HIGH POTENTIAL FOR IRON REDUCTION IN UPLAND SOILS FROM DIVERSE TERRESTRIAL ECOSYSTEMS

Changes in the redox state of iron (Fe) can be coupled to the biogeochemical cycling of carbon (C), nitrogen, and phosphorus. The importance of Fe in catalyzing redox-driven biogeochemical cycling has been underappreciated in terrestrial ecosystems because they are not typically thought of as anaerobic environments. However, upland soils can experience anaerobic conditions following rainfall events or in microsites of high biological oxygen consumption. Measurements of Fe reduction rates in soils are difficult to compare among studies from different ecosystems, so we developed a standardized assay to quantify potential Fe reduction. We collected soils from upland environments (annual grassland, drained peatland pasture, and a rainforest) that varied in poorly crystalline Fe and total C. We slurried the soils and incubated them in a glovebox with a dinitrogen headspace. To evaluate the role of C availability in potential Fe reduction, we added sodium acetate daily at rates up to 0.6 mg C/g soil/d. We measured methane (CH₄) production, acid- extractable Fe(II), citrate-ascorbate extractable Fe oxides, and pH over 5 days to determine the timing and magnitude of Fe reduction.

In relatively dry soils (< 20 % gravimetric soil moisture), Fe reduction began after one day of anaerobic incubation as slurries, but all of the soils demonstrated high Fe reduction potential. On day 3, Fe reduction rates for the 0.05 mg C/g soil/d treatment were 1535 ± 51 μg Fe(III) g^-1 d^-1 in the annual grassland soil, 1205 ± 42 μg Fe(III) g^-1 d^-1 in the drained peatland soil, and 826 ± 54 μg Fe(III) g^-1 d^-1 in the rainforest soil. This contrasts with the trend in poorly crystalline Fe oxide pools across the sites: 3.87 ± 0.06 μg Fe(III) g^-1 in the annual grassland, 7.49 μg Fe(III) g^-1 in the drained peatland, and 20.84 ± 0.19 μg Fe(III) g^-1 in the rainforest soil. Across all sites, small C additions (< 0.05 mg C/g soil/day) increased Fe reduction rates while larger C additions decreased Fe reduction. Iron reduction rates typically decreased by day 5, associated with an increase in CH₄ concentrations suggesting that potentially reducible Fe was depleted. Our results suggest that upland soils have the potential to exhibit high short-term rates of Fe reduction that may play a role in driving C oxidation and other soil biogeochemical cycles during periods of anaerobiosis.