Methane oxidation in landfill cover soils

Large amounts of methane, an important greenhouse gas, are released from landfills. However, microbial oxidation in aerated parts of the landfill cover soils may limit methane fluxes. This thesis reports the results of an investigation assessing the role of microbial methane oxidation in landfill cover soils and the environmental factors controlling methane emissions from landfills. At a landfill site lacking a gas extraction system, methane emissions showed a strong negative correlation with temperature (r2=0.85), suggesting that summer emissions were low owing to a positive effect of temperature on methanotrophic acitivity in the cover soil. A diurnal flux measurement series at this site confirmed the negative impact of temperature on low (diffusive) methane fluxes (less than 55 mg CH4 m-2 h-1). Higher (convective) methane fluxes (between 3.45 and 6.82 g CH4 m-2 h-1) were not related to temperature, but they were negatively correlated with increases in atmospheric pressure. The microbial community structure in the cover soils was characterized based on analyses of PLFA (phospholipid fatty acid). These analyses indicated that oblique methanotrophic bacteria of types I and II accounted for most of the methane oxidation in the cover soils. Nitrous oxide emission rates from the landfills were no higher than what was expected, based on the nitrogen content of the cover soils, and they showed that landfills are not an important source of atmospheric N2O. Comparison of soil samples from landfill covers of different ages indicated that the establishment of methanotrophic populations occurred slowly. In view of this finding, together with the low efficiency of methane oxidation at low temperatures, it can be concluded that the use of methane oxidation as a separate management practice to mitigate emissions has very limited potential, at least in Northern Europe and areas with similar climates. It was estimated that the gas extraction system used at one of the landfill sites mitigated methane fluxes by 80-90%. Highly efficient gas extraction systems combined with the utilization of landfill gas for energy purposes would be the preferable management strategy.