The amounts and delta(13)C values of CH4 at subambient concentrations in soil gas were determined along depth profiles in a U.K. grassland (Bronydd Mawr) and woodland (Leigh Woods). The data were used to determine in situ kinetic isotope effects (KIEs) associated with uptake of atmospheric CH4 by high-affinity methanotrophic bacteria that inha bit soil. Three independent calculation approaches yielded similar mean KIEs of 1.0211 +/- 0.0020 (n=18) for Bronydd Mawr and 1.0219 +/- 0.0010 (n=24) for Leigh Woods. Soil methanotrophy KIEs were largely invariant among oak, beech, and pine forest soils of different ages at Leigh Woods but exhibited a statistically significant relationship with methanotroph biomass in individual plots at Bronydd Mawr and Leigh Woods quantified previously by 13C stable isotope probing. This finding, albeit based upon a small data set suggests that 13C and 12C partitioning associated with the global soil sink for atmospheric CH4 may occur in part as a result of biological as well as physical processes. An accurate assessment of the relative importance of each process to the total KIE requires confirmation that significant partitioning of (13)CH4 and (12)CH4 occurs in pore spaces as a result of differences in diffusion rates.