Limits and dynamics of methane oxidation in landfill cover soils

In order to understand the limits and dynamics of methane (CH₄) oxidation in landfill cover soils, we investigated CH₄ oxidation in daily, intermediate, and final cover soils from two California landfills as a function of temperature, soil moisture and CO₂ concentration. The results indicate a significant difference between the observed soil CH₄ oxidation at field sampled conditions compared to optimum conditions achieved through pre-incubation (60 days) in the presence of CH₄ (50 ml l⁻¹) and soil moisture optimization. This pre-incubation period normalized CH₄ oxidation rates to within the same order of magnitude (112-644 μg CH₄ g⁻¹ day⁻¹) for all the cover soils samples examined, as opposed to the four orders of magnitude variation in the soil CH₄ oxidation rates without this pre-incubation (0.9-277 μg CH₄ g⁻¹ day⁻¹).Using pre-incubated soils, a minimum soil moisture potential threshold for CH₄ oxidation activity was estimated at 1500 kPa, which is the soil wilting point. From the laboratory incubations, 50% of the oxidation capacity was inhibited at soil moisture potential drier than 700 kPa and optimum oxidation activity was typical observed at 50 kPa, which is just slightly drier than field capacity (33 kPa). At the extreme temperatures for CH₄ oxidation activity, this minimum moisture potential threshold decreased (300 kPa for temperatures <5 °C and 50 kPa for temperatures >40 °C), indicating the requirement for more easily available soil water. However, oxidation rates at these extreme temperatures were less than 10% of the rate observed at more optimum temperatures (∼30 °C). For temperatures from 5 to 40 °C, the rate of CH₄ oxidation was not limited by moisture potentials between 0 (saturated) and 50 kPa. The use of soil moisture potential normalizes soil variability (e.g. soil texture and organic matter content) with respect to the effect of soil moisture on methanotroph activity. The results of this study indicate that the wilting point is the lower moisture threshold for CH₄ oxidation activity and optimum moisture potential is close to field capacity.No inhibitory effects of elevated CO₂ soil gas concentrations were observed on CH₄ oxidation rates. However, significant differences were observed for diurnal temperature fluctuations compared to thermally equivalent daily isothermal incubations.