Coastal wetlands serve as critical sinks for reactive nitrogen and carbon dioxide, helping to alleviate the impacts of human-induced loadings and global climate change. However, the factors governing nitrogen removal and carbon preservation, particularly the combined effects of salinity and sulfate availability, remain unclear. Using a 28-day microcosm incubation of mangrove sediments, we quantified nitrate reduction processes (denitrification, anammox, and DNRA) under a salinity gradient and examined biogeochemical dynamics of nitrogen, carbon, and sulfur cycling. Our results indicate that denitrification was the dominant process driving nitrogen loss, while anammox only accounted for 0-23%. Under ambient salinity (30 ppt), where sulfate was abundant, nitrogen removal efficiency was the highest (86.22%), fueled by sulfate reduction and sulfide-driven autotrophic denitrification. In contrast, lower salinity reduced the efficiency to 65.23% and increased nitrogen retention via DNRA. This shift was also accompanied by a greater role of heterotrophic denitrification in nitrogen loss, with methanogenesis emerging as a key carbon decomposition pathway, accelerating carbon breakdown. As climate change intensifies salinity fluctuations in coastal wetlands, a thorough understanding of the potential coupling and decoupling of nitrogen, carbon and sulfuring cycling is crucial for optimizing nitrogen removal, maintaining carbon sequestration and minimal greenhouse gas emissions.

Mangrove,sediment denitrification,anammox,DNRA,Blue carbon