| Summary: | In landfills, anaerobic biodegradation of solid wastes by methanogens is responsible for about 19% of the anthropogenic CH[subscript 4] - one of the most important greenhouse gases - introduced into the atmosphere each year. Among design advances to reduce CH[subscript 4] emissions, gas collection systems have greatly reduced the environmental impacts of new landfills and are now mandatory in most parts of the world. However, installation of a gas collection system in small or old landfills with low CH[subscript 4] production is not economically feasible and in new landfills, gas collection systems are not 100% efficient. This means that there will always be a certain amount of fugitive emissions. Therefore, any technology or approach that could help reducing atmospheric emissions of CH[subscript 4] from landfills - old or new - will be of great importance in connection with the atmospheric CH[subscript 4] budget. Proper use of techniques pertaining to the fields of geoenvironmental engineering and biotechnology can optimize the methane oxidation process within the landfill cover soil. More precisely, covering landfills using materials that might offer promising conditions to the development of methane oxidation bacteria or methanotrophs would be the equivalent of installing an immense biofilter above the waste mass, here referred to as a passive methane oxidation biocover (PMOB). PMOB efficiency depends on geotechnical soil conditions including the type of soil and porosity. Substrates must have a suitable pore volume to ensure the satisfactory supply of oxygen and methane as well as an adequate retention time for methane within the substrate. Substrate must also have a minimum of organic matter content and provide a satisfactory supply in nutrients that are essential prerequisites for the build-up of methanotrophic biomass. Natural CH[subscript 4] oxidation by methanotrophs, which have been isolated and characterized for a variety of soils, provides an important biological sink for both the atmospheric and the CH[subscript 4] that migrates through the landfill-cover soil. With the long-term goal of developing engineered top covers to optimize CH[subscript 4] abatement, 3 passive methane oxidation biocovers (PMOB) were built within the existing final cover of the St-Nicéphore MSW landfill, Quebec, Canada. As part of the main goals of this field experiment, we followed dynamics and changes in the potential activity and community structure of methanotrophs as a function of time and depth in two different materials: the substrate of one of the PMOBs (mix of compost and sand) and a reference soil (RS), i.e. the existing top cover (silt). The MPN and Q-PCR method was used for methanotrophs counts, while diversity was assessed by means of the DGGE fingerprinting method, and potential CH[subscript 4] oxidation was determined with soil microcosms. Over the monitoring period, changes in the number of methanotrophic bacteria in this PMOB exhibited different development phases: the first phase is an adaptation phases (3 months), in the second it is the actual growth phases and in third we characterise by a decline phases, because of temperature decline in December. The maximum of viable and culturable méthanotrophes were 1.5 x 10[superscript 9] Cells g dw[superscript -1] (1.7 « 3.0 x 10[superscript 8] Cells g dw[superscript -1]) and 500 target of pmoA g dw[superscript -1]. Methanotrophes counts show important variations with depth, while no observable change over time occurred in methanotroph number in the RS. Methanotrophs diversity revealed an overall dominance of Type I methanotrophs (especially genus Methylocaldum and Methylobacter) in both substrate and RS. As expected, the potential methane oxidation was higher in the PMOB substrate than in the RS. The maximum of potential rates were, respectively, 441.1 and 76.0 [micro]g CH[subscript 4] h[superscript -1] g dw[superscript -1].
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