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 (C H4) during microbial oxidation by an upland temperate soil from Colleg e Woods, New Hampshire, using both in situ and laboratory incubation m easurements. Uptake rates of 1-4.8 mg CH4/m2/d were measured during th e 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 facto r, calculated from field measurements, was alpha = 0.978 +/- .004. Thi s corresponds to a kinetic isotope effect (KIE) of k12/k13 = 1.022 +/- .004. Only a small dependence on temperature was noted for air temper atures between 281 and 296 K. Our results indicate that the KIE of soi l CH4 oxidation is controlled by physical parameters based on gaseous diffusion into the soil. The implications of these results are discuss ed with respect to the global CH4 budget, and balancing CH4 sources an d sinks through the use of delta(CH4)-C-13 measurements.