In soils, ecologically relevant iron reduction depends on the consistent formation of reducible Fe(III) phases through Fe(II) oxidation events. We exposed soils from an upland valley of the Bisley Watershed at the Luquillo Critical Zone Observatory, PR to a range of O2 fluxes by pulsing air at 100 ml h-1, 10 ml h-1, or 1 ml h-1 for 7 h during a 24 h oxidation period, following a 7 d anoxic period, in soil suspensions. This sequence was repeated three times for a total of three redox cycles with a constant 8 d frequency and length of oxic to anoxic time (1:7). We found that faster O2 flux during oxic conditions lead to faster net rates of Fe(III) reduction during subsequent anoxic periods. In a second experiment using an aqueous 57Fe isotope spike and Mössbauer spectroscopy (MB), we tracked Fe through a 24 h oxidation event and 7 d reducing event. After Fe oxidation, we found essentially all of the added 57Fe was incorporated into the solid phase predominately as a ferric phase did not magnetically order at 4.4 Kelvin in the MB spectrum. During the subsequent anoxic period, the aqueous 57/54Fe ratio increased consistent with the reduction and solubilization of the recently precipitated 57Fe(III) spike. However, after 7 days of reducing conditions, significant portions of the spike remained in the solid phase and contributed to an emergent ferrous doublet in the MB spectra that matched the abundance of Fe(II) in the 0.5 M HCl extract. These results suggest rapid oxidation of Fe(II) creates de novo Fe(III)-solids that are more available for Fe reduction than existing soil Fe(III) minerals. The likelihood for Fe reduction to play an important ecological role in soils thus depends on the delivery of oxygen for intermittently regenerating and modifying reactive Fe solid phases.

Oxygen flux modulates net rates of Fe reduction and Fe solid phase composition in redox dynamic soils

249th National Meeting (2015)

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