Abstract
Plants occupying coastal ecosystems draw in carbon dioxide (CO 2 ) from the air and water around them during photosynthesis. A fraction of this CO 2 becomes fixed into plant biomass and can eventually contribute to the blue carbon pool—organic carbon (C org ) sequestered in slow‐turnover sinks. An important step in protecting and enhancing this natural carbon sequestration pathway is determining the relative contributions of different coastal plants to this blue carbon pool in durable sinks. We compiled a global dataset of coastal soil carbon measurements and used a Bayesian hierarchical meta‐regression model to explore the relative contribution of local (autochthonous) versus external (allochthonous) sources of C org in the soils beneath tidal saltmarsh, mangrove, and seagrass wetlands. We found most soil C org in coastal wetlands came from allochthonous sources, rather than the habitat‐forming plants. Managing climate‐resilient blue carbon seascapes, therefore, requires an awareness of this portfolio of contributors to soil carbon sequestration. However, study design aspects such as soil sampling depth, levels of sample replication, and the modeling approach used to trace C org can have pronounced effects on the estimated contributions of different carbon sources. We outline a roadmap for improving how we track the various drivers of soil carbon sequestration in diverse mosaics of coastal vegetation.