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Simulated Hydrological Dynamics and Coupled Iron Redox Cycling Impact CH4 Production in an Arctic Soil

Introduction

Simulations of coupled carbon and Fe cycling show that the effect of Fe on CH4 production varies with substrate availability.

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The fate of organic C in permafrost soils is important to the climate system due to the large global stocks of permafrost C. Thawing permafrost can be subject to dynamic hydrology, making redox processes an important factor controlling soil organic matter (SOM) decomposition rates and greenhouse gas production. In Fe-rich permafrost soils, Fe(III) can serve as a terminal electron acceptor, promoting anaerobic respiration of SOM and increasing pH. Current large-scale models of Arctic C cycling do not include Fe cycling or pH interactions. Here, a geochemical reaction model was developed by coupling Fe redox reactions and C cycling to simulate SOM decomposition, Fe(III) reduction, pH dynamics, and greenhouse gas production in permafrost soils subject to dynamic hydrology. The team parameterized the model using measured CO2 and CH4 fluxes as well as changes in pH, Fe(II), and dissolved organic C concentrations from oxic and anoxic incubations of permafrost soils from polygonal permafrost sites in northern Alaska. In simulations of repeated oxic-anoxic cycles, Fe(III) reduction during anoxic periods enhanced CO2 production, while the net effect of Fe(III) reduction on cumulative CH4 fluxes depended on substrate C availability. With lower substrate availability, Fe(III) reduction decreased total CH4 production by further limiting available substrate. With higher substrate availability, Fe(III) reduction enhanced CH4 production by increasing pH. These results suggest that interactions among Fe-redox reactions, pH, and methanogenesis are important factors in predicting CH4 and CO2 production, as well as SOM decomposition rates in Fe-rich, frequently waterlogged Arctic soils. 

Citation: Sulman, B. N., F. Yuan, T. O’Meara, B. Gu, E. Herndon, J. Zheng, P. Thornton, and D. E. Graham. 2022. “Simulated hydrological dynamics and coupled iron redox cycling impact methane production in an Arctic soil.” Journal of Geophysical Research-Biogeosciences. In press.

Image with caption
Integrated CO2 and CH4 fluxes and the difference in cumulative fluxes between simulations with and without Fe redox reactions over a 150-day period

Integrated CO2 and CH4 fluxes and the difference in cumulative fluxes between simulations with and without Fe redox reactions over a 150-day period with varying maximum fermentation rate and length of each oxic-anoxic cycle. Colors show different maximum fermentation rates (representing substrate availabilities). Dotted lines show simulations with direct inhibition of methanogenesis by Fe(III), and solid lines show simulations without direct inhibition. (a) CO2 flux; (b) difference in CO2 fluxes between simulations with and without Fe redox reactions; (c) CH4 flux; (d) difference in CH4 flux between simulations with and without Fe redox reactions.

Contact(s)

For more information, please contact:

Benjamin Sulman

sulmanbn@ornl.gov
Project Phase(s)