Low O-2 level enhances CH4-derived carbon flow into microbial communities in landfill cover soils

CH4 oxidation in landfill cover soils plays a significant role in mitigating CH4 release to the atmosphere. Oxygen availability and the presence of co-contaminants are potentially important factors affecting CH4 oxidation rate and the fate of CH4-derived carbon. In this study, microbial populations that oxidize CH4 and the subsequent conversion of CH4-derived carbon into CO2, 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. CH4-derived carbon was primarily converted into CO2 and soil organic C in the landfill cover soils, accounting for more than 80% of CH4 oxidized. Under the O-2-sufficient condition, 52.9%-59.6% of CH4-derived carbon was converted into CO2 (CECO2-C), and 29.1%-39.3% was converted into soil organic C (CEorganic-C). A higher CEorganic-C and lower CECO2-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 CH4 into biomass C and organic C increased slightly, especially in the O-2-limited environment. A more complex microbial network was involved in CH4 assimilation in the O-2-limited environment than under the O-2-sufficient condition. DNA-based stable isotope probing of the community with (CH4)-C-13 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 CH4-derived carbon to be deposited into soils in the form of biomass C and organic C, thereby enhancing the contribution of CH4-derived carbon to soil community biomass and functionality of landfill cover soils (i.e. reduction of CO2 emission). (C) 2019 Elsevier Ltd. All rights reserved.