Although vernal pools are increasingly appreciated as critical habitat, their role in regulating biogeochemical cycles in forests is less understood. Vernal pools are small ephemeral wetlands with discrete spatiotemporal distributions, potentially creating sharp seasonal redox gradients. In temperate hardwood forests, vernal pools can represent a significant portion of the landscape during spring months. Moreover, pools often form in depressions with large accumulations of plant litter following autumn senescence. Vernal pools therefore may be underappreciated as regulators of nutrient cycling and biogeochemical transformations in forests. We studied trace-gas (greenhouse) dynamics and microbial community composition across vernal pool to non-vernal pool (unsaturated) soil transitions within three distinct ecosystem types in a NE Ohio temperate forest (upland, riparian, and disturbed ecosystems) to test the hypotheses that 1) vernal pools are biogeochemical hotspots with significant potential production and turnover of CO2, CH4 and N2O and 2) these patterns are associated with vernal pool microbial communities that are distinct from well-drained soil microbial communities. The experiment was conducted with soil from 18 paired vernal pool and well-drained soil samples that were incubated in vitro under either oxic or anoxic conditions. Microbial communities were examined using TRFLP and sequencing of methanogenic and methanotrophic functional genes.
Results/Conclusions
We found that potential CH4 production rates were initially higher in vernal pool soils, but the pattern was driven by high rates (45±25 nmol CH4 g soil-1 hr-1) in the upland ecosystem. Rates were low but consistent (5-10 nmol CH4 g soil-1 hr-1) in both vernal pool and well-drained soils in the disturbed ecosystem, and vernal pool soils in the riparian ecosystem. Rates were very low in the well-drained soil in upland and riparian ecosystems. However, rates increased significantly over time for all soils in all ecosystems. Initial aerobic CH4 consumption rates were significantly higher in vernal pool (~32 nmol CH4 g soil-1 hr-1) than in well-drained (~14 nmol CH4 g soil-1 hr-1) soil, but the pattern was again driven by high rates in the upland vernal pools (61±23 nmol CH4 g soil-1 hr-1). Again consumption rates increased significantly over time in all soils regardless of ecosystem type. Our analyzes suggest that CH4 production and consumption in vernal pools can be an important aspect of forest carbon cycling. However, viable populations of methanogens and methanotrophs exist in all forest soils and can respond to environmental conditions, suggesting that redox gradients, and not community composition, control rates over short time periods.
