Carbon accumulation in forested and riverine wetlands

Background/Question/Methods

High productivity and waterlogged conditions make wetland ecosystems significant carbon sinks. However, wetland types differ on their efficiency performing this task. The vegetation and organic input material, the location in the landscape, and the connection to the watershed are key factors determining wetland’s capacity to accumulate carbon in the soil. This study attempts to assess the effect of these variables on wetland soil carbon sequestration by comparing two natural wetlands in Ohio that belong to two distinct classification types: an 23-ha isolated forested wetland that receives water mainly via precipitation, and a 56-ha riverine flow-through wetland that receives water form an agricultural watershed. Composite soil cores were extracted in three different communities of both wetlands (open water, emergent, and edge) and analyzed for total carbon content to determine the wetland soil carbon pool. Sequestration rates were determined by radiometric dating with Cs-137 and Pb-210. Soil cores were also extracted in the forested upland area adjacent to the wetlands as a reference carbon pool of each site.

Results/Conclusions

Preliminary results indicate that both wetlands had similar accretion rates (3.4 mm yr^-1 in the forested wetland and 4 mm yr^-1 in the riverine one). The soil, however, had much higher organic content in the forested one (155 gC kg^-1, 53.7 gC kg^-1 in the riverine wetland) and therefore lower bulk density (0.57 g cm^-3 and 0.74 g cm^-3, respectively). The soil carbon in both wetlands was mostly organic, being 98% in the forested wetland and 83% in the riverine one. On average, the carbon sequestration rate of the forested wetland was 319 gC m^-2y^-1, a rate much higher than the riverine wetland (that was sequestering carbon at a rate of 142 gC m^-2y^-1). These preliminary results could be indication that, despite the semi-permanent flooded conditions of the forested wetland expose the soil and oxidize the carbon stored in it, the type and amount of organic matter entering the system is keeping the carbon pool high and the sequestration rate more efficient than the riverine wetland.