Effect of addition of organic carbon on greenhouse gas release and subsurface biogeochemistry in salt marshes

Vegetated coastal wetlands – comprising mangroves, salt marshes, and seagrass meadows – play an important role in the global carbon cycle due to their high sequestration rates of carbon (annual organic carbon burial rate 114-131 Tg C). The decomposition of organic carbon by microorganisms in these ecosystems causes greenhouse gas releases such as carbon dioxide (CO₂) and methane (CH₄). Understanding the rate and extent of microbially mediated greenhouse gas formation from coastal wetlands under current climate conditions is needed to predict greenhouse gas fluxes from these ecosystems with future climate change. Here, we investigate the processes that control the microbial decomposition of organic carbon at the Wadden Sea, northern Germany. Our preliminary field and laboratory results indicate that the degradation of organic carbon is not limited by the availability of electron acceptors such as sulfate, but rather by the concentrations and composition of the organic carbon itself. The objective of this project was therefore to test how the microbially mediated degradation of organic carbon and thus greenhouse gas fluxes change as a consequence of organic carbon input to the sediment. To do this, we conducted a field experiment in which we injected two different organic carbon sources separately into the sediment of the Wadden Sea and measured greenhouse gas fluxes over the course of six weeks. We choose acetate as a relatively labile organic carbon source and humic acids (purchased from the International Humic Substance Society) as a recalcitrant source. The in situ experiment was performed at two locations with differing tidal influence: (i) tidal flats, which are inundated twice a day during high tide, and (ii) pioneer marshes, which are inundated twice a month during spring tide. In addition to flux measurements, porewater, and sediment were sampled and used to study geochemical processes. For both marsh zones, an enhanced CO₂ flux was measured for the plots where labile organic carbon was injected relative to control plots in which no organic carbon was added. However, the addition of the recalcitrant organic carbon only caused an increase in the CO₂ flux in the tidal flat. Porewater data showed that the addition of the labile organic carbon promoted iron(III) reduction, especially in the pioneer marsh, while for the tidal flat, enhanced sulfate reduction was observed for both organic carbon sources. Overall, a significantly higher CO₂ flux was measured from plots enriched with labile organic carbon. The gained knowledge is important in the context of predicting how such an ecosystem reacts to an additional input of organic matter e.g., caused by eutrophication or mobilization of organic matter. Furthermore, it is also relevant for estimating the extent and rate of greenhouse gas fluxes from these ecosystems.