Biomarkers reveal vulnerability of plant and microbial carbon to sea-level rise in estuarine wetlands

Coastal wetlands are critical blue carbon sinks, yet contributions and stabilization mechanisms of key soil organic carbon components—plant-derived carbon, microbial necromass carbon, and glomalin-related soil protein—remain poorly quantified. Using a multi-biomarker approach (lignin, amino sugars, glomalin) across 0–100 cm soil profiles under four vegetation types along a salinity gradient in the Yellow River Delta Estuary, we found that contributions of plant-derived carbon (34% to 12%), microbial necromass carbon (17% to 11%), and glomalin (33% to 17%) declined significantly. Plant-derived carbon consistently dominated microbial necromass carbon in soil organic carbon. Glomalin may preserve both microbial and plant residues by acting as an iron carrier that enhances mineral-associated carbon stabilization. Carbon accumulation was jointly regulated by soil nutrients, geochemical properties, microbial community composition, and plant biomass. Plant-derived carbon had the strongest total effect; electrical conductivity and pH had negative indirect effects. These insights are critical for enhancing mineral-protected carbon accumulation under future sea-level rise.