Coupled Cycling of Iron and Carbon along a Climate Gradient of Forest Soils.
Jon Chorover and Aaron Thompson. Univ of Arizona, 429 Shantz Bldg, P.O. Box 210038, Tucson, AZ 85721-0038
Bacterially-driven iron reduction and oxidation events may bring about wide shifts in the concentration of colloidal and dissolved material because of either Fe mineral dissolution or pH shifts associated with Fe oxidation state changes. This can potentially have profound effects on the mobility of organic and metal constituents. We examined these phenomena by differential centrifugation in an Inceptisol A-horizon from Hawaii subjected to four consecutive 14 day reduction-oxidation cycles across the “soil-Fe” (Fe(OH)3)/Fe2+(aq)) equilibrium in triplicate reactors. Size fractionated samples were characterized based on elemental analysis of the framework elements (Si, C, Fe, Ti, and Al), as well as Zr, Nb, La and U. Intra-cycle oscillations in all colloidal (3 kDa to 160 nm) element concentrations peaked during the reduction half-cycles, mobilizing 10% of total soil Ti and from 1 – 5% of total soil Zr, Nb, La and U at peak dispersion. Colloid dynamics were coupled to the pH shifts accompanying the redox oscillations rather than the fluctuating solubility of Fe oxides. TEM/EDS and mass-balance calculations suggest a carbon-based colloid matrix with localized zones of metal enrichment. The cumulative effects of four full Fe redox cycles included an apparent increase in colloid stability.