Methane Production and Emissions from Four Reclaimed and Pristine Wetlands of Southeastern United States

Wetlands are significant contributors to global CHU emission. We measured CH» 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 CRi 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 Ctt, m' d~' and exhibited high spatial and temporal variability. Diffusive flux of CHt was calculated using dissolved CHt profiles in the soil pore water and accounted for <5% of the plant-mediated emissions. Potential CRi 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 CHt-C g" soil C d") 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 CHt m~ d~' 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 SO* concentrations of up to 20 mg SOj-S L', suggesting that methanogenesis occurred in the same soil layer as SO4 reduction, possibly in microsites where SO4 concentration was depleted. R ATTENTION has focused on the importance of CH4 as a greenhouse gas. Tropospheric CHt concentrations have increased during the last 10 yr at a rate >1 % per year and wetlands are considered a major source of atmospheric CHt (Bolle et al., 1986). Flooded soils, including rice paddies and natural wetlands, provide conditions where methanogenesis is favored, because they often have high C content due to the high productivity of wetland plant species and the slow rate of decomposition of plant matter under anaerobic conditions (Reddy and Patrick, 1984). In recent years, increased efforts have been made to quantify CH4 emissions from various wetlands (summarized by Bartlett and Harriss, 1993), and many of these studies have reported a high degree of variability both within and between wetland sites. Some of the variability of CFLt emissions from wetlands has been attributed to temperature (Crill et al., 1988; Lansdown et al., 1992), water depth (Sebacher et al., 1986: Harriss et al., 1988), soil salinity (Bartlett et al., 1987), CH4 oxidation rate (King et al., 1990), and vegetation presence and productivity (Whiting et L.A. Schipper, Landcare Research NZ Ltd, Private Bag 3127, Hamilton, New Zealand; and K.R. Reddy, Soil and Water Science Dep., Univ. of Florida, 106 Newell Hall, P.O. Box 11-0510, Gainesville, FL 32611-0510. Florida Agric. Exp. Stn. Journal Series no. R-03442. Received 1 Oct. 1993. *Corresponding author. Published in Soil Sci. Soc. Am. J. 58:1270-1275 (1994). al., 1991). However, few studies have examined the importance of soil physico-chemical properties as regulators of CH» production and emissions (Bartlett et al., 1987; Bachoon and Jones, 1992; Bridgham and Richardson, 1992). Harriss and Sebacher (1981) suggested that CHt emissions were correlated to C inputs and cycling hi cypress swamps. Methanogenesis is often the terminal step of organic matter decomposition in freshwater wetlands. Methanogenic bacteria are able to use only a limited array of low molecular weight C compounds for energy production and are dependent on a consortium of hydrolytic and fermentative bacteria to break down higher molecular weight compounds into lower molecular weight compounds (Oremland, 1988). In surface soils, the presence of electron acceptors such as NO3 and SO4 may result in methanogens being outcompeted by other anaerobic bacteria (Oremland, 1988). At lower soil depths, remaining C is likely to have already undergone considerable decomposition, and bioavailability of C may limit CHt production. The importance of electron acceptors and organic matter quality on methanogenesis hi wetland soil profiles has rarely been studied. About 30% of the total surface area of Florida is wetlands (Dahl, 1990). In spite of these vast areas of wetlands, few field measurements of CHt emissions have been made in southern Florida, and those were predominantly made in the Everglades (Sebacher et al., 1985; Harriss et al., 1988; Whiting et al., 1991). There is a growing interest in Florida and elsewhere to reclaim previously drained wetlands, which up until recently, were used for intensive agriculture (Lowe et al., 1989). Under drained conditions, these sites have had substantial losses of surface organic matter due to accelerated decomposition. These reclaimed wetlands have been reflooded and established with wetland plant species. Previous studies have not measured CH4 emissions from reclaimed wetlands, and additional measurements are required to obtain more reliable estimates of their contribution to global CH4 flux. In this study, we compared the production and emissions of CRt from selected reclaimed and pristine wetlands in Florida and Georgia. Our objectives were to determine (i) the diffusive and plant-mediated CH4 emissions; (ii) the CH» potential production in the soil profile; and (iii) the relationship between electron acceptor availability and potential CRt production. MATERIALS AND METHODS