Vegetation Affects Timing and Location of Wetland Methane Emissions

Common assumptions about how vegetation affects wetland methane (CH4) flux include acting as conduits for CH4 release, providing carbon substrates for growth and activity of methanogenic organisms, and supplying oxygen to support CH4 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 ofTyphaon CH4 flux using clear flux chamber measurements directly overTyphaplants ("whole-plant"), adjacent toTyphaplants (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 similar to 5 times higher CH4 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 CH4 concentrations were relatively low directly below whole-plant and in neighboring plant-adjacent treatments, while controls accumulated a highly concentrated reservoir of porewater CH4 . When the water table was drawn down to simulate seasonal drying, reserve porewater CH4 from control soil was released as a pulse, equaling the earlier higher CH4 emissions from whole-plants. Plant-adjacent treatments, which had neither plant-mediated CH4 transport nor a concentrated reservoir of porewater CH4 , had low CH4 flux throughout the study. Our findings indicate that in seasonal wetlands, vegetation affects the timing and location of CH4 emissions. These results have important mechanistic and methodological implications for understanding the role of vegetation on wetland CH4 flux.