Consumption of methane by a landfill cover soil under variable moisture and temperature conditions

Landfills are one of the largest anthropogenic sources of methane (CH₄), and hot-spots of CH₄ emissions in landfill cover soils can enrich microbes that oxidize CH₄ to carbon dioxide (CO₂). CH₄ oxidation rates are modulated by multiple variables including soil moisture and temperature, although the interactive effects of these factors on CH₄ oxidation rates have not been well-studied. Here, we conducted a closed-headspace batch experiment with cover soil from a former landfill in Ontario, Canada to measure CH₄ consumption and CO₂ efflux rates associated with variations in soil moisture and temperature simultaneously. Soil samples were incubated under a factorial design of 5 soil moisture contents ranging from 11 to 47% WFPS (water-filled pore space), and 6 temperatures ranging from 1 to 35°C. At each temperature and WFPS combination, CH₄ (812 nmol) was spiked into the headspace, and headspace CH₄ and CO₂ concentrations were measured over 2 hours to calculate CO₂ efflux and CH₄ consumption rates. The maximum CO₂ efflux rate was observed at the maximal WFPS and temperature conditions of this experiment (92 nmol h^-1 g dry wt.^-1 at 47% WFPS and 35°C), while the maximum CH₄ consumption rate was observed at intermediate WFPS and temperature conditions (1.9 nmol h^-1 g dry wt.^-1 at 25% WFPS and 25°C). The CO₂ efflux observed is primarily attributed to the oxidative degradation of soil organic matter. A diffusion-reaction model was fit to the observed data to represent the effects of temperature and soil WFPS on the CH₄ consumption and CO₂ efflux rates. The model predicted similar optimal conditions as those observed for both the CH₄ consumption and the CO₂ efflux rates. The modeling and experimental results show that the dominant controls on optimal soil moisture for CH₄ consumption are moisture limitation of microbial activity and of gas (CH₄ and oxygen) diffusion, versus the interactive effects of moisture limitation of gas (oxygen) diffusion and of solute mobility for CO₂ effluxes. These results provide insight into how seasonal changes in soil moisture and temperature could impact CH₄ oxidation rates, and therefore also net CH₄ emissions, in landfill cover soils and other environments.