MEPS prepress abstract  -  DOI: https://doi.org/10.3354/meps13009

Origin of organic carbon in the topsoil of Wadden Sea salt marshes

Peter Mueller***, Hai Thi Do**, Kai Jensen, Stefanie Nolte*

*Email: stefanie.nolte@uni-hamburg.de

ABSTRACT: Blue carbon ecosystems, including salt marshes, play an important role in the global carbon cycle because of their high efficiency storage of soil organic carbon (OC). Few studies focus on the origin of OC stored in salt-marsh soils, which comes from either allochthonous or autochthonous sources. The origin, however, has important implications for carbon crediting approaches because the alternative fate of allochthonous OC (AllOC), i.e. if it had not accumulated in the Blue C ecosystem, is unclear. Here, we assessed the origin of OC in two mainland salt-marsh sites of the European Wadden Sea, analyzing δ13C of topsoil (0–5 cm) samples, freshly deposited sediment (allochthonous source), and of above- and belowground biomass of vegetation (autochthonous sources). We tested for effects of geomorphological factors, including elevation and the distance to sediment sources, and of livestock grazing, as the most important land-use form, on the relative contributions of allochthonous versus autochthonous sources to the topsoil OC stock. A negative effect of distance to the creek on the relative contribution of AllOC was found at only one of the two salt marshes, probably due to differences in micro-topography between the two salt marshes. Additionally, the relative contribution of AllOC increased with increasing distance to the marsh edge in areas without livestock-grazing, while it decreased in grazed areas. Our findings demonstrate that spatial factors such as surface elevation and distance to a sediment source, which have been found to determine the spatial patterns of sediment deposition, are also important factors determining the relative contribution of AllOC to topsoil OC stocks of salt marshes. Furthermore, we provide the first evidence that livestock-grazing can reduce the relative contribution of AllOC to the soil OC stock. These findings thereby yield important implications for C crediting and land-use management.