Stable isotopic determination of methane oxidation: When smaller scales are better

Stable isotope measurements are an effective tool for evaluating methane (CH₄) consumption in landfill soils. However, determining the extent of CH₄ oxidation in soils using this approach can be inherently biased, depending on characteristics of the study site and the sampling strategy that is employed. In this study, we establish the unusual case that sampling at smaller scales captures a better representation of the degree of oxidation occurring in landfill cover soils. We did this by comparing three techniques (Plume, Probe, and Transect) that vary in the location of sampling within a site and in the areal footprint of each sample. The Plume method yielded estimates of CH₄ oxidation that were 13-16% lower than the Transect and Probe methods, respectively. The Probe and Transect methods, two relatively small-scale and high resolution methods, the latter of which has not been previously described, are best suited to quantify CH₄ oxidation in landfill soils as they demonstrably overcome the tendency of stable isotope methods to underestimate CH₄ oxidation at the landfill scale. We recommend the use of these two sampling methods for monitoring the efficacy of landfill CH₄ reduction strategies that are desired to help meet the goals of the Paris Agreement.