Mangroves, as pivotal blue carbon ecosystems in marine environments, demonstrate significant spatiotemporal variability in sediment carbon content, presenting considerable challenges for precise carbon stock quantification. This study systematically examined surface sediment carbon characteristics across different vegetation zones in the Aegiceras corniculatum-dominated tidal flats of the Nanliu River estuary to elucidate its spatial distribution patterns and accumulation mechanisms. The results reveal: (1) pronounced spatial heterogeneity in carbon content, exhibiting a distinct landward-to-seaward decreasing gradient from inner forest (1.01% ± 0.17%) > near-forest edge (0.58% ± 0.16%) > outer flat (0.52% ± 0.19%), with significant variations among biogeomorphological units: dense forest (0.99% ± 0.19%) > seedling area (0.68% ± 0.23%) > bare flat (0.49% ± 0.12%), showing strong coupling with aboveground vegetation structure; (2) carbon content was primarily regulated by fine particles (clay/silt), vegetation community traits, and organic input; (3) spatial differentiation primarily stems from distinct carbon input pathways - bare flats dominated by physical sedimentation of suspended particulate carbon, while forested areas are regulated through synergistic mechanisms including fine particle retention, organic matter input from litterfall, and soil redox conditions. Notably, mangrove aggregation effects and associated vegetation facilitate localized carbon enrichment in outer tidal flats. These findings provide theoretical foundations for mangrove restoration and blue carbon storage strategies while establishing a scientific basis for coastal carbon sink assessments.