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Title: Modeling CH 4 and CO 2 cycling using porewater stable isotopes in a thermokarst bog in Interior Alaska: results from three conceptual reaction networks

Abstract

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, δ13C-CO2 and δ13C-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 fivemore » 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.« less

Authors:
ORCiD logo [1];  [2];  [3];  [4];  [3]
  1. Univ. of Washington, Seattle, WA (United States)
  2. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States); U.S. Geological Survey, Menlo Park, CA (United States)
  3. U.S. Geological Survey, Menlo Park, CA (United States)
  4. Univ. of Guelph, ON (Canada)
Publication Date:
Research Org.:
Univ. of Washington, Seattle, WA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Biological and Environmental Research (BER) (SC-23)
Contributing Org.:
Bonanza Creek LTER Program, which is jointly funded by NSF (DEB 1026415) and the USDA Forest Service, Pacific Northwest Research Station (PNW01-JV112619320-16)
OSTI Identifier:
1344425
Grant/Contract Number:  
SC0010338
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Biogeochemistry
Additional Journal Information:
Journal Volume: 127; Journal Issue: 1; Related Information: Neumann, Rebecca B; Blazewicz, Steven J.; Conaway, C. H.; Turetsky, Merritt R; Waldrop, Mark P. 2015. Modeling CH4 and CO2 cycling using porewater stable isotopes in a thermokarst bog in Interior Alaska: Results from three conceptual reaction networks, Bonanza Creek LTER - University of Alaska Fairbanks. BNZ:610, http://www.lter.uaf.edu/data/data-detail/id/610; Journal ID: ISSN 0168-2563
Publisher:
Springer
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; carbon fluxes; homoacetogenesis; methanogenesis; methanotrophy; microbial rates; peat; model; 13CO2; 13CH4; carbon isotopes

Citation Formats

Neumann, Rebecca B., Blazewicz, Steven J., Conaway, Christopher H., Turetsky, Merritt R., and Waldrop, Mark P. Modeling CH4 and CO2 cycling using porewater stable isotopes in a thermokarst bog in Interior Alaska: results from three conceptual reaction networks. United States: N. p., 2015. Web. doi:10.1007/s10533-015-0168-2.
Neumann, Rebecca B., Blazewicz, Steven J., Conaway, Christopher H., Turetsky, Merritt R., & Waldrop, Mark P. Modeling CH4 and CO2 cycling using porewater stable isotopes in a thermokarst bog in Interior Alaska: results from three conceptual reaction networks. United States. doi:10.1007/s10533-015-0168-2.
Neumann, Rebecca B., Blazewicz, Steven J., Conaway, Christopher H., Turetsky, Merritt R., and Waldrop, Mark P. Wed . "Modeling CH4 and CO2 cycling using porewater stable isotopes in a thermokarst bog in Interior Alaska: results from three conceptual reaction networks". United States. doi:10.1007/s10533-015-0168-2. https://www.osti.gov/servlets/purl/1344425.
@article{osti_1344425,
title = {Modeling CH4 and CO2 cycling using porewater stable isotopes in a thermokarst bog in Interior Alaska: results from three conceptual reaction networks},
author = {Neumann, Rebecca B. and Blazewicz, Steven J. and Conaway, Christopher H. and Turetsky, Merritt R. and Waldrop, Mark P.},
abstractNote = {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, δ13C-CO2 and δ13C-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.},
doi = {10.1007/s10533-015-0168-2},
journal = {Biogeochemistry},
issn = {0168-2563},
number = 1,
volume = 127,
place = {United States},
year = {2015},
month = {12}
}

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    Works referencing / citing this record:

    Methanotrophy across a natural permafrost thaw environment
    journal, June 2018

    • Singleton, Caitlin M.; McCalley, Carmody K.; Woodcroft, Ben J.
    • The ISME Journal, Vol. 12, Issue 10
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    Effect of permafrost thaw on plant and soil fungal community in a boreal forest: Does fungal community change mediate plant productivity response?
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