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Preferential Microbial Reduction of Hematite Over Goethite in a Brazilian Oxisol

Empresa Brasileira de Pesquisa Agropecuária—Centro de Pesquisa Agropecuária dos Cerrados (EMBRAPA-CPAC), Planaltina, DF, 73300, Brazil

R. B. Bryant^*

Dep. of Agronomy, Cornell Univ., Ithaca, NY 14853

*Corresponding author.

ABSTRACT

Research was conducted to evaluate Fe transformations in a hydrosequence of Oxisols from the Central Plateau of Brazil. The best-drained member of the hydrosequence of soils selected for the study classifies as an Anionic Acrustox (Dark-Red Latosol) and has codominant hematitic and goethitic iron oxide mineralogy and reddish hues (2.5 YR). X-ray diffraction and chemical extraction techniques were used to make detailed analyses of the iron oxides. Peak positions for hematite are coincident with those reported for hematite without foreign ion substitution. Peak positions for goethite are shifted toward a lower d-spacing, which corresponds to 34 mole % of AlOOH substituting for FeOOH. Soil samples from the Bo horizon were subjected to reducing conditions in the laboratory over a range of time periods, using sucrose as a source of energy to speed up the activity of the soil microorganisms. The observed changes in soil mineralogies confirm the preferential reduction and removal of hematite over goethite. Mineralogies and soil colors after reduction treatments are comparable to that of the Anionic Acrustox (Red-Yellow Latosol) found in the wetter section of the hydrosequence. The results of this study support the hypothesis that differences in color and iron oxide mineralogy of soils in this hydrosequence are the result of the preferential reduction of hematite over Al-substituted goethite in the presence of a seasonally high water table.

Contribution from the Dep. of Agronomy, Cornell Univ. Research supported in part by EMBRAPA-CPAC, the TropSoil/Cornell Univ. Program, and Hatch project 409, USDA.

Received for publication April 13, 1988.

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