Evidence for Anaerobic CH₄ Oxidation in Freshwater Peatlands

Abstract

This study involved in vitro assays of peat soil to investigate the occurrence, importance and potential mechanism(s) of anaerobic methane oxidation (AOM) in several northern peatlands ranging from ombrotrophic bog to minerotrophic fen. Although strong evidence suggests that AOM is linked to sulfate reduction in marine sediments, very little is known about AOM in freshwater systems such as northern peatlands, which have large methane (CH ₄ ) production and are a significant source of atmospheric CH ₄ . Our results showed a mean net AOM rate of 17 ± 2.6 nmol kg ^{− 1} s ^{− 1} with a maximum rate of 176 nmol kg ^{− 1} s ^{− 1} for a minerotrophic fen in central New York. AOM was demonstrated with three independent methods to verify our results: (a) additions of methanogenic inhibitors, (b) stable isotope enrichment ( ¹³ C-CH ₄ ), and (c) natural abundance stable isotope analysis ( ¹³ C-CH ₄ ). These experiments confirmed that AOM occurs simultaneously with methanogenesis, consumes a significant portion of gross CH ₄ production, and significantly fractionates C isotopes (∼ −127‰). Experiments using a variety of potential electron acceptors demonstrated that Fe(III) and SO₄ ^{2 −} are not quantitatively important, while the role of NO ₃ ⁻ is uncertain and deserves more attention. The exact mechanism(s) for AOM in peat soils remains unclear; however the AOM rates reported in this study are similar to those reported for CH ₄ production and aerobic CH ₄ oxidation in northern peatlands, suggesting that AOM may be an important control on CH ₄ fluxes in northern peatland ecosystems.

Keywords:

• Anaerobic CH₄ oxidation
• peat soil
• wetland
• archaea
• electron acceptor
• methanogenesis
• stable isotopes
• diffusion

ACKNOWLEDGMENTS

Numerous people provided invaluable comments and insights on earlier drafts of this manuscript, including, Jason Demers, Timothy Fahey, Peter Groffman, Wendy Mahaney, Peter Weishampel, Tim Moore, and several anonymous reviewers. Stephen Zinder also provided comments and was the intellectual inspiration for this study. Joseph von Fischer helped with isotope analyses and calculations using the isotope-mixing model he developed, and stable isotope data and access to facilities were provided by David L. Valentine at UC Santa Barbara and Stanley C. Tyler at UC Irvine. Derek Lovley and Betsy Blount provided advice and training on laboratory assays; study site selection and sampling design ideas were given by Scott Bridgham, Sandy Verry and Brad Dewey (Minnesota), and Bosse Svensson and Mats Öqvist (Sweden). Jane Carlson worked numerous hours in the lab and the field. A National Science Foundation Doctoral Dissertation Improvement Grant provided funding for KAS and JBY.

Notes

¹ Smemo and Yavitt (2006)

²Dise (1991)

³Dise and Verry (2000)

⁴Santelmann (1991)

⁵ Bridgham et al. (1998)

⁶ Rosswall et al. (1975)

⁷Svenssonetal et al. (1999)

⁸S. Bridgham (personal communication)

¹Samples that had a r² > 0.6 for headspace CH₄ flux and ¹³C flux

²Based on a δ¹³C-CH₄ of −60‰ for CH₄ production

³Calculated for the entire incubation time of 180 hours

⁴Non-zero values due to instrument drift.

¹Treatment means for samples that had a r² > 0.6 for headspace CH₄ flux and ¹³C flux.

²Based on a δ¹³C-CH₄ of −60‰ for CH₄ production.

³Calculated for the entire incubation time of 96 hours.

⁴Small amount of production measured in controls due to slight GC drift.

¹Treatment means for samples that had a r² ≥ 0.6 for headspace CH₄ flux and ¹³C flux.

²Based on a δ¹³C─CH₄ of −60‰ for CH₄ production.

³Calculated for the entire incubation time of 106 hours.

⁴Negative values due to measured mass greater than calculated mass.

⁵Small amount of production or consumption measured in controls likely due to slight GC and MS drift, and/or incomplete kill.