Methane emission and methane oxidation in land-fill cover soil

The vertical profiles of methane and oxygen concentrations were measured in the cover soil at four sites in a restored and covered landfill. At sites 2 and 3 within the landfill area methane was detectable even to the soil surface and emission of methane occured at these two sites. Measured methane emission rates varied seasonally and appeared to be most influenced by soil water content. On an annual basis methane emissions at these two sites were 495 and 909 mol methane m−2 y−1, respectively. At sites 1 and 4 methane was detected in the cover soil but was not present in the immediate subsurface layer, and emission of methane did not occur. Oxidation of methane by bacteria within the soil profile at these two sites appeared to prevent methane emission from the surface. A methane-oxidising microflora had been enriched in the soils of all four landfill sites, as shown by counts of methanotrophs and methylotrophs garden soil not subjected to elevated methane. Counts of methanotrophs and methylotrophs were generally higher in those soil strata where methane concentrations were greatest. Methane oxidation rates were maximum at soil depths where gradients of methane and oxygen overlapped, usually 10–30 cm depth. The depth integrated rates of methane oxidation were very high at sites 2 and 3, the sites also where methane was emitted from the soil surface. A maximum oxidation rate of 450 mmol CH4 m−2 d−1 was measured at site 3. The data suggested that the microflora in the soil above landfill adapted to the presence of elevated methane concentrations by selection of a more methanotrophic community which was able to rapidly oxidise methane. Optimisation of microbial oxidation of methane by bacteria in landfill cover soil may provide a cheap management strategy to minimise the emissions of methane to the atmosphere from landfill.