Iron solid-phase differentiation along a redox gradient in basaltic soils

Iron compounds in soil are multifunctional, providing physical structure, ion sorption sites, catalytic reaction-centers, and a sink for respiratory electrons. Basaltic soils contain large quantities of iron that reside in different mineral and organic phases depending on their age and redox status. We investigated changes in soil iron concentration and its solid-phase speciation across a single-aged (400 ky) lava flow subjected to a gradient in precipitation (2200-4200 mm yr⁻¹) and hence redox history. With increasing rainfall and decreasing Eh, total Fe decreased from about 25% to <1% of the soil mass. Quantitative speciation of soil solid-phase iron was constrained by combining ⁵⁷Fe Mössbauer spectroscopy (MBS) at 295 and 4.2 K with powder X-ray diffraction, selective chemical extractions, and magnetic susceptibility measurements. This approach allowed us to partition iron into (1) nanoparticulate and microcrystalline Fe^III-(oxy)hydroxides, (2) microcrystalline and bulk Fe^III-oxides, (3) organic/silicate bound Fe^III, and (4) ferrous iron. The Fe^III-(oxy)hydroxide fraction dominated solid-phase Fe, exhibiting a crystallinity continuum based on magnetic ordering temperature. The continuum extended from well-ordered microcrystalline goethite through nanocrystalline Fe^III-(oxy)hydroxides to a nano Fe^III-(oxy)hydroxide phase of extremely low crystallinity. Magnetic susceptibility was correlated (R² = 0.77) with Fe^III-oxide concentration, consistent with a contribution of maghemite to the otherwise hematite dominated Fe-oxide fraction. The Fe^III-(oxy)hydroxide fraction of total Fe decreased with increasing rainfall and was replaced by corresponding increase in the organic/silicate Fe^III fraction. The crystallinity of the Fe^III-(oxy)hydroxides also decreased with increasing rainfall and leaching, with the most disordered members of the crystallinity continuum, the nano Fe^III-(oxy)hydroxides, gaining proportional abundance in the wetter sites. This finding runs counter to the conventional kinetic expectation of preferential removal of the most disordered minerals in a reductive dissolution-dominated environment. We suggest the persistence of highly disordered Fe phases reflects the dynamic redox conditions of these upland soils in which periods of anoxia are marked by high water-throughput and Fe²⁺(aq) removal, while periodic Fe oxidation events occur in the presence of high concentrations of organic matter. Our ⁵⁷Fe Mössbauer study shows basalt-derived nano-scale Fe^III phases are more disordered than current synthetic analogs and have nano-structural characteristics that are linked to their formation environment.