Potential Fe reduction: An assay of the role of Fe in soil metabolism

Background/Question/Methods

In environmental settings with predictable redox gradients, such as aquatic and marine sediments, Fe can be the dominant electron acceptor for carbon (C) oxidation.  However, in more heterogeneous upland soil environments much less is known about the role of Fe reduction as an electron acceptor.  In soils, there is currently no standard method of quantifying Fe reduction rates. Our goal was to develop a laboratory assay of potential Fe reduction.  We collected soils from three upland environments (annual grassland, drained peatland pasture, and tropical rainforest) that varied in poorly crystalline Fe and total C to develop the assay.  We slurried the soils (1:3 soil:water ratio) and incubated them in a glovebox with a dinitrogen headspace.  To evaluate the role of C availability, we added sodium acetate daily at rates up to 0.6 mg C g soil^-1 d^-1. We measured trace gas (carbon dioxide, nitrous oxide, and methane) production, 0.5 M HCl extractable Fe(II), citrate-ascorbate extractable Fe, and pH over 5 days to help determine the timing and magnitude of Fe reduction.  

Results/Conclusions

All of the soils demonstrated high Fe reduction potential.  Even in the driest (< 20 % gravimetric soil moisture), Fe reduction commenced after one day of anaerobic incubation as slurries, On day 3, Fe reduction rates for the 0.05 mg C g soil^-1 d^-1 treatment were 1535 ± 51 μg Fe g soil^-1 d^-1 in the annual grassland soil, 1205 ± 42 μg Fe g soil^-1 d^-1 in the drained peatland soil, and 826 ± 54 μg Fe g soil^-1 d^-1 in the rainforest soil. This contrasts with the trend in poorly crystalline iron oxide pools across the sites: 3.87 ± 0.06 μg Fe g soil^-1 in the annual grassland, 7.49 μg Fe g soil^-1 in the drained peatland, and 20.84 ± 0.19 μg Fe g soil^-1. Across all sites, small C additions (< 0.05 mg C g soil^-1 d^-1) increased Fe reduction rates, while larger C additions decreased Fe reduction. Iron reduction rates typically decreased by day 5 associated with an increase in methane concentrations suggesting that potentially reducible Fe was depleted. Tests with further soils will help to finalize the assay, but we conclude that an anaerobic slurry for 3 days with C additions of ~0.05 mg C g soil^-1 d^-1 will provide an assay to compare potential Fe reduction across diverse upland ecosystems.  Combined with information on soil redox status, this methodology will help to determine the importance of Fe reduction in C oxidation in soils.