Methane Gas Ebullition Dynamics From Different Subtropical Wetland Vegetation Communities in Big Cypress National Preserve, Florida Are Revealed Using a Multi-Method, Multi-Scale Approach

Methane (CH4) emissions from peat soils are highly variable in space and time and are influenced by changes in biogeochemical controls and other environmental factors. Areas or times with disproportionally high CH4 emissions in wetlands may develop where conditions are especially conducive for microbial processes like methanogenesis. Currently, eddy covariance methods are employed to quantify CH4 exchanges over several extensive subtropical forested wetland communities in the Big Cypress National Preserve, Florida. In this work, we investigate the importance of multi-scale measurements to characterize CH4 ebullition dynamics from subtropical wetlands. Our approach uses a combination of gas traps, time-lapse photography, and capacitance probes to characterize ebullition dynamics from two different wetland vegetation communities for comparison to eddy covariance CH4 measurements at the site. Ground-penetrating radar surveys and soil sampling are used to assess differences in subsurface properties between sites that influence ebullition. Our results show that the mean measurement bias between fluxes measured in this study and the eddy covariance measurements over the same period was 10-14 times larger during the wet season when ebullition rates were greatest, than the dry season, when ebullition rates were smallest. This suggests that eddy covariance measurements may underestimate the CH4 contribution of ebullition across heterogeneous wetland vegetation communities and that the comparability of CH4 fluxes from methods varying in spatio-temporal scale changes in response to subtropical Florida seasonality. Our work suggests that these methods can be used to complement eddy covariance measurements and improve the characterization of ebullition dynamics in subtropical wetlands. Methane (CH4) gas is emitted from wetlands as a result of a microbial process called methanogenesis. These CH4 gas emissions can be different depending on where and when they are measured because certain wetland areas may have the right conditions to promote methanogenesis more than others. Specialized instruments, called eddy covariance towers, are commonly used to measure CH4 gas emissions. However, because they measure the emissions over a large area, they may not discern smaller scale differences within their measurement area. In contrast, other methods are able to measure CH4 gas emissions at smaller spatial scales, like gas traps, time-lapse cameras, and capacitance probes. We tested the use of these methods at wetland locations in south Florida where eddy covariance measurements are ongoing. We found consistency between the different scales of measurement but differences from the eddy covariance data were largest during the wet season when CH4 emissions are greatest. Our results are important because they can help identify differences in CH4 gas emission rates for specific wetland environments that can be used in areas where eddy covariance is not available, helping to refine regional and global CH4 emission estimates from wetlands. Gas traps coupled with time-lapse cameras and capacitance probes reveal CH4 ebullition flux heterogeneities between different wetland soilsCombining different scales of measurement allows CH4 ebullition flux heterogeneities to be characterized in subtropical wetlandsComparability of CH4 fluxes from methods varying in spatio-temporal scale are influenced by subtropical seasonality in ebullition dynamics

Automated summary

The methane emissions from peat soils in wetlands vary greatly in space and time due to biogeochemical controls and environmental factors. Eddy covariance methods are commonly used to measure these emissions, but they may underestimate the methane contribution from ebullition in wetland vegetation communities. We used a combination of gas traps, time-lapse photography, and capacitance probes to characterize ebullition dynamics in subtropical wetlands, and our results showed that the bias between the measured fluxes and eddy covariance measurements was larger during the wet season. This suggests that the comparability of methane fluxes from different measurement methods changes with seasonality in subtropical Florida.