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Oxygen effects on methane production and oxidation in humid tropical forest soils

Authors

  • Yit Arn Teh,

    1. Department of Environmental Science, Policy, & Management, Ecosystem Sciences Division, 137 Mulford Hall #3114, University of California, Berkeley, CA 94720-3114, USA,
    2. Lawrence Berkeley National Laboratory, Center for Isotope Geochemistry, MS 70A-4418, Berkeley, CA 94720, USA
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  • Whendee L. Silver,

    1. Department of Environmental Science, Policy, & Management, Ecosystem Sciences Division, 137 Mulford Hall #3114, University of California, Berkeley, CA 94720-3114, USA,
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  • Mark E. Conrad

    1. Lawrence Berkeley National Laboratory, Center for Isotope Geochemistry, MS 70A-4418, Berkeley, CA 94720, USA
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Present address: Yit Arn Teh, Department of Geography, 507 McCone Hall, University of California, Berkeley, CA 94720-4740, USA, tel. +510 643 6984, fax +510 642 3370, e-mail: yit@nature.berkeley.edu

Abstract

We investigated the effects of oxygen (O2) concentration on methane (CH4) production and oxidation in two humid tropical forests that differ in long-term, time-averaged soil O2 concentrations. We identified sources and sinks of CH4 through the analysis of soil gas concentrations, surface emissions, and carbon isotope measurements. Isotope mass balance models were used to calculate the fraction of CH4 oxidized in situ. Complementary laboratory experiments were conducted to determine the effects of O2 concentration on gross and net rates of methanogenesis. Field and laboratory experiments indicated that high levels of CH4 production occurred in soils that contained between 9±1.1% and 19±0.2% O2. For example, we observed CH4 concentrations in excess of 3% in soils with 9±1.1% O2. CH4 emissions from the lower O2 sites were high (22–101 nmol CH4 m−2 s−1), and were equal in magnitude to CH4 emissions from natural wetlands. During peak periods of CH4 efflux, carbon dioxide (CO2) emissions became enriched in 13C because of high methanogenic activity. Gross CH4 production was probably greater than flux measurements indicated, as isotope mass balance calculations suggested that 48–78% of the CH4 produced was oxidized prior to atmospheric egress. O2 availability influenced CH4 oxidation more strongly than methanogenesis. Gross CH4 production was relatively insensitive to O2 concentrations in laboratory experiments. In contrast, methanotrophic bacteria oxidized a greater fraction of total CH4 production with increasing O2 concentration, shifting the δ13C composition of CH4 to values that were more positive. Isotopic measurements suggested that CO2 was an important source of carbon for methanogenesis in humid forests. The δ13C value of methanogenesis was between −84‰ and −98‰, which is well within the range of CH4 produced from CO2 reduction, and considerably more depleted in 13C than CH4 formed from acetate.

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