River deltaic floodplains, as one of the most productive and economically important landscapes, develop diverse wetland ecosystems, including salt, brackish and freshwater marshes. Under changing climate and increasing anthropogenic activities, floodplain wetlands have been experiencing land loss and deterioration due to accelerating sea level rise, increasing pollution and eutrophication. However, what is yet unknown is whether the roles of deltaic wetlands in the carbon budget of the Earth system will switch with changing conditions, and if so, to what extent. For example, a freshwater marsh may produce less methane and sequester more carbon dioxide under frequent saline water flooding. To address these questions, a reactive transport model – PFLOTRAN – was configured to represent key biogeochemical reactions in wetland soil columns across Barataria Bay in the Mississippi River deltaic plain. The reaction network includes organic matter decomposition, sulfatereduction, iron reduction, nitrification, denitrification, methanogenesis, and methanotrophy along with dissimilatory nitrate reduction to ammonium (DNRA). Three 36-cm soil columns were simulated, representing saline, brackish and freshwater marshes in Barataria Bay. Field measured soil properties including saturation, porosity, and bulk density were used in the model configuration. Unlike other meso or macrotidal environments, the flooding frequency and duration along the microtidal Mississippi deltaic plain are driven by waves, mixed diurnal tides, and river discharge along the river mouth region. To better establish wet-dry conditions for each soil column, long term field water level and salinity data from nearby CRMS gauge stations were applied instead of computing water level oscillations with local tidal constituents. Simulated surface methane flux compared well with site measurements across the salinity gradient. In model simulations, increases in salinity drove enhanced sulfate availability that suppressed methane production and increased methane consumption. With similar salinity influence, increases in flooding duration drove increased methane production. These results indicate the dominant role of wetland in the deltaic carbon budget is subject to change with increasing flooding duration and salinity level.