Low O level enhances CH-derived carbon flow into microbial communities in landfill cover soils.

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