Microbial Decomposition of Lignin to Methane Reduces Net Blue Carbon Benefit Across China's Saltmarshes.

CH4 emissions from mangrove, saltmarsh, and seagrass ecosystems partially offset carbon sequestration, potentially diminishing the climate mitigation capacity of these blue carbon habitats. However, a mechanistic understanding of the processes governing CH4 production potential across large spatial scales remains limited. By integrating incubation-based measurements from 116 sites, we reveal significant ecosystem-specific differences in CH4 production potential, with saltmarshes emerging as CH4 production hotspot relative to mangroves and seagrass meadows. Using an integrated analytical approach encompassing more than 30 environmental, biogeochemical, and microbial parameters, we demonstrate that CH4 production potential converges on sediment organic carbon availability, particularly plant-derived carbon, as a key regulatory axis. Additionally, metagenome-assembled genomes (MAGs) recovered from saltmarshes show a functional bias toward lignin degradation, thereby fueling downstream CH4 production via methylotrophic pathways. Lignin-addition and stable carbon isotope experiments further provide supportive evidence that lignin decomposition enhances Chinese saltmarsh CH4 production potential, revealing a pathway that may reduce net blue carbon benefit. Together, these findings underscore that saltmarsh plant-derived lignin is less stable than conventionally assumed, as microbial processing redirects stored carbon toward CH4 production, challenging current blue carbon accounting frameworks at a continental scale within China.