CARBON KINETIC ISOTOPE EFFECT ACCOMPANYING MICROBIAL OXIDATION OF METHANE IN BOREAL FOREST SOILS

Atmospheric methane (CH4) oxidation occurs in soils at sites in the Bo nanza Creek L.T.E.R. near Fairbanks, Alaska, USA, at rates less than o r 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 diox ide, and stable isotope (delta(13)C-CH4, delta(13)C-CO2) depth distrib utions were measured at two sites: South facing Aspen (AS2) and North facing Black Spruce (BS2). The combined effects of diffusion and oxida tion are similar at both sites and result in a CH4 concentration decre ase (1.8-0.1 ppm) and a delta(13)C-CH4 increase (-48 parts per thousan d to -43 parts per thousand) from the soil surface to 60-80 cm depth. Isotope flux ratio and diffusion-consumption models were used to estim ate the kinetic isotope effect (KIE); these results agree with the obs erved top-to-bottom difference in delta(13)C-CH4, which is the integra ted result of isotope fractionation due to diffusion and oxidation. Th e 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 lowe r soil respiration rate in the North facing Black Spruce soils is indi cated by fivefold lower soil CO2 concentrations. The similarity in CH4 oxidation at the two sites and the differences in inferred soil respi ration at the two sites suggest that soil CH4 oxidation and soil respi ration 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.