Spatial Variability of Greenhouse Gas (CH4, N20, and CO2) Soil Fluxes in Restored and Historic Marshes in California

Abstract:

Tidal wetlands exchange trace amounts of the potent greenhouse gases (GHGs) methane (CH₄) and nitrous oxide (N₂0) at the soil-atmosphere interface, potentially diminishing or reversing their role as net carbon dioxide (CO₂) sinks. Thus the effectiveness of wetland restoration and creation as a carbon-offset activity is to a large degree influenced by the magnitude of GHG flux rates and their ecological and management drivers. This study sought to evaluate the spatial variation of GHG emissions in a marsh complex by utilizing the static soil chamber technique and gas chromatography to estimate flux rates. We used stratified random sampling to collect flux measurements within a 1065 acre brackish wetland complex, of which 565 acres were restored, in Tomales Bay, CA. Salinities at sample locations ranged from 0.38 to 30ppt. Flux estimates and statistics were calculated using the HMR procedure (HMR package in R) for static chamber data. Briefly, the procedure categorizes data series and assigns the most appropriate data analysis: linear regression, nonlinear regression (Hutchinson and Mosier) or no significant flux. Daily methane emission rates were modeled using a linear regression of log-transformed methane flux data as the response variable and salinity as the predictor variable, yielding a relatively strong negative relationship (Log(CH₄ mg m^-2day^-1) = 2.69 - 0.0465 x Salinity (ppt), R²= 0.49, P<0.0001). Both methane and nitrous oxide flux rates were significantly higher in samples taken from the restored marsh area (N=46) than from locations in the adjacent reference marsh (N=24) (P=0.012, P=0.049 respectively), however, carbon dioxide flux rates were not statistically different between the two areas. The results of this study suggest that wetland restoration and creation activities for the purpose of carbon-offsets should consider utilizing polyhaline sites (18-30ppt) and long-term monitoring of GHG flux rates to verify net carbon sequestration.