Nitrogen removal and carbon mineralization under coastal salinity intrusion

Rapid population growth and intensification of human activities have led to a massive increase in the release of nitrogen (N) to the environment, often ending up in aquatic ecosystems. Coastal wetlands, a transition ecosystem in the freshwater-to-marine continuum, play a vital role in reducing nitrogen through natural processes, including denitrification and anaerobic ammonium oxidation (anammox). Considering denitrification's risk of producing nitrous oxide—a potent greenhouse gas—and anammox's efficient co-removal of ammonium and nitrite, it's crucial to identify what controls the balance between these two key processes. However, the identity of the drivers controlling the relative abundance of these two N-removal processes and their respective interactions with carbon (C) and sulfur cycles are not well-documented, especially in coastal wetlands.

This study investigated salinity's role in N reduction with carbon remineralization in coastal wetlands facing salinity intrusion. Using air-dried mangrove sediments mixed with anoxic artificial seawater of contrasting salinities (0, 10, 20, and 30 ppt) over a 28-day period, we monitored N and C transformation by the concentration of NH₄⁺, NO₂^-, NO₃^-, dissolved inorganic carbon (DIC) and total alkalinity in the supernatant, and microbial community adaptation in sediment by molecular analysis. We applied the revised ¹⁵N-paring isotope technique in slurry incubation to quantify the potential of N loss pathways.

Preliminary results indicate that significant N removal starts after a week of internal cycling between organic and inorganic N, with the maximum removal potential at 30 ppt salinity. Depletion of NO₃^- in the last week of incubation makes anammox stand out by utilizing NH₄⁺ and NO₂^-. The rate of DIC release decreased with increasing salinity, displaying an inverse pattern to that of N species. This decoupling points to the co-existence of autotrophic anammox, heterotrophic denitrification, and sulfate reduction processes. The stoichiometric ratio of total alkalinity to DIC suggests a shift of the predominant carbon decomposition process as salinity increased, from denitrification to sulfate reduction. This shift could enhance the total nitrogen removal potential while slowing carbon remineralization, indicating a positive feedback loop for both nitrogen removal and blue carbon storage in response to salinity intrusion. We will further focus on ¹⁵N₂ samples and microbial evidence to elucidate the interplay among nitrogen removal processes.