Hydrogen and carbon kinetic isotope effects during soil uptake of atmospheric methane

The hydrogen and carbon kinetic isotope effects (KJEs) occurring during uptake of atmospheric methane (CH 4 ) 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 α D soil = k (CH 4 )/ k (CH 3 D) = 1.099 ± 0.030 and 1.066 ± 0.007 for the grassland and forest, respectively. The carbon KIE of α C soil = k ( 12 CH 4 )/ k ( 13 CH 4 ) = 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 α soil was as large as the between‐ecosystem variability. The dependence of α soil on α ox and the KIE during diffusion is described. The apparent KIE associated with microbial oxidation, α ox , was determined from α soil and the relative rates of CH 4 oxidation and diffusion in the soil column, derived from observed steady state profiles of soil air CH 4 concentration. The apparent α ox ranged from 1.094 to 1.209 for α D ox and from 1.0121 to 1.0183 for α C ox . These are the first determinations of the hydrogen KIEs during soil uptake of atmospheric CH 4 and during aerobic microbial oxidation of CH 4 at or below atmospheric concentrations. The KIE during uptake of atmospheric CH 4 by soils is significantly different than the KIEs associated with the other sinks of atmospheric CH 4 . The interhemispheric asymmetry in the strength of the soil sink of atmospheric CH 4 suggests a difference of ∼6‰ between the overall hydrogen KIEs in the two hemispheres. Modeling studies of the global atmospheric CH 4 budget using deuterium as a tracer must therefore include α D soil .