Iron-oxide crystallinity increases during soil redox oscillations

An Inceptisol A-horizon from Hawaii was subjected to a series of reduction-oxidation cycles—14 d cycle length over a 56 d duration—across the “soil-Fe” [Fe(OH)3.Fe2+(aq), log Ko = 15.74] equilibrium in triplicate redox-stat reactors. Each reducing event simulated the flush of organic C and diminished O2 that accompanies a rainfall-induced leaching of bioavailable reductants from the forest floor into mineral soil. The soil contained considerable amounts of short-range ordered (SRO) minerals (e.g., nano-goethite and allophane) and organic matter (11% org-C). Room temperature and cryogenic 57Fe Mössbauer spectroscopy showed that the iron-bearing minerals were dominated by nano- to micro-scale goethite, and that ferrihydrite was not present. Over the four full cycles, fluctuations in Eh (from 200 to 700 mV) and pFe2+ (from 2.5 to 5.5) were inversely correlated with those of pH (5.5 to 4). Here, we focus on the solubility dynamics of the framework elements (Si, Fe, Ti, and Al) that constitute 35% of the oxygen-free soil dry mass. Intra-cycle oscillations in dissolved (<3 kDa) metals peaked during the reduction half-cycles. Similar intra-cycle oscillations were observed in the HCl and acid ammonium oxalate (AAO) extractable pools. The cumulative response of soil solids during multiple redox oscillations included: (1) a decrease in most HCl and AAO extractable metals and (2) a transformation of SRO Fe (as nano-goethite) to micro-crystalline goethite and micro-crystalline hematite. This may be the first direct demonstration that Fe oxide crystallinity increases during redox oscillations—an a priori unexpected result.