HR: 1340h
AN: B43B-0156    [Abstracts]
TI: Effects of Acetate Competition, pH and Soil Structure on the Rates and Pathways of Methane Formation in Tropical Rain Forest Soils
AU: * Teh, Y
EM: yit@nature.berkeley.edu
AF: Department of Environmental Science, Policy, and Management, 151 Hilgard Hall #3110 University of California, Berkeley, CA 94720-3110 United States
AU: * Teh, Y
EM: yit@nature.berkeley.edu
AF: Center for Isotope Geochemistry, MS 70A-4418 Lawrence Berkeley National Laboratory, Berkeley, CA 94720 United States
AU: Dubinsky, E
EM: dubinsky@nature.berkeley.edu
AF: Department of Environmental Science, Policy, and Management, 151 Hilgard Hall #3110 University of California, Berkeley, CA 94720-3110 United States
AU: Carlson, C M
EM: ccarlson@middlebury.edu
AF: Department of Chemistry and Biochemistry, Middlebury College, Middlebury, VT 05753 United States
AU: Silver, W L
EM: wlsilver@nature.berkeley.edu
AF: Department of Environmental Science, Policy, and Management, 151 Hilgard Hall #3110 University of California, Berkeley, CA 94720-3110 United States
AU: Conrad, M E
EM: MSConrad@lbl.gov
AF: Center for Isotope Geochemistry, MS 70A-4418 Lawrence Berkeley National Laboratory, Berkeley, CA 94720 United States
AB: The C isotopic composition of CH$_{4}$ emissions are strongly influenced by the pathway of CH$_{4}$ formation. Contrary to data from other freshwater systems, soil gas and surface flux measurements made in the tropical rain forests of Puerto Rico strongly suggest that CH$_{4}$ produced in these environments was derived from CO$_{2}$ reduction, rather than from acetate consumption. This study explored the effects of bacterial competition for acetate, pH, and soil structure on the pathways of CH$_{4}$ formation in tropical rain forest soils. Our goal was to test two principal hypotheses: (1) ferric iron-reducing bacteria out-competed methanogens for acetate, resulting in greater CO$_{2}$ reduction rather than aceticlastic methanogenesis, and (2) the low pH of tropical rain forest soils favors CO$_{2}$ reduction rather than aceticlastic methanogenesis. In addition, this study also investigated the effect of destroying soil aggregate structure on the pathways and rates of CH$_{4}$ production. Ferric iron-reducing bacteria out-competed methanogens for acetate, reducing CH$_{4}$ production rates by a factor of 3. However, competition for acetate with iron-reducing bacteria did not alter the partitioning of C between different methanogenic pathways, challenging our first hypothesis. Approximately 58 % of the CH$_{4}$ produced in the first 10 days of incubation was derived from acetate in all treatments. Lowering soil pH to 4 significantly reduced CH$_{4}$ production and total acetate utilization, while increasing soil pH to 6 doubled CH$_{4}$ production and increased total acetate utilization. Disruption of soil aggregate structure through slurry conversion negatively affected CH$_{4}$ production. Methanogenesis declined by a factor of 17 after intact, well-aggregated soils were converted to slurries. Slurry conversion also negatively affected the ability of methanogens to compete with other bacteria for acetate. At least 9 times more $^{13}$C-tracer was recovered in CO$_{2}$ compared to CH$_{4}$ after soils were converted to slurries, suggesting that less of the $^{13}$C-labeled acetate was consumed by methanogens after slurry conversion. Lastly, the isotopic data indicate that the relative partitioning of C between aceticlastic and hydrogentrophic pathways was unchanged after conversion of soils to slurries, regardless of the other effects of slurry formation on methanogenesis. The discrepancy between our field and laboratory data suggest that most of the $^{13}$C-depleted CH$_{4}$ was produced from deeper in the soil. Surface emissions were probably dominated by the upward advection of more $^{13}$C-depleted material, rather than by CH$_{4}$ production from the 0-15 cm layer.
DE: 1040 Isotopic composition/chemistry
DE: 1055 Organic geochemistry
DE: 1610 Atmosphere (0315, 0325)
DE: 1615 Biogeochemical processes (4805)
DE: 0315 Biosphere/atmosphere interactions
SC: Biogeosciences [B]
MN: 2004 AGU Fall Meeting