Carbon sequestration is not inhibited by livestock grazing in Danish salt marshes

Climate change has created a need for solutions that can counteract greenhouse gas emissions. One is the expansion and maintenance of natural carbon (C)-sequestration habitats, such as forests and coastal and marine vegetated ecosystems, socalled "blue carbon ecosystems" including salt marshes, which represent large and long-term C-storage potential. While salt marsh blue carbon is gaining international attention, Baltic and Nordic salt marshes have been neglected in the blue carbon context. Here, we quantified C-stocks and C-sequestration rates in grazed and nongrazed sites of three Danish salt marshes, and evaluated whether grazing livestock, the standard management to increase biodiversity of the marshes, affect C-stocks and sequestration rates. The aboveground biomass and vegetation height in nongrazed salt marshes were significantly larger than for grazed salt marshes, but this did not lead to significantly enhanced overall sediment- or C-accumulation rates. Detailed model analyses of sediment profiles even indicated significantly higher C-densities in surface layers at grazed sites. Averaged C-densities, C-stocks (top 43 cm sediment) and sequestration rates estimated from Pb-210 profiles ranged 0.011-0.022 g C cm(-3), 4228-8178 g C m(-2), and 17-45 g C m(-2) yr(-1), respectively, with the latter being low in the global context. While this pioneering study for the Baltic and Nordic regions showed a neutral to positive effect of grazing on C-sequestration, there is a need to explore the overall C-footprint of this practice, including effects on net greenhouse gas emissions and coastal defense capacity, and define blue carbon management strategies for salt marshes to maximize their climate change mitigation and adaptation capacity while supporting biodiversity.