Drivers of variability in Blue Carbon stocks and burial rates across European estuarine habitats.

The implementation of climate change mitigation strategies based on the conservation and restoration of Blue Carbon ecosystems requires a deep understanding of the magnitude and variability in organic carbon (C) storage across and within these ecosystems. This study explored the variability in soil C stocks and burial rates across and within intertidal estuarine habitats of the Atlantic European coast and its relation to biotic and abiotic drivers. A total of 136 soil cores were collected across saltmarshes located at different tidal zones (high marsh, N = 45; low marsh, N = 30), seagrass meadows (N = 17) and tidal flats (N = 44), and from the inner to the outer sections of five estuaries characterized by different basin land uses. Soil C stocks were higher in high-marsh communities (65 ± 3 Mg ha) than in low-marsh communities (38 ± 3 Mg ha), seagrass meadows (40 ± 5 Mg ha) and unvegetated tidal flats (46 ± 3 Mg ha) whereas C burial rates also tended to be higher in high marshes (62 ± 13 g m y) compared to low marshes (43 ± 15 g m y) and tidal flats (35 ± 9 g m y). Soil C stocks and burial rates decreased from inner to outer estuarine sections in most estuaries reflecting the decrease in the river influence towards the estuary mouth. Higher soil C stocks were related to higher content of silt and clay and higher proportion of forest and natural land within the river basin, pointing at new opportunities for protecting coastal natural carbon sinks based on the conservation and restoration of upland ecosystems. Our study contributes to the global inventory of Blue Carbon by adding data from unexplored regions and habitats in Europe, and by identifying drivers of variability across and within estuaries.