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 NO 3 - and SO 4 2 - have increased due to nutrient loading and saltwater intrusion. This could affect C mineralization and the emission of the major greenhouse gases carbon dioxide (CO 2) and methane (CH 4). In this investigation, a laboratory microcosm study was carried out to elucidate the effects of NO 3 - and SO 4 2 - on CO 2 and CH 4 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 NO 3 - (0, 3.2 and 5 mM) or SO 4 2 - (0, 2, and 5 mM) concentrations, and incubated for 214 d under anaerobic conditions. The results showed that the presence of NO 3 - (especially at 3.2 mM) significantly decreased CO 2 productions whereas SO 4 2 - did not. On the other hand, both NO 3 - and SO 4 2 - treatments decreased CH 4 production but the NO 3 - almost completely inhibited CH 4 production (>99%) whereas the SO 4 2 - treatments reduced CH 4 production by 78–90%. The overall C mineralization rate constant under the NO 3 - 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.