Modeling Interactions of Iron Reduction, Methanogenesis, and Organic Matter Degradation in Permafrost Soils

Abstract:

Carbon dioxide (CO₂) and methane (CH₄) releases from thawing permafrost soils depend on interactions between organic matter, nutrients, microbial decomposers, and terminal electron acceptors such as ferric iron (Fe(III)) and iron oxide minerals. Redox cycling of iron (Fe) plays an important role in regulating the amount and relative proportion of CO₂ and CH₄ emissions. Reduction of Fe(III) to Fe(II) provides a preferential pathway to organic matter mineralization over methanogenesis, potentially decreasing methane production rates while Fe(III) is available. Fe(III) availability is dependent on soil minerals and pH as well as total dissolved Fe concentrations. Oxidation of Fe(II) at oxic/anoxic interfaces and during cycles of inundation and drainage can replenish Fe(III) availability and accelerate organic matter mineralization. Land surface models such as the E3SM Land Model (ELM) currently do not simulate dynamics of either terminal electron acceptors or pH, instead assuming that water table and time since inundation can be used as proxies for controls on anaerobic decomposition pathways. We simulated organic matter degradation, Fe(III) reduction, Fe(II) oxidation, and methanogenesis in the reactive transport model PFLOTRAN coupled to ELM. Model simulations were compared with measurements of Fe cycling and methane production from laboratory incubations of permafrost soils. Simulations of inundation/oxygenation cycles showed that Fe availability and iron oxide mineral properties drove the temporal dynamics of methanogenesis, with the onset of maximum methane production controlled by the rate of Fe(III) depletion. Incorporation of redox processes into ELM via PFLOTRAN will improve model simulations of CO₂ and CH₄ emissions from soil systems with dynamic hydrology and biogeochemistry such as permafrost landscapes.