Organic carbon mineralization, stock, and sequestration in an unvegetated mud flat and adjacent Suaeda japonica marsh of Garolim Bay, Yellow Sea: insights from δ13C–DIC

Abstract

To elucidate the biogeochemical linkage between organic carbon (OC) mineralization and sequestration in coastal wetlands, we investigated sediment and pore-water biogeochemistry, 210 Pb-derived sedimentation rates, and δ 13 C–DIC (Keeling plot) signatures in an unvegetated mud flat (UMF) and an adjacent Suaeda japonica -vegetated marsh (SJ). OC mineralization (OC min ) was consistently higher at SJ than that at UMF in both seasons, with a pronounced summer surface hotspot coinciding with peak root biomass, whereas Keeling-plot intercepts diverged more strongly from bulk δ 13 C–TOC at UMF, indicating that the DIC produced there was isotopically more distinct from bulk sedimentary OC. Dissolved Ca 2+ measured during the incubations constrained carbonate dissolution to less than 2% of depth-integrated DIC production in summer, indicating that this isotopic offset does not arise primarily from carbonate dissolution. OC stocks and sequestration rates at both sites were well below global blue carbon means, and OC min exceeded sequestration at both sites in both seasons, corresponding to burial efficiencies of 8.8-23.8%. These results indicate that the presence of S. japonica was associated with a higher supply and microbial turnover of OC relative to the adjacent unvegetated mud flat, and that carbon stock inventories alone are insufficient to characterize the carbon balance of vegetated and unvegetated tidal flats.