METHANE PRODUCTION AND EMISSIONS FROM 4 RECLAIMED AND PRISTINE WETLANDS OF SOUTHEASTERN UNITED-STATES

Wetlands are significant contributors to global CH4 emission. We measured CH4 emissions at two pristine wetlands [Okefenokee swamp and the Everglades (Water Conservation Area 2A)] and two reclaimed wetlands (Sunny Hill Farm and Apopka Marsh) in Southeastern USA, and we attempted to relate emissions to CH4 production rates of the soil and the soil's biological and chemical properties. Methane emissions through cattail [Typha sp.] and waterlily [Nymphaea ordorata (L.)] ranged from 0.09 to 1.7 g CH4 m-2 d-1 and exhibited high spatial and temporal variability. Diffusive flux of CH4 was calculated using dissolved CH4 profiles in the soil pore water and accounted for <5% of the plant-mediated emissions. Potential CH4 production rates were measured as a function of depth using soil samples obtained at 2-cm increments. Methane production rates were the same order of magnitude at all sites (<1-70 ng CH4-C g-1 soil C d-1) and were highest in the surface soils (0-6 cm) at three of the wetland sites, indicating that the predominant source of C available to methanogens was in the surface soils. Methane production rates in the top 24 cm ranged from 0.3 to 1.1 g CH4 M-2 d-1 and annual C losses due to anaerobic decomposition accounted for between 0.68 and 3.7% of the total C in the surface 24-cm soil depth. Methane production was observed at sites with porewater SO4 concentrations of up to 20 Mg SO4-S L-1, suggesting that methanogenesis occurred in the same soil layer as SO4 reduction, possibly in microsites where SO4 concentration was depleted.