Low O2 level enhances CH4-derived carbon flow into microbial communities in landfill cover soils
CH 4 oxidation in landfill cover soils plays a significant role in mitigating CH 4 release to the atmosphere. Oxygen availability and the presence of co-contaminants are potentially important factors affecting CH 4 oxidation rate and the fate of CH 4 -derived carbon. In this study, microbial populations that oxidize CH 4 and the subsequent conversion of CH 4 -derived carbon into CO 2 , soil organic C and biomass C were investigated in landfill cover soils at two O 2 tensions, i.e., O 2 concentrations of 21% (“sufficient”) and 2.5% (“limited”) with and without toluene. CH 4 -derived carbon was primarily converted into CO 2 and soil organic C in the landfill cover soils, accounting for more than 80% of CH 4 oxidized. Under the O 2 -sufficient condition, 52.9%–59.6% of CH 4 -derived carbon was converted into CO 2 (CE CO2-C ), and 29.1%–39.3% was converted into soil organic C (CE organic-C ). A higher CE organic-C and lower CE CO2-C occurred in the O 2 -limited environment, relative to the O 2 -sufficient condition. With the addition of toluene, the carbon conversion efficiency of CH 4 into biomass C and organic C increased slightly, especially in the O 2 -limited environment. A more complex microbial network was involved in CH 4 assimilation in the O 2 -limited environment than under the O 2 -sufficient condition. DNA-based stable isotope probing of the community with 13 CH 4 revealed that Methylocaldum and Methylosarcina had a higher relative growth rate than other type I methanotrophs in the landfill cover soils, especially at the low O 2 concentration, while Methylosinus was more abundant in the treatment with both the high O 2 concentration and toluene. These results indicated that O 2 -limited environments could prompt more CH 4 -derived carbon to be deposited into soils in the form of biomass C and organic C, thereby enhancing the contribution of CH 4 -derived carbon to soil community biomass and functionality of landfill cover soils (i.e. reduction of CO 2 emission).