, Volume 127, Issue 1, pp 57–87 | Cite as

Modeling CH4 and CO2 cycling using porewater stable isotopes in a thermokarst bog in Interior Alaska: results from three conceptual reaction networks

  • Rebecca B. Neumann
  • Steven J. Blazewicz
  • Christopher H. Conaway
  • Merritt R. Turetsky
  • Mark P. Waldrop


Quantifying rates of microbial carbon transformation in peatlands is essential for gaining mechanistic understanding of the factors that influence methane emissions from these systems, and for predicting how emissions will respond to climate change and other disturbances. In this study, we used porewater stable isotopes collected from both the edge and center of a thermokarst bog in Interior Alaska to estimate in situ microbial reaction rates. We expected that near the edge of the thaw feature, actively thawing permafrost and greater abundance of sedges would increase carbon, oxygen and nutrient availability, enabling faster microbial rates relative to the center of the thaw feature. We developed three different conceptual reaction networks that explained the temporal change in porewater CO2, CH4, δ 13C–CO2 and δ 13C–CH4. All three reaction-network models included methane production, methane oxidation and CO2 production, and two of the models included homoacetogenesis—a reaction not previously included in isotope-based porewater models. All three models fit the data equally well, but rates resulting from the models differed. Most notably, inclusion of homoacetogenesis altered the modeled pathways of methane production when the reaction was directly coupled to methanogenesis, and it decreased gross methane production rates by up to a factor of five when it remained decoupled from methanogenesis. The ability of all three conceptual reaction networks to successfully match the measured data indicate that this technique for estimating in situ reaction rates requires other data and information from the site to confirm the considered set of microbial reactions. Despite these differences, all models indicated that, as expected, rates were greater at the edge than in the center of the thaw bog, that rates at the edge increased more during the growing season than did rates in the center, and that the ratio of acetoclastic to hydrogenotrophic methanogenesis was greater at the edge than in the center. In both locations, modeled rates (excluding methane oxidation) increased with depth. A puzzling outcome from the effort was that none of the models could fit the porewater dataset without generating “fugitive” carbon (i.e., methane or acetate generated by the models but not detected at the field site), indicating that either our conceptualization of the reactions occurring at the site remains incomplete or our site measurements are missing important carbon transformations and/or carbon fluxes. This model–data discrepancy will motivate and inform future research efforts focused on improving our understanding of carbon cycling in permafrost wetlands.


Carbon fluxes Homoacetogenesis Methanogenesis Methanotrophy Microbial rates Peat Model 13CO2 13CH4 Carbon isotopes 



We thank Julie Shoemaker for input and advice on the reaction network modeling; Burt Thomas for input and advice on the peeper method; Monica Haw, Torren Campbell and Sabrina Sevilgen for laboratory assistance; Lily Cohen and Sarah Wood for field assistance; Jack McFarland for sharing oxygen data; Eugénie Euskirchen, Jennifer Harden, and David McGuire for their participation in the APEX research program; and Jeff Chanton, Larry Miller and an anonymous reviewer for input that improved the manuscript. This material is based upon work supported, in part, by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under Award Number DE-SC-0010338; the U.S. National Aeronautics and Space Administration NASA grant NNX11AR16G; the USGS Climate Science Center and USGS Climate and Land R&D Program; and the USGS Mendenhall Postdoctoral Fellowship program. Research Experiences for Undergraduates (REU) funding and considerable logistic support were provided by the Bonanza Creek LTER Program, which is jointly funded by NSF (DEB 1026415) and the USDA Forest Service, Pacific Northwest Research Station (PNW01-JV112619320-16). Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Data used in this publication are available on the Bonanza Creek LTER website (

Supplementary material

10533_2015_168_MOESM1_ESM.pdf (1.7 mb)
Supplementary material 1 (PDF 1770 kb)


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Copyright information

© Springer International Publishing Switzerland 2015

Authors and Affiliations

  • Rebecca B. Neumann
    • 1
  • Steven J. Blazewicz
    • 2
    • 4
  • Christopher H. Conaway
    • 2
  • Merritt R. Turetsky
    • 3
  • Mark P. Waldrop
    • 2
  1. 1.Department of Civil and Environmental EngineeringUniversity of WashingtonSeattleUSA
  2. 2.U.S. Geological SurveyMenlo ParkUSA
  3. 3.Department of Integrative BiologyUniversity of GuelphOntarioCanada
  4. 4.Lawrence Livermore National LaboratoryLivermoreUSA

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