate sands of coral reefs are not clear. Taking the coral reef area of Weizhou Island, which has a relatively high N concentration, as the research object, we conducted a series of flow-through reactor experiments to investigate the fluxes of different N forms at the interface of sediment and seawater and their regulation mechanism by environmental factors. The fluxes of dissolved inorganic and organic N (DIN and DON) at different stations were −0.39–0.12 mmol/(m²·h) and −0.18–0.39 mmol/(m²·h), respectively. Denitrification (0.11–0.25 mmol/(m²·h)) was closely coupled to nitrification, which was limited by the availability of organic matter and its degradation product (i.e., ${\rm{NH}}_4^+ $). Thus, the excessive ${\rm{NO}}_3^– $ might be reduced to ${\rm{NH}}_4^+ $ by dissimilatory nitrate reduction to ammonium, rather than to N₂ by denitrification. ${\rm{NO}}_3^– $ reduction peaked at intermediate advection rates (96 L/(m²·h)) and flow path lengths (10 cm), but the release of DON also peaked at the same condition. In addition, climate warming would significantly affect sedimentary N dynamics at Weizhou Island. These results may help address the broader issue of the N cycle in coral reef ecosystems under the dual pressure of climate warming and anthropogenic activities, and these results are beneficial to coral reef protection and local ecological management.