Soil organic carbon preservation and decay trends in tidal marsh, mangrove and seagrass blue carbon ecosystems

The management of coastal blue carbon ecosystems can contribute to mitigate anthropogenic greenhouse gas emissions. However, limited information on soil organic carbon (OC) decay rates in tidal marsh, mangrove and seagrass soils hinders their inclusion in climate strategies and carbon-crediting schemes. Here, we analyzed downcore OC trends in 3,733 soil cores from blue carbon ecosystems worldwide. A decrease in OC content with soil depth was measured in 63% of the cores, whereas stable and increasing trends were observed in 23% and 14% of the cores, respectively. Based on 75 profiles where OC decay could be modelled, the OC decay rate in blue carbon ecosystems was 0.024±0.002 yr^-1 over the last 100 years and 0.007±0.0007 yr^-1 over the last 1,000 years. This results in the stabilization of 9% and 0.1% of the soil OC inputs 100 and 1,000 yr after burial, respectively, showcasing the long residence time of OC in the sinks associated to blue carbon ecosystems. The models provided can inform baseline scenarios towards the implementation of carbon-crediting schemes.