The management of coastal blue carbon ecosystems can help mitigate anthropogenic greenhouse gas emissions. However, limited information on soil organic carbon (SOC) decomposition rates in salt marshes, mangroves, and seagrasses hinders their inclusion in climate strategies and carbon credit schemes. Here, we analyse SOC trends in 3,761 soil cores from blue carbon ecosystems worldwide. A decrease in SOC content with soil depth was measured in 63% of the cores, while stable and increasing trends were observed in 22% and 15% of the cores, respectively. Based on 85 profiles where SOC decomposition could be modelled, the SOC decomposition rate in blue carbon ecosystems was 0.023 \u00b1 0.002 year\u22121 over the past 100 years and 0.007 \u00b1 0.0007 year\u22121 over the past 1,000 years. This results in the stabilization of 9% and 0.1% of SOC inputs to the soil 100 and 1,000 years after burial, respectively, demonstrating the long residence time of SOC in sinks associated with blue carbon ecosystems. This study provides benchmark estimates of SOC decomposition under natural conditions in the short and long term, and assesses carbon reduction in addition to the contribution of carbon offset projects in blue carbon ecosystems.<\/p>\n