Abstract
Common assumptions about how vegetation affects wetland methane (CH)
flux include acting as conduits for CH release, providing carbon
substrates for growth and activity of methanogenic organisms, and
supplying oxygen to support CH oxidation. However, these effects may
change through time, especially in seasonal wetlands that experience
drying and re-wetting, or change across space, dependent on proximity to
vegetation. In a mesocosm study, we assessed the impacts of on CH flux
using clear flux-chamber measurements directly over plants
(‘whole-plant’), adjacent to 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 flux rates (51.78±8.16
mg-C md) than plant-adjacent or control treatments, which was primarily
due to plant-mediated transport, with little contribution from
diffusive-only flux. However, high fluxes from whole-plants depleted
porewater CH concentrations both directly below whole-plant and in
neighboring plant-adjacent treatments, while controls accumulated a
highly concentrated reservoir of porewater CH. When the water table was
drawn down to simulate seasonal drying, reserve porewater CH from
control soil was released as a pulse, equaling the earlier higher CH
emissions from whole-plants. Plant-adjacent treatments, which had
neither plant-mediated CH transport nor a concentrated reservoir of
porewater CH, had low CH flux throughout the study. Our findings
indicate that in seasonal wetlands, vegetation affects the timing and
location of CH emissions. These results have important mechanistic and
methodological implications for understanding the role of vegetation on
wetland CH flux.