Factors determining seagrass Blue Carbon across bioregions and geomorphologies

Seagrass meadows rank among the most significant organic carbon (C) sinks on earth. We examined the variability in seagrass soil C stocks and composition across Australia and identified the main drivers of variability, applying a spatially hierarchical approach that incorporates bioregions and geomorphic settings. Top 30 cm soil C stocks were similar across bioregions and geomorphic settings (min-max: 20–26 Mg C ha), but meadows formed by large species (i.e., Amphibolis spp. and Posidonia spp.) showed higher stocks (24–29 Mg C ha) than those formed by smaller species (e.g., Halodule, Halophila, Ruppia, Zostera, Cymodocea, and Syringodium; 12–21 Mg C ha). In temperate coastal meadows dominated by large species, soil C stocks mainly derived from seagrass C (72 ± 2%), while allochthonous C dominated soil C stocks in meadows formed by small species in temperate and tropical estuarine meadows (64 ± 5%). In temperate coastal meadows, soil C stocks were enhanced by low hydrodynamic exposure associated with high mud and seagrass C contents. In temperate estuarine meadows, soil C stocks were enhanced by high contributions of seagrass C, low to moderate solar radiation, and low human pressure. In tropical estuarine meadows formed by small species, large soil C stocks were mainly associated with low hydrodynamic energy, low rainfall, and high solar radiation. These results showcase that bioregion and geomorphic setting are not necessarily good predictors of soil C stocks and that site-specific estimates based on local environmental factors are needed for Blue Carbon projects and greenhouse gases accounting purposes.