­Deciphering coupled mineralogical-redox controls on methane evolution and metal cycling in variably saturated soils and sediments

Dominant controls on soil-sedimentary trace gas evolution and metal cycling often emerge through coupled physical, chemical, and biological processes, many of which remain enigmatic. For example, saturated soils and sediments are important sources of methane to the Earth's atmosphere, yet the extent and magnitude of soil-sediment methane emissions are difficult to constrain owing to complex biogeochemical-physical controls on methanogenic activity. Methanogenesis may be inhibited in the presence of terminal electron acceptors that provide a greater free energy yield; in low-sulfate systems, these typically consist of reducible Fe(III)-(hydr)oxides. However, Fe(III) solids (dominantly iron (hydr)oxides such as ferrihydrite, goethite, and hematite), possess a range of thermodynamic properties that impart different inhibitory effects on methanogenesis. Moreover, a recently proposed mechanism that predicts coupled iron reduction-methanogenesis—direct interspecies electron transfer (DIET)—may further convolute predictions of methane evolution in systems containing iron hydr(oxides). It is possible that, under energetically favorable conditions, metal reducing bacteria contribute an electron to a (semi)conductive iron (hydr)oxide, which enters the conduction band and is subsequently utilized by another organism to produce methane. To explore Fe(III)-(hydr)oxide thermodynamic and semi-conducting effects on soil methanogenic activity, several iron (hydr)oxide minerals were inoculated with wetland sediment, and soluble/solid-phase Fe(II) and methane were monitored throughout the course of the experiments. According to thermodynamic predictions, enhanced methanogenic activity is observed in the presence of crystalline iron (hydr)oxides relative to ferrihydrite, a less thermodynamically stable solid. Hence, the inhibitory effects of solid-phase, reducible "Fe(III)" on methanogenesis is predicted to be dependent on soil-sedimentary iron (hydr)oxide mineralogy. Ongoing measurements examining interspecies electron transfer through iron (hydr)oxide solids are also presented, and implications on the evolution of methane and cycling of redox-sensitive metals in variably saturated mineral soils and surficial sediments are discussed.