How bad are wetland burps? Quantifying methane emission via ebullition in a temperate emergent marsh

Background/Question/Methods

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.

Results/Conclusions

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.