Online

Lienhart, Peyton

Civil and Environmental Engineering

2023

M.S.

Landfills are the third largest source of anthropogenic methane emissions in the United States and have also been found to produce nitrous oxide, an even more potent greenhouse gas than methane. Intermediate cover soils (ICS) play a major role in reducing greenhouse gas emissions from landfills because they harbor methanotrophic bacteria that degrade methane. However, the extent of methanotrophs in landfill cover soils and the roles of microbial communities in these complex ecosystems is only beginning to be understood. Additionally, the role of ICS in producing nitrous oxide is poorly understood. While methane consumption is a desirable trait, genes involved in methane oxidation may also promote nitrous oxide production. To better understand the role of ICS microbial communities in nitrogen and carbon cycling, samples were collected from twelve locations and two depths from ICS at an active municipal solid waste landfill. Metagenomic analyses of these samples were performed. Further, composite soil samples were incubated with methane and varying amounts of ammonia to assess the impacts of ammonia on nitrous oxide production. After an initial stabilization period, the incubations showed consumption of approximately 100,000 ppm of methane in the headspace in 9 days. All incubations with methane produced small amounts of nitrous oxide (5 - 8 ppm) even when ammonia was not supplemented. Incubations without methane added, however, produced less nitrous oxide. The methanotrophs Methylobacter and unclassified Methylococcaceae were present in the original ICS samples and the incubation samples, and their abundances increased in the incubation with Methylobacter being the dominant methanotroph. Other candidate methanotrophs were also enriched, including Verrucomicrobia. Genes encoding particulate methane monooxygenase/ammonia monooxygenase (pMMO/AMO) were much more abundant than genes encoding soluble methane monooxygenase (sMMO) across the landfill ICS, but sMMO genes were enriched during the incubations with methane. Genes associated with nitrous oxide production via ammonia oxidation (the first process in nitrification) and denitrification were also present where only certain genes were enriched during the incubations. Genes encoding hydroxylamine oxidoreductase (the second step in ammonia oxidation) were largely absent. In total, these results suggest that ammonia oxidation via methanotrophs may result in low levels of nitrous oxide production, but ICS microbial communities have the potential to greatly reduce the overall global warming potential of landfill emissions.