HR: 0830h
AN: B31E-0359    [PDF]
TI: Effect of CH$_{4}$ and O$_{2}$ variations on rates of CH$_{4}$ oxidation and stable isotope fractionation in tropical rain forest soils
AU: * Teh, Y
EM: yit@nature.berkeley.edu
AF: University of California at Berkeley, Ecosystem Sciences Division, Department of ESPM, 151 Hilgard Hall, Berkeley, CA 94720-3110 United States
AU: Conrad, M
EM: MSConrad@lbl.gov
AF: Lawrence Berkeley National Laboratory, Center for Isotope Geochemistry, Earth Sciences Division, MS 70A-4418, Berkeley, CA 94720 United States
AU: Silver, W L
EM: wsilver@nature.berkeley.edu
AF: University of California at Berkeley, Ecosystem Sciences Division, Department of ESPM, 151 Hilgard Hall, Berkeley, CA 94720-3110 United States
AU: Carlson, C M
EM: ccarlson@middlebury.edu
AF: Middlebury College, Department of Chemistry and Biochemistry, Middlebury, VT 05753 United States
AB: Methane-oxidizing bacteria are the primary sink for CH$_{4}$ in reduced soils, and account for as much as 90% of all CH$_{4}$ produced. Methanotrophic bacteria strongly discriminate against the heavy isotopes of carbon, resulting in CH$_{4}$ emissions that are significantly more $^{13}$C-enriched than the original source material. Previous studies have used an isotope mass balance approach to quantify CH$_{4}$ sources and sinks in the field, based on the assumption that the fractionation factor for CH$_{4}$ oxidation is a constant. This study quantifies the effect of systematic variations in CH$_{4}$ and O$_{2}$ concentrations on rates of CH$_{4}$ 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$_{4}$ (100ppmv, 500 ppmv, 1000 ppmv and 5000 ppmv) or O$_{2}$ (3%, 5%, 10% and 21%). The isotope fractionation factor for CH$_{4}$ oxidation was calculated for each incubation using a Rayleigh fractionation model. Rates of CH$_{4}$ oxidation varied significantly between CH$_{4}$ treatments, with the 100 ppmv CH$_{4}$ treatment showing the lowest rate of CH$_{4}$ uptake, and the other 3 treatments showing similar rates of CH$_{4}$ uptake. Rates of CH$_{4}$ oxidation did not vary significantly between the different O$_{2}$ treatments. The fractionation factor for CH$_{4}$ oxidation varied significantly between the different CH$_{4}$ treatments, with the 5000 ppmv CH$_{4}$ treatment showing the largest $^{13}$C-enrichment of residual CH$_{4}$. In treatments where CH$_{4}$ concentration was not rate-limiting ($>$500 ppmv CH$_{4}$), the fractionation factor for CH$_{4}$ oxidation was negatively correlated with CH$_{4}$ oxidation rate (P$<$0.003, r$^{2}$ = 0.86). A multiple regression model that included initial CH$_{4}$ concentration and CH$_{4}$ oxidation rate as independent variables accounted for 94% of the variability in the isotope fractionation data, suggesting that both factors are important in determining the extent of isotopic fractionation (P$<$0.002, r$^{2}$ = 0.94). The fractionation factor for CH$_{4}$ oxidation did not vary significantly between the different O$_{2}$ treatments. These results challenge the assumption that the isotope fractionation factor for CH$_{4}$ oxidation remains constant, regardless of metabolic activity or CH$_{4}$ pool size.
DE: 0315 Biosphere/atmosphere interactions
DE: 0400 Biogeosciences
DE: 1040 Isotopic composition/chemistry
DE: 4805 Biogeochemical cycles (1615)
SC: Biogeosciences [B]
MN: 2003 Fall Meeting