Vegetation Affects Timing and Location of Wetland Methane Emissions
Sheel Bansal, Olivia F. Johnson, Jacob Meier, Xiaoyan Zhu
Abstract
Abstract Common assumptions about how vegetation affects wetland methane (CH 4 ) flux include acting as conduits for CH 4 release, providing carbon substrates for growth and activity of methanogenic organisms, and supplying oxygen to support CH 4 oxidation. However, these effects may change through time, especially in seasonal wetlands that experience drying and rewetting, or change across space, dependent on proximity to vegetation. In a mesocosm study, we assessed the impacts of Typha on CH 4 flux using clear flux chamber measurements directly over Typha plants (“whole‐plant”), adjacent to Typha plants (where roots were present but no stems; “plant‐adjacent”), and plant‐free soils (“control”). During the establishment phase of the study (first 30 days), the whole‐plant treatment had ~5 times higher CH 4 flux rates (51.78 ± 8.16 mg‐C m −2 day −1 ) than plant‐adjacent or control treatments, which was primarily due to plant‐mediated transport, with little contribution from diffusive‐only flux. However, porewater CH 4 concentrations were relatively low directly below whole‐plant and in neighboring plant‐adjacent treatments, while controls accumulated a highly concentrated reservoir of porewater CH 4 . When the water table was drawn down to simulate seasonal drying, reserve porewater CH 4 from control soil was released as a pulse, equaling the earlier higher CH 4 emissions from whole‐plants. Plant‐adjacent treatments, which had neither plant‐mediated CH 4 transport nor a concentrated reservoir of porewater CH 4 , had low CH 4 flux throughout the study. Our findings indicate that in seasonal wetlands, vegetation affects the timing and location of CH 4 emissions. These results have important mechanistic and methodological implications for understanding the role of vegetation on wetland CH 4 flux.