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PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Biogeochemical modeling of CO2 and CH4 production in anoxic Arctic soil microcosms

Peter E. Thornton
Oak Ridge National Laboratory


21 September 2016
Explicit aquatic phase redox and pH added to decomposition cascade model.

The Science                       
Explicit aqueous phase redox, pH, and mineral interaction dynamics were coupled to the Converging Trophic Cascade (CTC) decomposition model, enabling prediction of CO2 and CH4 production from Arctic polygonal tundra soils under laboratory incubations over a range of temperatures.

The Impact
The extended model captures pH dynamics reasonably well in Arctic soil incubations. Temperature and pH sensitivity for microbial reactions is highlighted as an important area for further research.

Soil organic carbon turnover and CO2 and CH4 production are sensitive to redox potential and pH. However, land surface models typically do not explicitly simulate the redox or pH, particularly in the aqueous phase, introducing uncertainty in greenhouse gas predictions. To account for the impact of availability of electron acceptors other than O2 on soil organic matter (SOM) decomposition and methanogenesis, we extended an existing decomposition cascade model (the Converging Trophic Cascade model, or CTC) to link complex polymers with simple substrates and add Fe(III) reduction and methanogenesis reactions. Because pH was observed to change substantially in the laboratory incubation tests and in the field and is a sensitive environmental variable for biogeochemical processes, we use the Windermere Humic Aqueous Model (WHAM) to simulate pH buffering by SOM. To account for the speciation of CO2 among gas, aqueous, and solid (adsorbed) phases under varying pH, temperature, and pressure values, as well as the impact on typically measured headspace concentration, we use a geochemical model and an established reaction database to describe observations in anaerobic microcosms incubated at a range of temperatures (-2, +4, and +8 °C). Our results demonstrate the efficacy of using geochemical models to mechanistically represent the soil biogeochemical processes for Earth system models. The modeling approach demonstrated in this work will be evaluated against additional field and laboratory data, and will be incorporated in new Earth system modeling development to improve prediction of greenhouse gas fluxes in Arctic tundra environments.

Contacts (BER PM)
Daniel Stover and Jared DeForest, Dorothy Koch
Daniel.Stover@science.doe.gov (301-903-0289) and Jared.DeForest@science.doe.gov (301-903-1678), Dorothy.Koch@science.do.egov (301-903-0105)
(PI Contact)
Peter E. Thornton, Environmental Science Division and Climate Change Science Institute, Oak Ridge National Laboratory. thorntonpe@ornl.gov, 865-241-3742


Tang, G., J. Zheng, X. Xu, Z. Yang, D.E. Graham, B. Gu, S. Painter, P.E. Thornton, 2016. Biogeochemical modeling of CO2 and CH4 production in anoxic Arctic soil microcosms. Biogeosciences, 13, 5021-5041. doi:10.5194/bg-13-5021-2016


 Extension of the Converging Trophic Cascade (CTC) decomposition model (Thornton and Rosenbloom, 2005) to include a labile DOC pool (LabileDOC). A portion of the original respiration fraction is assumed to produce labile DOC, which undergoes fermentation, Fe reduction, and methanogenesis to release CO2 and CH4. FeRB, MeGA, and MeGH denote microbial mass pools for Fe reducers, acetoclastic methanogens, and hydrogenotrophic methanogens, respectively. τ is the turnover time. Used with permission.


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