Hydrologic gradient and vegetation controls on CH4 and CO2 fluxes in a spring-fed forested wetland

Abstract

Four different habitats in a spring-fed forested wetland (Clear Springs Wetland, Panola County, Mississippi, USA) varying in hydrologic regime were examined for methane and carbon dioxide fluxes from soils over 15 and 9 months, respectively. There was an increasing gradient of CH4 flux rates from an unflooded upper-elevation forest site to an occasionally flooded bottomland forest site to a shallow permanently flooded site, and then to a deeper-water permanently flooded site. Depending on the time of year, all sites were sources of methane but only at the upper-elevation forest site, when gravimetric soil moisture content fell below 54%, was atmospheric methane consumed. On average, summer CH4 emission rates were higher than those in other seasons. A multiple regression model with soil temperature and soil redox potential as independent variables could explain 65% of the variation in CH4 flux rates. In the flooded zone, variation in CH4 flux rates was correlated with aboveground plant biomass and stem density of emergent vascular plants, and plant-mediated CH4 transport depended on plant type. The efflux of CH4 to plant biomass (Eff:B) ratio was generally lower in Hydrocotyle umbellata compared to Festuca obtusa. Compared to several other freshwater forested wetlands in the southeastern USA, this spring-fed forested wetland ecosystem was a strong source of atmospheric CH4, likely due to a long hydroperiod and high soil organic matter content. Carbon dioxide fluxes show a reverse spatial pattern than CH4 fluxes with highest CO2 emissions in the non-flooded zone at all times of the year, indicating the dominance of aerobic soil respiration. A multiple regression model also revealed a strong dependency of CO2 fluxes (r 2 = 0.73) on soil temperature and soil redox potential.