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

The hydrogen and carbon kinetic isotope effects (KIEs) 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 alpha(soil)(D) = 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 alpha(soil)(c) = 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 alpha(soil) was as large as the between-ecosystem variability. The dependence of alpha(soil) on alpha(ox) and the KIE during diffusion is described. The apparent KIE associated with microbial oxidation, alpha(ox), was determined from alpha(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 alpha(ox) ranged from 1.094 to 1.209 for alpha(ox)(D) and from 1.0121 to 1.0183 for alpha(ox)(C). These are the first determinations of the hydrogen KIEs during soil uptake of atmospheric CH, and during aerobic microbial oxidation of CH, 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 similar to 6 parts per thousand 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 alpha(soil)(D).