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.