Carbon dioxide release driven by organic carbon in minerogenic salt marshes

Abstract. Coastal wetlands play an important role in the global carbon cycle by sequestering carbon (referred to as “blue carbon”). At the same time, organic carbon (OC) in the subsurface is decomposed, releasing greenhouse gases (GHGs) such as carbon dioxide (CO₂) and methane (CH₄). To predict how this carbon balance in salt marshes will change under future climate scenarios (e.g., higher temperatures, sea level rise), it is essential to understand the controls on OC decomposition in these systems. Here, we investigated OC turnover and CO₂ release in a minerogenic salt marsh at the Wadden Sea, Germany. We first characterized the porewater and sediment of a pioneer marsh and adjoining intertidal flat to identify key biogeochemical processes. We then performed an in situ experiment by injecting two OC sources (labile (acetate)/complex (humic acid)) and subsequently monitored GHG release over four injection cycles along with subsurface geochemistry. Overall, we found that the microbially mediated CO₂ release was limited by OC availability and composition, and not by electron acceptor availability, as evidenced by the presence of aqueous sulfate (SO₄^2-) at all depths and the lack of CH₄. Following the addition of labile OC, CO₂ release in the pioneer marsh increased by up to 47.4 ± 36.4 % compared to the control, with a generally similar trend in the intertidal flat. The CO₂ release from the complex OC treatment was similar to the control. The results of our work improve understanding of minerogenic salt marsh OC dynamics in temperate zones and enable better prediction of future changes.