TRANSFORMATIONS IN THE Feo/Fed RATIO OF PEDOGENIC IRON OXIDES AFFECTED BY IRON-REDUCING BACTERIA

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Abstract
In model experiments under controlled conditions of pH, particle size and amount of glucose, changes in the free Fe oxides (Fed) and acid oxalate soluble Fe (Feo) of six soils were studied after inoculation with iron-reducing, nitrogen-fixing Clostridia. In all soils, the total amount of Fed as well as crystalline Fe (Fed-Feo) decreased immediately and significantly, while Fe(II) in solution increased sigmoidally and was inversely proportional to changes in pH. Feo remained relatively constant throughout the incubation period. As a consequence, the Feo/Fed ratio increased in all samples. At the end of the reduction process, total dissolved Fe(II) correlated with the original Feo content and showed a negative correlation to Fed-Feo. These results are explained by a mechanism in which active non-crystalline iron oxides are reduced in preference to the crystalline forms. The latter are probably transformed and reduced to non-crystalline forms but not until all available active non-crystalline oxides have been exhausted. In the discussion, the reliability of acid oxalate-soluble Fe as a measure of the non-crystalline, active pedogenic iron forms is outlined. Further, the significance of a lowered redox potential (Eh or rH) in reductive transformations of flooded soils is analyzed.
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