Carbon kinetic isotope effect accompanying microbial oxidation of methane in boreal forest soils

Atmospheric methane (CH4) oxidation occurs in soils at sites in the Bonanza Creek L.T.E.R. near Fairbanks, Alaska, USA, at rates less than or equal to 2 mg CH4 m(-2) d(-1); the maximum CH4 oxidizing activity is located in loess at a depth of similar to 15 cm. Methane, carbon dioxide, and stable isotope (delta(13)C-CH4, delta(13)C-CO2) depth distributions were measured at two sites: South facing Aspen (AS2) and North facing Black Spruce (BS2). The combined effects of diffusion and oxidation are similar at both sites and result in a CH4 concentration decrease (1.8-0.1 ppm) and a delta(13)C-CH4 increase (-48 parts per thousand to -43 parts per thousand) from the soil surface to 60-80 cm depth. Isotope flux ratio and diffusion-consumption models were used to estimate the kinetic isotope effect (KIE); these results agree with the observed top-to-bottom difference in delta(13)C-CH4, which is the integrated result of isotope fractionation due to diffusion and oxidation. The KIE for CH, oxidation determined from these measurements is 1.022-1.025, which agrees with previous KIE determinations based on changes in headspace CH4 concentration and delta(13)C-CH4 over time. A much lower soil respiration rate in the North facing Black Spruce soils is indicated by fivefold lower soil CO2 concentrations. The similarity in CH4 oxidation at the two sites and the differences in inferred soil respiration at the two sites suggest that soil CH4 oxidation and soil respiration are independent processes. The soil organic matter responsible for the CO2 flux has a delta(13)C estimated to be -27 to -28 parts per thousand. Copyright (C) 1997 Elsevier Science Ltd.