TRANSFORMATIONS IN THE Fe o /Fe d RATIO OF PEDOGENIC IRON OXIDES AFFECTED BY IRON-REDUCING BACTERIA

Abstract

In model experiments under controlled conditions of pH, particle size and amount of glucose, changes in the free Fe oxides (Fe d ) and acid oxalate soluble Fe (Fe o ) of six soils were studied after inoculation with iron-reducing, nitrogen-fixing Clostridia. In all soils, the total amount of Fe d as well as crystalline Fe (Fe d -Fe o ) decreased immediately and significantly, while Fe(II) in solution increased sigmoidally and was inversely proportional to changes in pH. Fe o remained relatively constant throughout the incubation period. As a consequence, the Fe o /Fe d ratio increased in all samples. At the end of the reduction process, total dissolved Fe(II) correlated with the original Fe o content and showed a negative correlation to Fe d -Fe o . 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.