Carbon isotope fractionation by methane-oxidizing bacteria in tropical rain forest soils

[ 1] Humid tropical forests have the potential to be significant sources or sinks of atmospheric methane ( CH4), a radiatively important trace gas. Methane oxidation can consume a large fraction of the CH4 produced in tropical soils, although controls on this process are poorly understood. Using soil incubation experiments, we investigated the effects of CH4 and oxygen ( O-2) concentrations on C isotope fractionation and CH4 oxidation in tropical rain forest soils. We also explored the effects of these environmental variables on the isotope fractionation factor for CH4 oxidation ( a), which is widely used to evaluate the relative contributions of CH4 production and oxidation to the atmospheric CH4 pool. Methane oxidation was sensitive to CH4 at lower CH4 concentrations (< 850 ppmv) and insensitive to O-2 concentrations between 3 and 21%. Maximum rates of CH4 oxidation were between 8.2 +/- 1.2 and 11.3 +/- 1.5 nmol CH4 hour(-1) g dry soil(-1). Measured values for a were sensitive to both CH4 oxidation rate and CH4 concentration. Alpha was inversely proportional to CH4 oxidation rate ( r(2) = 0.86, P < 0.001) and positively correlated with CH4 concentration ( r(2) = 0.52, P < 0.01). A multiple regression model that included CH4 oxidation rate, CH4 concentration, and the interaction of the two terms explained a high proportion of the variability in a ( r(2) = 0.94, P < 0.0001). These data suggest that it is possible to accurately determine a, allowing for more precise estimates of CH4 oxidation by isotope mass balance.