Selenium is present in the environment in both organic and inorganic forms. The organic form originates from partially decayed seleniferous vegetation. Its form generally determines the mobility and availability of Se in soils. Selenite (SeO3-2, Se IV) is the main mobile inorganic form of Se in soils of humid regions. Under alkaline conditions, Se is present as selenate (SeO4-2, Se VI), which does not form highly insoluble salts or stable complexes with sesquioxide. The low solubility of Se in acid and neutral soils can be attributed to its presence as selenite in combination with ferric iron or basic ferric selenite, which is slightly soluble (Adriano 1986). Other soil properties that influence the mobility and solubility of Se in soils include: organic carbon, clay and CaCO3 content, cation exchange capacity, allophane content, and free forms of SiO2, Al2O3, and Fe2O3 (Adriano 1986).
Selenium also biomethylates, producing organic metabolites that are more volatile than the parent organic forms. Plants, fungi, bacteria, microorganisms, and animals can produce methylated forms of Se from inorganic and certain organic forms. Non-accumulator plants, such as cabbage and alfalfa, give off (CH3)2Se or other volatile Se compounds, and accumulator species, such as Astragalus racemosus, produce dimethyldiselenide when exposed to selenite (Adriano 1986). The formation of methylated Se compounds by animals appears to be one mechanism for Se detoxification as the toxicity of dimethyl selenide is 500 to 1 000 times lower than the toxicity of Se2- (Vokal-Borek 1979).
Several microorganisms have been isolated that are capable of methylating inorganic Se. Conversion of selenite, selenate, and indigenous Se to volatile products was enhanced in soils amended with organic matter. Conversion of inorganic and organic Se compounds by microorganisms to volatile compounds, such as dimethylselenide and dimethyldiselenide, has also been observed in the lake sediments. Biomethylation products have also been detected in sewage sludge (Adriano 1986).