Factors Determining Seagrass Blue Carbon Across Bioregions and Geomorphologies

Abstract Seagrass meadows rank among the most significant organic carbon (C org ) sinks on earth. We examined the variability in seagrass soil C org 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 org stocks were similar across bioregions and geomorphic settings (min‐max: 20–26 Mg C org ha −1 ), but meadows formed by large species (i.e., Amphibolis spp. and Posidonia spp.) showed higher stocks (24–29 Mg C org ha −1 ) than those formed by smaller species (e.g., Halodule, Halophila, Ruppia, Zostera, Cymodocea, and Syringodium ; 12–21 Mg C org ha −1 ). In temperate coastal meadows dominated by large species, soil C org stocks mainly derived from seagrass C org (72 ± 2%), while allochthonous C org dominated soil C org stocks in meadows formed by small species in temperate and tropical estuarine meadows (64 ± 5%). In temperate coastal meadows, soil C org stocks were enhanced by low hydrodynamic exposure associated with high mud and seagrass C org contents. In temperate estuarine meadows, soil C org stocks were enhanced by high contributions of seagrass C org , low to moderate solar radiation, and low human pressure. In tropical estuarine meadows formed by small species, large soil C org 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 org stocks and that site‐specific estimates based on local environmental factors are needed for Blue Carbon projects and greenhouse gases accounting purposes.