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

The hydrogen and carbon kinetic isotope effects (KIEs) occurring during upt ake of atmospheric methane (CH4) by soils were measured using in situ stati c flux chambers in a native grassland and a temperate forest in Washington State. The hydrogen KIE was alpha(soil)(D) = k(CH4)/k(CH3D) = 1.099 +/- 0.0 30 and 1.066 +/- 0.007 for the grassland and forest, respectively. The carb on KIE of alpha(soil)(c) = k((CH4)-C-12)/k((CH4)-C-13) = 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 variabil ity in alpha(soil) was as large as the between-ecosystem variability. The d ependence of alpha(soil) on alpha(ox) and the KIE during diffusion is descr ibed. The apparent KIE associated with microbial oxidation, alpha(ox), was determined from alpha(soil) and the relative rates of CH4 oxidation and dif fusion in the soil column, derived from observed steady state profiles of s oil air CH4 concentration. The apparent alpha(ox) ranged from 1.094 to 1.20 9 for alpha(ox)(D) and from 1.0121 to 1.0183 for alpha(ox)(C). These are th e first determinations of the hydrogen KIEs during soil uptake of atmospher ic CH, and during aerobic microbial oxidation of CH, at or below atmospheri c concentrations. The KIE during uptake of atmospheric CH4 by soils is sign ificantly different than the KIEs associated with the other sinks of atmosp heric CH4. The interhemispheric asymmetry in the strength of the soil sink of atmospheric CH4 suggests a difference of similar to 6 parts per thousand between the overall hydrogen KIEs in the two hemispheres. Modeling studies of the global atmospheric CH4 budget using deuterium as a tracer must ther efore include alpha(soil)(D).