Limits and dynamics of methane oxidation in landfill cover soils

Authors: Spokas, Kurt; BOGNER, J - Landfills+, Inc

Submitted to: Waste Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2009
Publication Date: 3/16/2011
Citation: Spokas, K.A., Bogner, J. 2011. Limits and dynamics of methane oxidation in landfill cover soils. Waste Management. 31(5):823-832.

Interpretive Summary: Rates for methane (CH4) oxidation in soils vary over several orders of magnitude. However, more details are needed about the controlling mechanisms on these oxidation rates. In order to assess these controlling mechanisms, soils were collected from two landfills in California. Landfill soils are unique since they possess high potentials for methane oxidation activity and thereby are well suited for the evaluation of the controlling mechanisms. In this research, we investigated the limits and dynamics of CH4 oxidation in daily, intermediate, and final cover soils from two California landfills as a function of temperature, soil moisture and CO2 concentrations. Typical profiles for CH4 oxidation activity were observed when the soils were incubated without any modifications from field conditions, indicating that the rate of oxidation was a function of depth with maximum rates observed below the surface. However, after a 60 d pre-incubation period with adjusted soil moisture and in the presence of CH4 (5%), the CH4 oxidation rates were consistently higher and more uniform across all depths and cover types from the two locations. The variability in the observed oxidation rates after the pre-incubations was reduced to the same order of magnitude, versus the 4 orders of magnitude observed without pre-incubation periods. These findings suggest that previously-reported oxidation rates for landfill cover soils are related to moisture, temperature and CH4 concentration differences and not true differences in the ultimate potential for CH4 oxidation activity. In this research soil moisture potential was used versus soil moisture contents, since soil texture influences the behavior of moisture in the soil. By utilizing soil moisture potential, these differences across soils of different textures can be accounted for. Overall, a minimum soil moisture threshold for CH4 oxidation activity was estimated at -1400 kPa. There was no distinct moisture optimum observed for CH4 oxidation behavior. However, the rate of CH4 oxidation was not limited by moisture potentials above -500 kPa. No inhibitory effects of elevated CO2 concentrations were observed on CH4 oxidation rates. The results of this study indicate that CH4 exposure histories as well as annual soil temperature and moisture cycles (focusing on soil moisture potential rather than absolute soil moisture content) need to be factored into assessments of CH4 oxidation potential and more importantly need to be accounted for in any modeling or assessment of the annual rates of oxidation for landfill cover soils. This research is of importance in the study of methane oxidation activity in soils, and not just limited to landfill environments. This research will contribute to improving annual greenhouse gas inventory guidelines where the soil processes need to be estimated for the assessment of the net emissions of methane to the atmosphere. Inclusion of these dynamic relationships coupled with appropriate modeling of soil temperature and moisture conditions will improve annual greenhouse gas inventory assessments.

Technical Abstract: 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 to 644 ug 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 to 277 ug 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 – 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.