Oxidation soil microorganisms

Mercury and the noble metals are found in nature in their elemental forms however, they are generally unreactive and so their occurrence in the soil solution is limited. Some elements, such as sulfur, can be reduced to their elemental state (see Figure 4.8) by soil microorganisms however, they can also easily be both oxidized and the oxidized forms reduced and so are rarely found in their elemental form in soil. 

Molinate has a low toxicity to rats, oral LDso=720 mg/kg, and is rapidly metabolized by plants to CO2 (1) (5) and naturally occurring plant constituents (1). Molinate is also readily metabolized by soil microorganisms (6). After incubation of molinate with Bacillus sp. 24, Nocardia sp. 119, and Micrococcus sp. 22r which were isolated from Russian garden soils and rice field drains (7,8), it was found that molinate was completely degraded into various hydroxy and oxidized products in the medium. Molinate can be metabolized to its corresponding sulfoxide in the mouse in vivo and by the microsome-NADPH system of mouse liver (9, 10). Hubbell et al. (11) and DeBaun et al. (12) also found molinate sulfoxide along with other polar and nonpolar metabolites in rat urine. 

Blackmer, A. M., Bremner, J. M., and Schmidt, E. L. (1980). Production of nitrous oxide by ammonia-oxidizing chemoautotrophic microorganisms of soil. Appl. Environ. Microbiol. 40, 1060-1066. 

In another study (Watwood, White Dahm, 1991), benzene mineralization occurred in soils incubated under an inert gas for 4 weeks. No attempt was made to remove residual oxygen from these soils and the possibility exists that benzene mineralization may have been linked to the consumption of oxygen. Alternately, it may have been that benzene was partially oxidized by microorganisms and the resulting product was amenable to anaerobic decay. An earlier study (Van Beelen Van Keulen, 1990) showed an extremely rapid rate of benzene mineralization 2% mineralized in 1 h and 5% in 7 days. No samples were taken between 1 and 7 days and further benzene mineralization was not observed. 

Sjoblad, R. D., and Bollag, J.-M. (1981). Oxidative coupling of aromatic compounds by enzymes from soil microorganisms. In Soil Biochemistry, Vol. 5, Paul, E. A., and Ladd, J. N., eds., Marcel Dekker, NewYork, 113-125. 

However, controlled or specific environmental degradation sometimes is necessary for herbicidal action. For example, the phenoxy herbicide sesone (sodium 2,4-dichlorophenoxyethyl sulfate) has no effect on plants until it can be oxidized to 2,4-D by a specific soil microorganism, Bacillus cereus (38). The growth regulator ethophon (Ethrel) relies upon slow environmental conversion into ethylene for its activity (39). And metham (Vapam) depends upon hydrolysis in soil to release toxic methyl isothiocyanate (40). 

The atmosphere is a major source of soil acidity. Even in unpolluted environments rainwater is slightly acidic, having a pH of about 5.7 due to the dissolution of atmospheric CO2 to form the weak carbonic acid (see Worked example 5.4). The CO2 concentration in the partially enclosed soil pore system can be significantly higher (typically up to about 10 times) than in the free atmosphere due to respiration of soil microorganisms and plant roots. This results in a lower pH. In areas affected by industrial pollution, sulfur dioxide and nitrogen oxides dissolve in rainwater to produce sulfuric and nitric acids (acid rain), which are both strong acids and cause even more acidity. 

Dichloral urea (DCU) is typical of the substituted ureas. These compounds vary in water solubility from about 5 to about 3000 p.p.m. Consequently, some of them are readily leached into the soil. They are stable to oxidation and hydrolysis and are relatively persistent. They are slowly broken down by soil microorganisms and are likely to remain active in the soil for several seasons. 

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