Vegetation Loss Following Vertical Drowning of Mississippi River Deltaic Wetlands Leads to Faster Microbial Decomposition and Decreases in Soil Carbon

Wetland ecosystems hold nearly a third of the global soil carbon pool, but as wetlands rapidly disappear the fate of this stored soil carbon is unclear. The aim of this study was to quantify and then link potential rates of microbial decomposition after vertical drowning of vegetated tidal marshes in coastal Louisiana to known drivers of anaerobic decomposition altered by vegetation loss. Profiles of potential CH4 and CO2 production (surface to 60 cm deep) were measured during anaerobic incubations, organic matter chemistry was assessed with infrared spectroscopy, and soil porewater nutrients and redox potentials were measured in the field along a chronosequence of wetland loss. After vertical drowning, pond soils had lower redox potentials, higher pH values, lower soil carbon and nitrogen concentrations, lower lignin: polysaccharide ratios, more NH4+ and PO43-, and higher rates of potential CO2 release than vegetated marsh soils. Potential CH4 production was similar in vegetated marshes and open water ponds, with depth-dependent decreases in CH4 production as soil carbon concentrations increased. In these anoxic soils, vegetation loss exerts a primary control on decomposition rates because flooding drives sustained increases in porewater nutrient availability (NH4+ and PO43, dissolved organic carbon) and decreases in redox potential (from -150 to -500 mV) that lead to higher potential CO2 fluxes within a few years. Without new carbon inputs following wetland loss, the sustained decomposition in open water ponds may lead to losses of stored soil carbon and could influence global carbon budgets. Coastal wetlands capture and store large amounts of carbon in soil and vegetation, but wetland ecosystems are being lost at a rapid rate. Coastal wetlands like tidal marshes can be lost when they flood and sink, which kills the marsh grasses in a process called vertical drowning. We found that microbial production of greenhouse gases was higher after marshes in coastal Louisiana vertically drown. These potential soil carbon losses were related to changing environmental conditions caused by the loss of marsh grasses, specifically increases in nutrient and organic matter availability and decreases in soil reduction-oxidation potential. A first impulse would be to think that at higher elevations, vegetated wetland soils would have faster rates of microbial decomposition than soils submerged under 10-70 cm of water. Our results show that the relationship between flooding and organic matter decomposition is more complex in tidal wetlands and suggest that faster decomposition rates combined with lower vegetation inputs following wetland loss could lead to sustained losses of previously stored soil carbon. The vertical drowning of coastal wetlands leads to stimulated rates of decomposition and higher production of CO2 Higher CO2 production is mediated by vegetation loss, which is associated with lower redox potential and greater nutrient availability Without carbon inputs, sustained decomposition in open water ponds could lead to substantial losses of stored carbon