Ahtract. Field measurement of landfill methane emissions indicates natural variability spanning more than seven orders of magnitude, from approximately 0.0004 to more than 4000 g m day . This wide range reflects net emissions resulting fiom production (methanogenesis), consumption (methanotrophic oxidation), and gaseous transport processes. The determination of an "average" emission rate for a given field site requires sampling designs and statistical techniques which consider spatial and temporal variability. Moreover, particularly at sites with pumped gas recovery systems, it is possible for methanotrophic microorganisms in aerated cover soils to oxidize all of the methane from landfill sources below and, additionally, to oxidize methane d f i s i n g into cover soils from atmospheric sources above. In such cases, a reversed soil gas concentration gradient is observed in shallow cover soils, indicating bidirectional diffusional transport to the depth of optimum methane oxidation. Rates of landfill methane oxidation from field and laboratory incubation studies range up to 166 g m day , among the highest for any natural setting, providing an effective natural control on net emissions. It has been shown that methanotrophs in landfill soils can adapt rapidly to elevated methane concentrations with increased rates of methane oxidation related to depth of oxygen penetration, soil moisture, and the nutrient status of the soil.Estimates of worldwide landfill methane emissions to the atmosphere have ranged fiom 9 to 70 Tg yr-', differing mainly in assumed methane yields from estimated quantities of landfilled refuse. At highly controlled landfill sites in developed countries, landfill methane is often collected via vertical wells or horizontal collectors. Recovery of landfill methane through engineered systems can provide both environmental and energy benefits by mitigating subsurface migration, reducing surface emissions, and providing an alternative energy resource for industrial boiler use, on-site electrical generation, or upgrading to a substitute natural gas. Manipulation of landfill cover soils to maximize their oxidation potential could provide a complementary strategy for controlling methane emissions, particularly at older sites where the methane concentration in landfill gas is too low for energy recovery or flaring. For the future, it is necessary to better quanti@ net emissions relative to rates of methane production, oxidation, and transport. Field measurements, manipulative studies, and model development are currently underway at various spatial scales in several countries. Introduction and BackgroundGlobal landfilling practices vary widely and are a major determinant of methane generation and emission rates at a given site. At most controlled landfills, rehse is placed in discrete cells covered by replaced natural soils of local derivation; thus it is appropriate to investigate the range of physical, chemical, and microbiological soil processes active in landfill settings. The rates and controlling var...

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