Redox properties of particulate electron acceptors affect anaerobic microbial respiration under oxygen-limited conditions in floodplain soils

Mountain floodplains are characterized by spatiotemporal variations in soil redox conditions that arise due to dynamic hydrological and resulting biogeochemical states. Under oxygen-depleted conditions, solid phase Fe(III) can serve as terminal electron acceptor (TEA) in anaerobic microbial respiration. It remains unclear, however, to what degree the redox properties of Fe(III) phases limit rates of anaerobic respiration and hence organic matter degradation. Here, we assess such limitations in iron-rich soils collected across a gradient in native redox states from the Slate River floodplain (Colorado, U.S.A.). We incubated soils under anoxic conditions and quantified electron transfer to TEAs, TEA reactivity toward electrochemical reduction, and CO₂ production. Fe(III) reduction occurred together with CO₂ production in native oxic soils; no Fe(II) nor CO₂ production was observed in native anoxic soils. Initial CO₂ production rates increased as the reactivity of TEAs toward electrochemical reduction increased across all soil depths and, thus, native soil redox states. The low redox reactivity of TEAs was likely caused by higher acid-extractable Fe(II) concentrations rather than higher crystallinity of Fe(IIII) mineral phases based on analysis of Fe(III) mineral identity and crystallinity using Mössbauer spectroscopy. Our findings indicate that the low redox reactivity of TEAs limited microbial respiration rates in our incubation experiments. This work advances our understanding of controls on anaerobic microbial respiration and can help anticipate organic matter degradation under future hydrological conditions.