| Summary: | Methane generated from small-rate releases of ethanol-blended fuels into the vadose zone potentially poses health and safety risks. Ubiquitous methane-oxidizing bacteria (MOB) in soils can convert CH into CO, potentially alleviating these risks. Understanding MOB ecology can help to better predict where subsurface CH production may pose health and safety risks and inform site management by identifying environmental conditions not conducive to CH mitigation. We established a densely monitored field site previously unexposed to high CH concentrations to allow the controlled release of CH into the vadose zone and monitoring of subsurface gas migration, surface efflux, and changes to MOB communities by quantitative polymerase chain reaction. During the initial stages of CH injection, soil conditions were very dry, and a large portion of the injected CH reached the ground surface as efflux. During this time, the composition of MOB remained similar to pre-experimental conditions, with the group dominating. Following a period of rainfall and increased soil moisture conditions, efflux dropped, and only approximately 1% of injected CH was detected as efflux. The composition of the MOB community measured immediately following the drop in efflux had shifted so that near the injection point, the group of MOB was now dominant. This behavior followed the predictions of the competitor-stress-tolerator-ruderal (CSR) ecological framework, which suggests that is a stress-tolerating group while is a competitor group capable of degrading large amounts of CH but poorly suited for surviving stressful conditions.
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