Abstract: Blue carbon ecosystems (BCEs) possess efficient carbon sequestration and storage capabilities, yet the characteristics of soil organic carbon (SOC) composition and sources in BCEs remain unclear. Therefore, this study selected seagrass meadow, mangroves, and adjacent bare mudflat along the coast of Liusha Bay, Leizhou Peninsula to compare the differences in SOC composition and stability across various BCEs. The characteristics of carbon sources were also identified by 13C and Bayesian mixing models. Results showed that along the gradient from bare mudflat to seagrass meadow and mangroves, both soil carbon density and SOC content increased, while the proportion of labile carbon fractions (including easily oxidizes organic carbon, dissolved organic carbon, and microbial biomass carbon) exhibited a decreasing trend. The Bayesian mixing models further revealed that SOC in bare mudflat was primarily derived from marine sources, SOC in seagrass meadow was mainly composed of both marine- and seagrass-derived organic carbon, while SOC in mangroves was dominated by autochthonous organic carbon. In addition, mangrove-derived organic carbon was predominantly enriched in mangrove areas, seagrass-derived organic carbon was mainly deposited in both seagrass meadow and mangrove areas. This study highlights the advantages of typical BCEs (particularly mangroves) in carbon storage and emphasizes the importance of ecosystem connectivity for blue carbon accumulation, providing a scientific basis for blue carbon management in BCEs.

Key words: Liusha Bay, blue carbon, mangrove, seagrass bed, SOC stability, SOC source