Relevance of soil physical properties for the microbial oxidation of methane in landfill covers

The microbial oxidation of methane in landfill cover soils offers great potential to reduce methane emissions from landfills. High methane degradation rates can only be accomplished if the supply of atmospheric oxygen to the methanotrophic community is adequate. Thus, if environmental variables such as pH or nutrient status are not limiting, system performance is suggested to be governed by the share of pores available for gas transport. Diffusion tests as well as column studies were conducted to investigate the effect of air-filled porosity and degree of compaction on diffusivity and methane oxidation efficiency. Results show that the effective diffusion coefficient governing oxygen migration through soil is exponentially related to air-filled porosity space and can be significantly decreased by compaction. Discontinuity and tortuosity of the pore system strongly impeded diffusive migration at air-filled porosities below 10%. In the column study, soil gas composition and methane oxidation rates correlated with both the degree of compaction and the magnitude of advective bottom flux. Low aeration and hence low methane oxidation rates prevailed at high compaction rates and/or high bottom fluxes whereas high rates could be maintained at lower fluxes and/or low compaction rates. At a low degree of compaction (75% of the Proctor density), fluxes of 3.5 g CH₄ m⁻² h⁻¹ could be fully oxidized at all times by a sandy loam, the capacity limit of which was not reached during the experiment. Our studies suggest that soils intended for use as methane-oxidizing biocovers are to maintain an air-filled porosity of at least 14 vol.%. At low and medium degree of compaction, this is provided by sands, loamy sands, sandy loams and some of the coarsely textured loams.