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

Abstract

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 (O2) 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 (α), 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 O2 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 α were sensitive to both CH4 oxidation rate and CH4 concentration. Alpha was inversely proportional to CH4 oxidation rate (r2 = 0.86, P < 0.001) and positively correlated with CH4 concentration (r2 = 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 α (r2 = 0.94, P < 0.0001). These data suggest that it is possible to accurately determine α, allowing for more precise estimates of CH4 oxidation by isotope mass balance.