C-13/C-12 FRACTIONATION OF METHANE DURING OXIDATION IN A TEMPERATE FORESTED SOIL

We have made measurements of the C-13/C-12 fractionation of methane (CH4) during microbial oxidation by an upland temperate soil from College Woods, New Hampshire, using both in situ and laboratory incubation measurements. Uptake rates of 1-4.8 mg CH4/m2/d were measured during the active season in New Hampshire while rates of uptake were 2.6-6.8 mg CH4/m2/d in jars used for incubation studies. The fractionation factor, calculated from field measurements, was alpha = 0.978 +/- .004. This corresponds to a kinetic isotope effect (KIE) of k12/k13 = 1.022 +/- .004. Only a small dependence on temperature was noted for air temperatures between 281 and 296 K. Our results indicate that the KIE of soil CH4 oxidation is controlled by physical parameters based on gaseous diffusion into the soil. The implications of these results are discussed with respect to the global CH4 budget, and balancing CH4 sources and sinks through the use of delta(CH4)-C-13 measurements.