EVIDENCE FOR NONMICROBIAL OXIDATION OF MANGANESE IN SOIL

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Abstract
Bacteria, isolated from Vermont soil samples, oxidized Mn(II) on a yeast extractmineral salts agar medium. Oxidation was found only in the stationary phase of growth, usually after at least 1 week of incubation. None of these bacteria caused oxidation when inoculated into sterilized soil. Oxidation was found, however, when a small amount of fresh soil suspension was incubated with the sterile soil. Oxidation of Cr(III) to Cr(VI) by Mn oxides in soil samples was used as a sensitive indicator of changes in levels of oxidized Mn in the soils. Manganous sulfate, added to field-moist Nellis loam (Typic Eutrochrept) and Eldridge loamy fine sand (Aquic Udorthent) samples, rapidly oxidized. Streptomycin, chloramphenicol, cyclohexamide, HgCl2, and toluene had little or no effect on this rapid oxidation. Sodium azide inhibited the oxidation of added Mn(II). But sodium azide and chloroform each reduced Mn oxides in untreated soils, indicating that inhibition of Mn oxidation by these substances cannot serve as proof for microbial oxidation of Mn. Oxidation of added Mn(II) by 14 northeast soil samples was shown to be proportional to the level of existing reactive Mn oxides. Arrhenius plots of rates of oxidation in a Nellis loam of added Mn(II) between 1 and 35°C showed that oxidation had nonbiological characteristics. Higher temperatures caused reduction of existing Mn oxides. Generally, oxidation of added Mn(II) was initially rapid, with no lag period. Oxidation is theorized to be autocatalytic involving specific adsorption of Mn(II) on existing Mn oxide surfaces.

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