Unveiling distribution and drivers of soil organic carbon in coastal China: Salt marshes and tidal flats as major carbon storage contributors

• SOC stocks of China’s BCEs were underestimated. • Combined, salt marshes and tidal flats store more SOC than mangroves at national scale. • Tidal flats should be included in classical blue carbon ecosystem. • Vegetation and soil properties (SWC and pH) are key drivers of SOC stock variability. Understanding the characteristics, distribution and drivers of soil organic carbon (SOC) stock in coastal wetlands are of great significance to effective management and sustainable development of blue carbon ecosystems (BCEs). However, current assessments of their SOC stocks at national to regional scales remain inconsistent and uncertain. Here, we conduct a comprehensive meta -analysis of SOC stock (0–1 m depth) compiled from 1990 to 2024, covering 436 sampling sites of the classical BCEs (mangroves, salt marshes, seagrasses) and often-overlooked unvegetated tidal flats across China’s coastline. Among these ecosystems, mangroves exhibit highest average SOC stock (192 ± 130 Mg C/ha), followed by salt marshes (111 ± 65 Mg C/ha), while unvegetated tidal flats (78 ± 55 Mg C/ha) are comparable to seagrasses (74 ± 75 Mg C/ha). Despite their lower SOC stocks, combined, salt marshes and tidal flats surpass mangroves and dominate national total soil carbon storage (88 % of 54.56 Tg C), highlighting their dominant contribution to climate mitigation among BCEs. SOC stock in China’s BCEs exhibited a significant latitudinal pattern, decreasing linearly with elevated latitude. The spatial patterns were driven by vegetation structure and soil properties. Our findings provide updated national estimates of SOC stocks for China, thereby informing domestic conservation and restoration strategies while contributing to the global understanding of blue carbon capacity in diverse coastal systems.