Evaluating Soil and Plant Controls on the Contribution of Ebullition to Methane Emission from Fens

Abstract:

Wetlands are the largest natural source of methane (CH₄) that is produced in anoxic conditions in wetland soils and emitted via diffusion through soil and water, plant-mediated transport and ebullition. The sporadic nature of ebullition makes it difficult to quantify. While ebullition has been shown to represent a substantial portion of total CH₄ emission from open water wetlands and bogs, its contribution in fens is not well known. We measured ebullition in a temperate calcareous fen using gas traps and investigated environmental controls (water table, temperature, peat properties, and plant community) on gas volume and mass of CH₄ released. To further investigate vegetation controls, we conducted a greenhouse experiment to compare the contribution of ebullition to total CH₄ flux from monoliths of bare peat, dense sedge cover or sedge-willow composition. In the field, ebullition contributed 16% of total CH₄ emissions between June and October and was consistently highest at the wettest location where peat bulk density was lowest and organic matter content the highest. Greatest ebullition flux occurred in July and August, coinciding with the warmest soil temperatures. A model that included soil temperature, precipitation, and pore water CH₄ concentration explained 65% of the variation in ebullition flux of CH₄. In the greenhouse, dense sedge cover significantly decreased ebullition flux with ebullition accounting for < 2% of total emissions from sedge cores. Total ebullition flux was similar at bare and willow-sedge cores but contributed 47 and 26% of total CH₄ flux, respectively, largely due to higher rates of steady CH₄ flux from willow-sedge cores. Results indicate that ebullition can remain an important pathway of CH₄ emission in fens but declines as sedge cover increases. Future work should aim to quantify thresholds of sedge cover at which ebullition rates decline substantially and whether this pattern is species-dependent.