The atmospheric concentration of CH₄, an important greenhouse gas, has increased by a factor of 2.5 since the pre-industrial era. 13 % of the global anthropogenic emissions derive from landfills. Microbial methane oxidation in the recultivation layer of landfill sites reduces CH₄ emissions from landfills. Understanding the microbial organisms involved helps to improve the design of an appropriate landfill cover.

Samples were collected from four different depths of 0.6 m deep lysimeters filled with a recultivation substrate (compost - gravel mixture). The lysimeters were fumigated with an artificial landfill gas (100 L CH₄ m^-2 d^-1) and irrigated with landfill leachate for two years. The microbial methane oxidizing community was investigated by measurements of phospholipid fatty acids (PLFAs), ergosterol and respiratory quinones. In addition, incorporation of methane into PLFAs of soil microorganisms was studied by isotopic means owing to the fact, that the isotopic ratio of the artificial methane supplied was lower (d¹³C = -50 ‰) than the recultivation substrate (d¹³C = -26 ‰).

The total amount of bacterial PLFAs and especially the PLFAs including type I methanotrophs (14:0 PLFA, 16:1 isomers) and type II methanotrophs (18:1 isomers) increased by a factor of 6, 13 and 11 respectively under fumigation. The methanotrophic biomarker 16:1w8c and 18:1w8c PLFAs were detected but in many cases not baseline separated. The 16:1w8c PLFA pattern was consistent and reflected in 14:0 and 16:1 PLFAs, which was not the case for the type II methanotrophic biomarker (18:1w8c PLFA). Concerning the soil depth distribution, 18:1 PLFAs (type II methanotrophs) were found at lower soil depth than type I methanotrophs. Under landfill leachate irrigation, the methanotrophic population shifted closer to the surface, and 18:1 PLFA contents were decreased. Ubiquinone-8 and ubiquinones-10 mirrored the results obtained by 16:1 and 18:1 PLFA measurements, respectively. Although methane addition increased both 18:1 and 16:1 PLFA concentrations, 18:1 isomers were not depleted in ¹³C. In contrast, 14:0 and 16:1 PLFAs showed d¹³C values of -45 to -50 ‰. Assumingly, type I methanotrophs, e.g. Methylomonas sp. (14:0 PLFA) assimilated methane derived carbon. The fungal biomarkers (18:2w6,9 PLFA and ergosterol) increased up to five times in the upper layer under landfill gas fumigation, but fungi growth was inhibited under landfill leachate irrigation. However, fungal PLFA was not depleted in ¹³C. Therefore, fungi unlikely took part in the carbon circling deriving from methane. Contrarily, gram positive bacteria, which are predominately carbon degraders, were depleted in ¹³C and their d¹³C distribution followed the depth profile of type I methanotrophic PLFAs. Secondary depletion of ¹³C might have occurred either through degradation of methanotrophic bacteria, uptake of depleted CO₂ or exopolymeric substances excreted by type I methanotrophs. Cy17:0 PLFA, which relates to sulfate reducing bacteria, was ¹³C depleted at lower soil depth indicating involvement in anaerobic methane oxidation. No effect of landfill leachate irrigation was found on the depletion of ¹³C in methanotrophs.

To conclude, an active methanotrophic population established during two years landfill gas fumigation. Secondary turnover of the bound carbon involved gram positive bacteria but no fungi.