The importance of an estuarine salinity gradient on soil organic carbon stocks of tidal marshes
Tidal marshes are sedimentary environments that are among the most productive ecosystems on earth. As a consequence tidal marshes, and vegetated coastal ecosystems in general, have the potential to reduce atmospheric greenhouse gas concentrations as they efficiently sequester soil organic carbon (SOC). In the past decades, most research has focused on salt marshes, leaving carbon dynamics in brackish- and freshwater marshes largely understudied and neglecting the diversity among tidal marshes.
... ong tidal marshes. Moreover, most existing studies underestimate total organic carbon (OC) stocks due to shallow soil sampling, which also influences reported patterns in OC storage along estuaries. We find that SOC stocks vary significantly along the salinity gradient of a temperate estuary (Scheldt estuary, Belgium and The Netherlands), from 46&thinsp;kg OC&thinsp;m<sup>&minus;2</sup> in freshwater marshes to 10&thinsp;kg OC&thinsp;m<sup>&minus;2</sup> in saltmarshes. In all tidal marsh sediments the OC concentration has a constant value from a certain depth below the surface downward. However, this concentration decreases with increasing salinity, indicating that the amount of stabile SOC decreases from the upper estuary towards the coast. Although net primary production of macrophytes differs along the estuary, our data suggest that these differences in OC storage are caused mainly by variations in suspended sediment concentration and stable particulate OC (POC) content in the water along the estuary. The fraction of suspended sediments and POC that is transported downstream the maximum turbidity zone is very limited, contributing to smaller amounts of long term OC sequestration in brackish- and saltmarsh sediments. In addition, high rates of sediment deposition on freshwater tidal marshes in the maximum turbidity zone promote efficient burial of OC in these marsh sediments.