764 / 2024-09-19 11:54:23

Freshening of coastal wetlands weakens nitrogen removal and carbon preservation potential: the role of sulfate

Mangrove,sediment denitrification,anammox,DNRA,Blue carbon

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.