Response of Atmospheric-Methane Oxidation to Methane-Flux Manipulation in a Laboratory Soil-Column Experiment

Abstract:

Upland soils are an important sink for atmospheric methane (CH₄). Uptake of atmospheric CH₄ in soils is generally diffusion limited, and is mediated by aerobic CH₄ oxidizing bacteria (MOB) that possess a high-affinity form of a key enzyme, allowing CH₄ consumption at near-atmospheric concentrations (≤ 1.9 µL/L). As cultivation attempts for these high-affinity MOB have shown little success, there remains much speculation regarding their functioning in different environments. For example, it is frequently assumed that they are highly sensitive to physical disturbance, but their response in activity and abundance to changes in substrate availability remains largely unknown.

We present results of a laboratory column experiment conducted to investigate the response in activity and abundance of high-affinity MOB to an increase in CH₄ flux. Intact soil cores, collected at a field site where atmospheric CH₄ oxidation activity is frequently quantified, were transferred into two 1-m-long, 12-cm-dia. columns. The columns were operated at constant temperature in the dark, their headspace being continuously flushed with air. Diffusive gas-transport conditions were maintained in the reference column, whereas CH₄ flux was increased in several steps in the treatment column by inducing advective gas flow using a diaphragm pump. Soil-gas samples periodically collected from ports installed along the length of the columns were analyzed for CH₄ content. Together with measurements of soil-water content, atmospheric CH₄ oxidation was quantified using the soil-profile method. First results indicate that atmospheric CH₄ oxidation activity comparable with the field was maintained in the reference column throughout the experiment. Moreover, high-affinity MOB quickly adjusted to an increase in CH₄ flux in the treatment column, efficiently consuming CH₄. Quantification of MOB abundance is currently ongoing. Our data provide new insights into controls on atmospheric CH₄ oxidation in soils.