Characterization of methanotrophic microbial communities in Biochar Amended Soils in a Novel Biogeochemical Cover System

Biochar amended soils have been explored recently to enhance microbial methane (CH4) oxidation in landfill cover soil by utilizing the biologically favorable physico-chemical properties of biochar such as high moisture retention, internal porosity and specific surface area, and adsorption capacity for gases. Recently, research priorities have expanded to include the mitigation of other components of landfill gas such as carbon dioxide (CO2) and hydrogen sulfide (H2S) along with CH4. To address the release of multiple gases of concern, we have proposed a novel biogeochemical cover system which incorporates steel slag and biochar amended soil layers. Steel slag can mitigate CO2 and H2S through direct geochemical reactions while biochar amended soil can mitigate CH4 through microbial oxidation. In this study, column tests were performed to simulate biogeochemical covers to evaluate the effect of cover configuration on microbial methane oxidation and community composition. Biogeochemical covers included a control soil cover, a biochar-amended soil cover (10% w/w), and methanotroph-enriched activated biochar amended soil covers (5% or 10% w/w). The primary outcome measures of interest were methane oxidation rates and the structure and abundance of methane-oxidizing bacteria in the covers. All column reactors were active in methane oxidation, but columns containing activated biochar amended soils had higher CH4 oxidation rates (133 to 143 ug CH4 g-1 day-1) than those containing non-activated biochar amended soil (maximum of 50 ug CH4 g-1 day-1) or no-biochar soil (maximum of 43 ug CH4 g-1 day-1). All treatments showed significant increases in the relative abundance of methanotrophs from an average relative abundance of 5.6% before incubation to a maximum of 45% following incubation. In activated biochar, the abundance of Type II methanotrophs, primarily Methylocystis and Methylosinus, was greater than that of Type I methanotrophs (Methylobacter) due to which activated biochar amended soils also showed higher abundance of Type II methanotrophs. Conversely, in the control soil and non-activated biochar columns, Type I methanotrophs were dominant. A positive correlation was observed between methane oxidation rate and relative abundance of Type II methanotrophs (R2 = 0.75, p< 0.001) and ratio of Type II/Type I methanotrophs (R2 = 0.95, p < 0.001). Overall, biogeochemical cover profiles showed promising potential for methane oxidation without any adverse effect on microbial community composition and methane oxidation. Biochar activation led to an alteration of the dominant methanotrophic communities and increased methane oxidation.