Reduction-oxidation (redox) reactions involving iron (Fe) are coupled to the release of nutrients, carbon (C), and greenhouse gases in terrestrial soils. A pilot study showed that several different soils exhibited high rates of Fe reduction when subjected to anaerobic conditions, including soils from dry, temperate environments where redox-sensitive processes are not generally thought to be biogeochemically important. However, Fe reduction rates were not strongly correlated to the availability of poorly crystalline Fe oxides. We conducted a survey study to test the potential for Fe reduction in a wide range of terrestrial ecosystems with varying climate, geologic parent material, and C and nutrient availability to elucidate the possible controls on microbial Fe reduction in upland soils. Soil slurries were incubated in an anoxic atmosphere for six days with varying rates of C and Fe substrate addition in the form of acetate and hydrous ferric oxide, respectively. Soils were sampled daily for acid-extractable Fe(II) and Fe(III) to determine the Fe reduction rate, as well as for carbon dioxide and methane production to determine C mineralization rates. Soils were characterized based on their total Fe and poorly crystalline Fe pools, pH, mineralogy, and total C and dissolved organic C concentrations.
Results/Conclusions
Preliminary results suggest that soils with moderate to high C content exhibit the highest rates of Fe reduction despite their relatively low concentrations of reducible Fe available to microbes. In soil slurries that received no substrate amendments, a temperate hardwood forest soil had the highest Fe reduction rate at 601 ± 7.04 μg Fe(III) g soil-1 day-1. Soil from an Arctic site, which had the highest C content at 40.5 ± 0.46% C, also had a high peak Fe reduction rate of 547 ± 12.2 μg Fe(III) g soil-1 day-1. In contrast, a temperate rainforest soil with high total Fe content (25.4 ± 0.64 mg Fe/g soil) and low C content (3.01 ± 0.11% C) had Fe reduction rates of less than 200 μg Fe(III) g soil-1 day-1.