Effect of pH on Methane Oxidation and Community Composition in Landfill Cover Soil

Municipal solid waste (MSW) landfills are regarded as one of the major sources of greenhouse gas (GHG) emissions across the world. An innovative and sustainable biogeochemical cover system that consists of soil, biochar, and basic oxygen furnace (BOF) slag is being developed to mitigate fugitive landfill emissions such as methane (CH4), carbon dioxide (CO2), and hydrogen sulfide (H2S). Biochar-amended soil can mitigate CH4 emissions by oxidizing CH4 with the help of methanotrophs (CH4-consuming microorganisms), whereas BOF slag can mitigate CO2 and H2S emissions by adsorption and various mineralogical reactions. However, BOF slag is highly alkaline in nature, with pH values usually above 12, and the effect of such high pH on the overall performance of biogeochemical cover system is not known. This study aims at investigating the effect of pH on CH4 oxidation and methanotrophic community structure in landfill cover soil and cultivated consortia through laboratory incubation experiments. In this regard, soil suspension and enrichment cultures were incubated at pH values ranging from 2 to 12, CH4 oxidation rates were measured, and the microbial community structure was analyzed using 16S rRNA gene amplicon sequencing. The optimal pH for CH4 consumption was found to be 7 in enrichment culture and 7.6 in soil suspensions. Very low or no CH4 consumption was observed at extreme pH values of 2 (enrichment culture) and 12 (enrichment culture and soil suspension). A shift in microbial community structure was observed in enrichment cultures initiated at different pH values. Type II methanotrophs were enriched under acidic pH conditions and Type I methanotrophs were enriched in incubations from pH 4 to 10. Soil suspensions were more stable, but also showed slight shifts in the microbial community dominated by Type I methanotrophs and methylotrophs at pH 7.6-10.0. These results demonstrate that at an extreme alkaline pH (similar to 12), CH4 oxidation is inhibited as growth of methane-oxidizing bacteria (MOB) is arrested in the landfill cover soil.