Modeling the Interaction of H2 on Root Exudate Degradation and Methanogenesis in Wetland Sediments

Abstract:

CH₄ is produced in wetland sediments from the microbial degradation of organic carbon through multiple fermentation steps and methanogenesis pathways. There are many potential sources of carbon for methananogenesis; in vegetated wetland sediments, microbial communities consume root exudates as a major source of organic carbon. In many methane models propionate is used as a model carbon molecule. This simple sugar is fermented into acetate and H₂, acetate is transformed to methane and CO₂ while the H₂ and CO₂ is synthesized to form an additional CH₄ molecule. The hydrogenotrophic pathway involves the equilibrium of two dissolved gases, CH₄ and H₂.

In an effort to limit CH₄ emissions from wetlands, there has been growing interest in finding ways to limit plant transport of soil gases through root systems. While this may decrease the direct emissions of methane, there is little understanding about how H₂ dynamics may feedback into overall methane production. Since H₂ is used in methane production and produced in propionate fermentation, increased subsurface H₂ concentrations can simultaneously inhibit propionate fermentation and acetate production and enhance hydrogenotrophic methanogenesis.

For this study, we incubated soil samples from vegetated wetland sediments with propionate or acetate and four different hydrogen concentrations. The headspaces from these incubations were simultaneously analyzed for H₂ and CH₄ at multiple time points over two months. The comparison of methane production between different hydrogen concentrations and different carbon sources can indicate which process is most affected by increased hydrogen concentrations. The results from this study were combined with a newly formulated steady-state model of propionate degradation and formation of methane, that also accounts for the venting off both gases via plants. The resulting model indicates how methane production and emissions would be affected by plant volatilization.