September 21, 2016

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Biogeochemical Modeling of CO2 and CH4 Production in Anoxic Arctic Soil Microcosms

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 carbon dioxide (CO2) and methane (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.

Summary

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 oxygen (O2) on soil organic matter (SOM) decomposition and methanogenesis, the research team extended an existing decomposition cascade model (Converging Trophic Cascade model or CTC) to link complex polymers with simple substrates and add iron [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, the researchers used 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, they used 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). The study’s 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 incorporated in new Earth system modeling development to improve prediction of greenhouse gas fluxes in Arctic tundra environments.

Principal Investigator

Peter E. Thornton
Oak Ridge National Laboratory
thorntonpe@ornl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This work was supported by the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science, through the Oak Ridge National Laboratory Terrestrial Ecosystem Science Scientific Focus Area and Earth System Modeling (Accelerated Climate Model for Energy project).

References

Tang, G., J. Zheng, X. Xu, and Z. Yang, et al.. "Biogeochemical modeling of CO2 and CH4 production in anoxic Arctic soil microcosms". Biogeosciences 13 (17), 5021–5041  (2016). https://doi.org/10.5194/bg-13-5021-2016.