An Integrative Model for Soil Biogeochemistry and Methane Processes. II: Warming and Elevated CO₂ Effects on Peatland CH₄ Emissions

Peatlands are one of the largest natural sources for atmospheric methane (CH₄), a potent greenhouse gas. Climate warming and elevated atmospheric carbon dioxide (CO₂) are two important environmental factors that have been confirmed to stimulate peatland CH₄ emissions; however, the mechanisms underlying enhanced emissions remain elusive. A data-model integration approach was applied to understand the CH₄ processes in a northern temperate peatland under a gradient of warming and doubled atmospheric CO₂ concentration. We found that warming and elevated CO₂ stimulated CH₄ emissions through different mechanisms. Warming initially stimulated but then suppressed vegetative productivity while stimulating soil organic matter (SOM) mineralization and dissolved organic carbon (DOC) fermentation, which led to higher acetate production and enhanced acetoclastic and hydrogenotrophic methanogenesis. Warming also enhanced surface CH₄ emissions, which combined with warming-caused decreases in CH₄ solubility led to slightly lower dissolved CH₄ concentrations through the soil profiles. Elevated CO₂ enhanced ecosystem productivity and SOM mineralization, resulting in higher DOC and acetate concentrations. Higher DOC and acetate concentrations increased acetoclastic and hydrogenotrophic methanogenesis and led to higher dissolved CH₄ concentrations and CH₄ emissions. Both warming and elevated CO₂ had minor impacts on CH₄ oxidation. A meta-analysis of warming and elevated CO₂ impacts on carbon cycling in wetlands agreed well with a majority of the modeled mechanisms. This mechanistic understanding of the stimulating impacts of warming and elevated CO₂ on peatland CH₄ emissions enhances our predictability on the climate-ecosystem feedback.