How bad are wetland burps? Quantifying methane emission via ebullition in a temperate emergent marsh
Quantifying wetland greenhouse gas exchange is necessary to understand the overall radiative forcing effect of wetland fluxes and their potential role in feedbacks to climate change. However measuring methane (CH4) fluxes in wetlands is difficult due to high spatial and temporal variability, and multiple transport pathways of emission. Transport of biogenic soil gas via highly sporadic ebullition (bubbling) events is often ignored or quantified poorly in wetland greenhouse gas budgets, but can rapidly release bubbles containing high concentrations of CH4 to the atmosphere in discrete events. To quantify a statistically robust annual bubble flux we measured ebullition rates continuously for a year (2013-2014) using custom-built chambers deployed in a restored emergent wetland located in the Sacramento-San Joaquin Delta, CA. We combined bubble flux rates with observations of bubble gas concentrations to estimate ebullition emissions of CH4 and compare flux rates to whole-ecosystem exchange CH4 measured simultaneously by eddy covariance.
Mean daily bubble release rate was approximately 29 ml m-2 d-1, and we estimated an annual cumulative bubble release of 10.6 L m-2. Bubble fluxes exhibited significant temporal variability on weekly timescales, but monthly-seasonal variability was less pronounced, despite high seasonality in ecosystem productivity and temperature. In contrast we found the CH4 concentrations of emitted bubbles to vary strongly with season across 3 orders of magnitude, from ~0.01 % from December-March, to >10 % during the peak growing season (June-August). As a result of strong seasonality in bubble CH4 concentrations, overall ebullitive CH4 releases were approximately 0.3 g C-CH4 m-2 yr-1; representing a relatively small fraction of whole-ecosystem CH4 emissions at the annual timescale. Our data indicate that while bubble production may be relatively constant year-round, CH4 emission via ebullition may be largest during the summer growing season when the CH4 content of bubble releases is highest.