Redoximorphic Bt horizons of the Calhoun CZO soils exhibit depth-dependent iron-oxide crystallinity

Abstract

Iron (Fe) oxyhydroxides and their degree of ordering or crystallinity strongly impact the role that Fe plays in ecosystem function. Lower crystallinity phases are generally found to be more reactive than higher crystallinity phases as sorbents for organic matter and chemical compounds, as electron acceptors for organic matter mineralization or as electron donors for dysoxic respiration. We investigated Fe solid phase speciation as a function of soil depth in a redoximorphic upland soil profile. We examined a redoximorphic upland soil profile, which displayed alternating Fe-enriched and Fe-depleted zones of the Bt horizons with platy structure from 56 to 183 cm depth at the Calhoun Critical Zone Observatory in South Carolina, USA. Redoximorphic Fe depletion and enrichment zones were sampled to enable a detailed investigation of Fe mineralogy during redox transformations. All samples were characterized by total elemental analysis, X-ray diffraction, and 57Fe Mossbauer spectroscopy. Total Fe in the Fe-enriched and Fe-depleted zones was 26.3 – 61.2 and 15.0 – 22.7 mg kg−1 soil, respectively, suggesting periodic redox cycling drives Fe redistribution within the upland soil profile. The Mossbauer data clearly indicated goethite (56 – 74% of total Fe) and hematite (7 – 31% of total Fe) in the Fe-enriched zones, with the proportion of hematite increasing with depth at the expense of goethite. In addition, the overall crystallinity of Fe phases increased with depth in the Fe-enriched zones. In contrast to Fe-enriched zones, Fe-depleted zones contained no hematite and substantially less goethite (and of a lower crystallinity) but more aluminosilicates-Fe(III) (e.g., hydroxy-interlayered vermiculite, biotite, kaolinite) with XRD and Mossbauer data suggesting a shift from oxidized biotite-Fe(III) at depth to hydroxy-interlayered vermiculite plus low-crystallinity goethite in the Fe-depleted zones in the upper Bt. Our data suggest the varied crystalline states of hematite and goethite may be important for Fe reduction over long-term time scales. The persistence of low-crystallinity Fe phases in Fe depletion zones suggests that both dissolution and re-precipitation events occur in the Fe-depleted layers. These variations in Fe phase abundance and crystallinity within similar redoximorphic features suggest that Fe likely shifts ecosystem roles as a function of soil depth and likely has more rapid Fe cycling in the upper Bt horizons in upland soils, while serving as a weathering engine at depth.