Use of methanotrophically activated biochar in novel biogeochemical cover system for carbon sequestration: Microbial characterization.

Biochar-amended soils have been explored to enhance microbial methane (CH) oxidation in landfill cover systems. Recently, research priorities have expanded to include the mitigation of other components of landfill gas such as carbon dioxide (CO) and hydrogen sulfide (HS) along with CH. In this study, column tests were performed to simulate the newly proposed biogeochemical cover systems, which incorporate biochar-amended soil for CH oxidation and basic oxygen furnace (BOF) slag for CO and HS mitigation, to evaluate the effect of cover configuration on microbial CH oxidation and community composition. Biogeochemical covers included a biochar-amended soil (10% w/w), and methanotroph-enriched activated biochar amended soil (5% or 10% w/w) as a biocover layer or CH oxidation layer. The primary outcome measures of interest were CH oxidation rates and the structure and abundance of methane-oxidation bacteria in the covers. All column reactors were active in CH oxidation, but columns containing activated biochar-amended soils had higher CH oxidation rates (133 to 143 μg CH g day) than those containing non-activated biochar-amended soil (50 μg CH g day) and no-biochar soil or control soil (43 μg CH g day). 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. Overall, biogeochemical cover profiles showed promising potential for CH 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 CH oxidation.