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Abstract
Methane is a potent greenhouse gas generated when organic carbon is broken down under anaerobic conditions. Natural and anthropogenic sources, including wetlands, aquatic environments, landfills, lowland rice production and animal production are the largest contributors to the global methane budget. One poorly understood methane-producing system is the degradation of gasoline and biofuels in soil surrounding small-rate vapor releases of fuels into soils, which may also pose health and safety risks. Because the physical environment and methane production rates differ from more well-understood systems, such as wetlands and landfills, basic questions of methane transport in the vadose zone remain. For example, the rate of methane migration in the subsurface and to the atmosphere, the volume of soil impacted by point-source methane production, and environmental controls on methane migration require further study. Methane-oxidizing bacteria, or methanotrophs, are unique microorganisms capable of consuming methane and are an important control on methane migration and atmospheric methane levels. However, a better understanding of methanotroph ecology and controls on their activity will help better predict their ability to mitigate methane emissions now and under changing land use and climate expected in the future. Although methane is only directly used by a narrow group of microorganisms, this one carbon compound may have indirect impacts on the broader microbial community through ecological interactions between methanotrophs and non-methanotrophic bacteria and archaea.
We instrumented a field site allowing direct continuous injection of methane into shallow subsurface soils and monitoring of gas efflux to the atmosphere, soil gas concentrations, temperature and soil moisture. We monitored these parameters during an injection period lasting approximately 7 months, including varying methane injection rates. We also sampled soils throughout the duration of the methane release at different distances from the gas injection, and at different depths, allowing us to examine the methanotroph community using quantitative polymerase chain reaction (qPCR) assays targeting 4 different groups of methanotrophs through the pmoA gene coding for the alpha subunit of the enzyme responsible for the first step of methane oxidation. To assess indirect effects on the broader microbial community, we surveyed the bacterial and archaeal communities using 16S rRNA gene amplicon sequencing.
Methane efflux was high and relatively stable for most of the injection period under low moisture conditions. The proportion of methane reaching the atmosphere increased from approximately 10–11% of injected methane during the low-rate injection periods to approximately 34–52% of injected methane during the higher-rate injection periods. Shortly after a period of precipitation and increased soil moisture began, efflux rapidly dropped to approximately 1% of injected methane, suggesting that soil moisture was a primary control on methane migration. While physical diffusion limitations from increased water-filled pore space likely played a role, tracer and inhibitor experiments strongly suggested that biological methane oxidation also played an important role.
qPCR results demonstrated the presence of Methylosinus pmoA genes prior to methane injection, likely from high-affinity methane oxidizers. This methanotroph group dominated for the duration of the methane injection until the appearance of Methylobacter/Methylosarcina group methanotrophs during the final sampling event, which followed the observed drop in efflux. The increases were localized to sampling points nearest the injection point. The shift in methanotroph communities supports the application of the competitor/stress-tolerator/ruderal model to methanotrophs as proposed by Ho et al. (2013).
Despite the strong impact of methane injection on the methanotroph community, we were unable to detect a strong influence of methane injection on the larger microbial community. This may be due to limitations of the methods and experimental design, or due to a truly narrow impact of methane inputs in this system. Overall, microbial communities resembled those reported in other agricultural and grassland systems and was dominated by Actinobacteria and Proteobacteria with Gemmatamonadetes, Acidobacteria, Planctomycetes, Verrucomicrobia, Bacteroidetes, Chloroflexi, Firmicutes, and Thaumarchaeota also present at > 2% relative abundance. Compared with lateral distance from the injection point, moisture content, or sampling date, depth was most strongly associated with changes in the microbial community.