Carbon gas production under different electron acceptors in a freshwater marsh soil

Dynamics of carbon (C) gas emission from wetlands influence global C cycling. In many freshwater systems such as Louisiana freshwater marsh, soil contents of NO3- and SO42- have increased due to nutrient loading and saltwater intrusion. This could affect C mineralization and the emission of the major greenhouse gases carbon dioxide (CO2) and methane (CH4). In this investigation, a laboratory microcosm study was carried out to elucidate the effects of NO3- and SO42- on CO2 and CH4 production from a freshwater marsh soil located in the Barataria Basin of Louisiana coast, which has been subjected to the Mississippi River diversion and seawater intrusion. Composite soil samples were collected from top 50 cm marsh profile, treated with different levels of NO3- (0, 3.2 and 5 mM) or SO42- (0, 2, and 5 mM) concentrations, and incubated for 214 d under anaerobic conditions. The results showed that the presence of NO3- (especially at 3.2 mM) significantly decreased CO2 productions whereas SO42- did not. On the other hand, both NO3- and SO42- treatments decreased CH4 production but the NO3- almost completely inhibited CH4 production (>99%) whereas the SO42- treatments reduced CH4 production by 78–90%. The overall C mineralization rate constant under the NO3- presence was also low. In addition, the results revealed that a large proportion (95%) of anaerobic carbon mineralization in the untreated freshwater soil was unexplained by the reduction of any of the measured major electron acceptors.