Supplementary Data for Manuscript: Changes in salinity impact nitrogen removal and carbon preservation in coastal wetlands sediment

Coastal wetlands naturally remediate nitrogen (N) pollution through microbial pathways that either remove reactive N via denitrification and anammox, or retain it via dissimilatory nitrate reduction to ammonium (DNRA). The balance among three processes is closely linked to the carbon (C) cycle, as both denitrification and DNRA partially consume organic C. Climate-driven salinity changes can disrupt nitrate reduction processes through direct ionic stress and indirect effects from the sulfur (S) cycle. However, the overall impact on N removal and C preservation services remains unclear. Here, we used microcosm experiments to examine how a large salinity gradient (0-30 ppt) influenced pathways supporting N removal and C preservation in mangrove sediments. Freshening from the ambient 30 ppt to 10 ppt decreased N removal efficiency by ~20%. The decline was caused by reduced denitrification, whereas anammox and DNRA were unaffected. Meanwhile, low salinity stimulated C decomposition, likely due to enhanced heterotrophic respiration. Critically, the impact of salinity and S cycling on the balance between N removal and retention was nitrate-dependent. Only under nitrate limitation, higher sulfate drove the ecosystem toward greater retention. Together, our study demonstrates that coastal water freshening may weaken wetlands’ ability to remove N and preserve C, highlighting how tightly coupled biogeochemical cycling shapes coastal ecosystem functioning.