Effect of CH4 and O2 variations on rates of CH4 oxidation and stable isotope fractionation in tropical rain forest soils
Methane-oxidizing bacteria are the primary sink for CH{sub 4} in reduced soils, and account for as much as 90 percent of all CH{sub 4} produced. Methanotrophic bacteria strongly discriminate against the heavy isotopes of carbon, resulting in CH{sub 4} emissions that are significantly more enriched in {sup 13}C than the original source material. Previous studies have used an isotope mass balance approach to quantify CH{sub 4} sources and sinks in the field, based on the assumption that the fractionation factor for CH{sub 4} oxidation is a constant. This study quantifies the effect of systematic variations in CH{sub 4} and O{sub 2} concentrations on rates of CH{sub 4} 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 CH{sub 4} (100 ppmv, 500 ppmv, 1000 ppmv, and 5000 ppmv) or O{sub 2} (3 percent, 5 percent, 10 percent, and 21 percent). The isotope fractionation factor for CH{sub 4} oxidation was calculated for each incubation using a Rayleigh fractionation model. Rates of CH{sub 4} oxidation varied significantly between CH{sub 4} treatments, with the 100 ppmv CH{sub 4} treatment showing the lowest rate of CH{sub 4} uptake, and the other 3 treatments showing similar rates of CH{sub 4} uptake. Rates of CH{sub 4} oxidation did not vary significantly between the different O{sub 2} treatments. The fractionation factor for CH{sub 4} oxidation varied significantly between the different CH{sub 4} treatments, with the 5000 ppmv CH{sub 4} treatment showing the largest {sup 13}C-enrichment of residual CH{sub 4}. In treatments where CH{sub 4} concentration was not rate-limiting (> 500 ppmv CH{sub 4}), the fractionation factor for CH{sub 4} oxidation was negatively correlated with CH{sub 4} oxidation rate (P < 0.003, r{sup 2} = 0.86). A multiple regression model that included initial CH{sub 4} concentration and CH{sub 4} 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, r{sup 2} = 0.94). The fractionation factor for CH{sub 4} oxidation did not vary significantly between the different O{sub 2} treatments. These results challenge the assumption that the isotope fractionation factor for CH{sub 4} oxidation remains constant, regardless of metabolic activity or CH{sub 4} pool size.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- NASA Earth System Science Fellowship (NGT5-50331), NSF Doctoral Dissertation Improvement Grant No.DEB-0206547 (US)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 835168
- Report Number(s):
- LBNL-55579; R&D Project: 465103; TRN: US200435%%99
- Resource Relation:
- Conference: AGU Fall Meeting, San Francisco, CA (US), 12/08/2003--12/12/2003; Other Information: PBD: 1 Oct 2003
- Country of Publication:
- United States
- Language:
- English
Similar Records
Atmospheric
Carbon isotope fractionation by methane-oxidizing bacteria intropical rain forest soils