Abstract:
Vegetated coastal wetlands such as salt marshes can store large amounts of organic carbon (OC) belowground. The accumulation of OC is favored due to water-saturated, anoxic subsurface conditions, which suppress OC decomposition. Due to their substantial OC sequestration rate, coastal wetlands have an important role in the global carbon cycle.
OC decomposition in salt marshes includes aerobic and anaerobic respiration (mainly iron(III) and sulfate reduction). The frequent inundation of salt marshes by seawater introduces sulfate into the sediment and thus supplies the system regularly with an electron acceptor. However, it is not known whether the OC decomposition in a minerogenic salt marsh is limited by the availability of electron acceptors or electron donors. Addressing this knowledge gap is important to reliably predict how the carbon cycle of these ecosystems will response to climate changes e.g., sea level rise, warmer temperatures, and eutrophication of coastal waters.
This work focuses on the carbon cycle and its link to iron and sulfur cycles in a salt marsh at the Wadden Sea coast. Specifically, the presented project integrates field and laboratory experiments, showing that the availability of electron donors as labile OC controls CO2 release from minerogenic salt marshes. This influence was stronger when labile OC was combined with a nitrogen source.
The field experiment comprised the injection of two contrasting OC sources (labile (acetate)/complex (humic acid)) into the sediment of a minerogenic salt marsh. Greenhouse gas (GHG) release – used as a proxy for OC decomposition – was subsequently monitored along with subsurface geochemistry. Overall, CO2 release was limited by OC availability and composition. This was reflected in higher CO2 release in treatments amended with labile OC compared to the control, while electron acceptor availability was unlikely to be the primary limiting factor, as indicated by the presence of aqueous sulfate and the lack of methane (CH4) as a flux or in the porewater.
Furthermore, simulating eutrophication of coastal waters confirmed that CO2 release is limited by available labile OC. Moreover, co-occurring nutrients (e.g., ammonium) in algal-derived organic matter significantly influenced CO2 release. Treatments with algal-derived organic matter emitted 113.6±21.0% more CO2 than the control, while the OC only treatment (without nutrients) emitted 77.16±19.2% more CO2, despite the same OC concentrations. Hence, predicting CO2 release from minerogenic salt marshes based solely on OC would likely underestimate CO2 release. In summary, this work links GHG release and subsurface biogeochemical processes of a minerogenic salt marsh.
