Landfill cover soil methane removal and microbial community composition response to applied rainfall in experimental soil columns

Municipal solid waste (MSW) landfills contribute ∼20% of global anthropogenic methane (CH₄) emissions which can be mitigated using methanotrophic bacteria that become enriched in "hotspots" in the landfill's oxic cover soil. Methanotrophic activity is highly influenced by soil moisture, and local precipitation patterns can therefore have an important impact on CH₄ removal rates and efficiencies. In this study, soil columns simulating MSW landfill "hotspots" were constructed using cover soil from a former MSW landfill site. Columns were incubated over a 35-day experiment under landfill conditions by injecting synthetic landfill gas (48% CH₄ and 52% CO₂) to the bottom. Synthetic rainfall events were performed weekly by adding 146 mL to the top of each column to simulate the regional rainfall conditions. The aim was to delineate the effects of soil moisture dynamics on CH₄ consumption, with the hypothesis that rainfall would gradually decrease CH₄ consumption rates as soil moisture increased. CH₄ removal rates ranged from 108 to 248 g CH₄ m^-2 d^-1 while CH₄ removal efficiency (MRE) ranged from 37 to 100% and decreased by approximately 25% between the first and last rainfall events. Gas concentration depth profiles further supported a reduction in CH₄ oxidation with increased moisture. Changes in gas cycling were not reflected in the soil microbial communities, which were dominated by methanotrophic lineages, and which remained essentially unchanged over the course of the experiment. The relatively high initial MREs observed when moisture conditions were optimal, combined with the unchanging microbial community composition, suggest that there was a well-established methanotroph community in the landfill soil used. Overall, our findings support the need for soil moisture considerations in MSW landfill mitigation plans, particularly for those located in regions with high and variable precipitation which could exacerbate CH₄ emissions.