The hydrogen and carbon kinetic isotope effects (KIEs) occurring during uptake of atmospheric methane (CH4) by soils were measured using in situ static flux chambers in a native grassland and a temperate forest in Washington State. The hydrogen KIE was &agr;soilD=k(CH4)/k(CH3D)=1.099¿0.030 and 1.066¿0.007 for the grassland and forest, respectively. The carbon KIE of &agr;soilC=k(12CH4)/k(13CH4)=1.0173¿0.0010 and 1.0181¿0.0004 for the grassland and forest, respectively, compares well to previous determinations in other ecosystems. Local spatial variability in &agr;soil was as large as the between-ecosystem variability. The dependence of &agr;soil on &agr;ox and the KIE during diffusion is described. The apparent KIE associated with microbial oxidation, &agr;ox, was determined from &agr;soil and the relative rates of CH4 oxidation and diffusion in the soil column, derived from observed steady state profiles of soil air CH4 concentration. The apparent &agr;ox ranged from 1.094 to 1.209 for &agr;oxD and from 1.0121 to 1.0183 for &agr;oxC. These are the first determinations of the hydrogen KIEs during soil uptake of atmospheric CH4 and during aerobic microbial oxidation of CH4 at or below atmospheric concentrations. The KIE during uptake of atmospheric CH4 by soils is significantly different than the KIEs associated with the other sinks of atmospheric CH4. The interhemispheric asymmetry in the strength of the soil sink of atmospheric CH4 suggests a difference of ~6? between the overall hydrogen KIEs in the two hemispheres. Modeling studies of the global atmospheric CH4 budget using deuterium as a tracer must therefore include &agr;soilD. ¿ 2000 American Geophysical Union