Lawrence Berkeley National Laboratory

Methane-oxidizing bacteria are the primary sink for CH4 in reduced soils, and account for as much as 90 percent of all CH4 produced. Methanotrophic bacteria strongly discriminate against the heavy isotopes of carbon, resulting in CH4 emissions that are significantly more enriched in 13C than the original source material. Previous studies have used an isotope mass balance approach to quantify CH4 sources and sinks in the field, based on the assumption that the fractionation factor for CH4 oxidation is a constant. This study quantifies the effect of systematic variations in CH4 and O2 concentrations on rates of CH4 oxidation and stable isotope fractionation in tropical rain forest soils. Soils were collected from the 0-15 cm depth, and incubated with varying concentrations of CH4 (100 ppmv, 500 ppmv, 1000 ppmv, and 5000 ppmv) or O2 (3 percent, 5 percent, 10 percent, and 21 percent). The isotope fractionation factor for CH4 oxidation was calculated for each incubation using a Rayleigh fractionation model. Rates of CH4 oxidation varied significantly between CH4 treatments, with the 100 ppmv CH4 treatment showing the lowest rate of CH4 uptake, and the other 3 treatments showing similar rates of CH4 uptake. Rates of CH4 oxidation did not vary significantly between the different O2 treatments. The fractionation factor for CH4 oxidation varied significantly between the different CH4 treatments, with the 5000 ppmv CH4 treatment showing the largest 13C-enrichment of residual CH4. In treatments where CH4 concentration was not rate-limiting (> 500 ppmv CH4), the fractionation factor for CH4 oxidation was negatively correlated with CH4 oxidation rate (P < 0.003, r2 = 0.86). A multiple regression model that included initial CH4 concentration and CH4 oxidation rate as independent variables accounted for 94 percent of the variability in the isotope fractionation data, suggesting that both factors are important in determining the extent of isotopic fractionation (P < 0.002, r2 = 0.94). The fractionation factor for CH4 oxidation did not vary significantly between the different O2 treatments. These results challenge the assumption that the isotope fractionation factor for CH4 oxidation remains constant, regardless of metabolic activity or CH4 pool size.