TY - JOUR
T1 - Iron-oxide crystallinity increases during soil redox oscillations
AU - Thompson, Aaron
AU - Chadwick, Oliver A.
AU - Rancourt, Denis G.
AU - Chorover, Jon
N1 - Funding Information:
We thank Peter Vitousek for his leadership on the Hawaii Ecosystems Project, Heraldo Ferrington for logistical and sampling assistance in the field and Mary Kay Amistadi for ICP-MS analysis. This project was supported by National Research Initiative Competitive Grant No. 2003-35107-13663 from the USDA Cooperative State Research, Education, and Extension Service; National Science Foundation Grant No. DEB-0212245; and The Pennsylvania State University BRIE (IGERT) Grant No. DGE-9972759. The Mössbauer measurements were funded by a Strategic Project Grant from the Natural Sciences and Engineering Research Council of Canada to co-author D.G.R. Finally, we thank Daniel Richter and two anonymous reviewers for helpful comments on the manuscript.
PY - 2006/4/1
Y1 - 2006/4/1
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=33645006656&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=33645006656&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2005.12.005
DO - 10.1016/j.gca.2005.12.005
M3 - Article
AN - SCOPUS:33645006656
SN - 0016-7037
VL - 70
SP - 1710
EP - 1727
JO - Geochmica et Cosmochimica Acta
JF - Geochmica et Cosmochimica Acta
IS - 7
ER -