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

Abstract

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., NH4+ ). Thus, the excessive NO3- might be reduced to NH4+ by dissimilatory nitrate reduction to ammonium, rather than to N2 by denitrification. NO3- 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.