Dynamic interactions between methanogenesis and microbial iron reductions drive carbon release from Arctic tundra

Arctic tundra stores large quantities of organic matter. Increasing temperature is expected to accelerate rates of carbon decomposition and release of greenhouse gases CO₂ and CH₄ from Arctic soils. Methanogenesis and microbial iron reduction are two dominant anaerobic microbial reactions and major contributors to Arctic carbon cycling. The relative importance of microbial iron reduction and methanogenesis vary substantially across varying topographic features and geochemical gradients, posing a significant challenge to accurately predict the fate of carbon in Arctic soils. Here we present a systematic investigation on the dynamic relationship between methanogenesis and iron reduction. Although traditional models assume that the reactions are segregated into distinct zones defined by competition, numerous studies demonstrate cooccurrence of methanogenesis and iron reduction. We identified that the broad range of soil pH in the Arctic soils exerts significant control over the relative contribution of methanogenesis and iron reduction. pH affects both the kinetics of iron-reducing reactions and their significance relative to other microbial reactions. We conducted a series of anoxic incubations with fixed soil pH ranging from 4.5 to 8.5 to illustrate difference in pH responses for methanogenesis and iron reduction and their potential feedbacks on soil pH. Molecular analyses of the microbial community revealed opposite trends of methanogens and iron reducers in response to pH gradient. The dynamic interplay of iron reduction and methanogenesis is tightly coupled with pH, which could only be captured by mechanistic models with explicit representation of microbial functions and aqueous phase chemistry feedbacks. Thus, we constructed a microbial explicit dynamic model with explicit parameterization of pH response for iron reducers and methanogens for more accurate simulation of carbon flow partitioned among these two different electron accepting processes. This study directly addresses our knowledge gap of the interactions between methanogenesis and microbial iron reduction, and advances our understanding of environmental controls on microbial mediated carbon cycling with a model-experiment integration method.