Assessing the long-term carbon-sequestration potential of the semi-natural salt marshes in the European Wadden Sea

Salt marshes and other blue carbon ecosystems have been increasingly recognized for their carbon (C)-sink function. Yet, an improved assessment of organic carbon (OC) stocks and C-sequestration rates is still required to include blue C in C-crediting programs. Particularly, factors inducing variability in the permanence of sequestration and allochthonous contributions to soil OC stocks require an improved understanding. This study evaluates the potential for long-term C sequestration in the semi-natural salt marshes of the European Wadden Sea (WS), conducting deep (1.3 m) down-core OC-density assessments in sites with known site histories and accretion records. Because these young marshes have developed from tidal-flat ecosystems and have undergone rapid succession during the last 80-120 yr, the identification of different ecosystem stages down-core was crucial to interpret possible changes in OC density. This was conducted based on the down-core distribution of different foraminiferal taxa and grain sizes. Comparisons of historic and recent accretion rates were conducted to understand possible effects of accretion rate on down-core changes in OC density. delta C-13 in OC was used to assess the origin of accumulated OC (autochthonous vs. allochthonous sources). We show that large amounts of short-term accumulated OC are lost down-core in the well-aerated marsh soils of the WS region and thus emphasize the importance of deep sampling to avoid overestimation of C sequestration. Despite steep declines in OC-density down-core, minimum values of OC density in the salt-marsh soils were considerably higher than those of the former tidal-flat sediments that the marshes were converted from, illustrating the greater C-sequestration potential of the vegetated ecosystem. However, our data also suggest that marine-derived allochthonous OC makes up a large fraction of the effectively, long-term preserved OC stock, whereas atmospheric CO2 removal by marsh vegetation contributes relatively little. The implication of this finding for C-crediting approaches in blue C ecosystems has yet to be clarified.