Index >>Mineral Transformation in Soil >>Sulphur Cycle

(iv) the reduction of sulphates and other ions to sulphides. summarizes the various transformations that sulphur undergoes in soil. Plants obtain their sulphur requirement from sulphur compounds while animals do so by feeding on plant materials or other animals.

In these systems sulphur is found mostly as a component of sulphur containing amino acids such as methionine and cysteine and to a small extent in some vitamins. When plant and animal proteins are degraded, the sulphur is released from the amino acids and accumulates in the soil which is then oxidized in the presence of oxygen while under anaerobic conditions H2S may accumulate.

H2S can also accumulate during the reduction of sulphates under anaerobic conditions which can be oxidized to sulphate under aerobic conditions. Plants utilize sulphur in the form of sulphates and then reduce it within the cells to H2S before it is utilized mainly in the synthesis of sulphur amino acids and vitamins (biotin, thiamine, lipoic acid). The assimilation of sulphur therefore, in many ways resembles the assimilation of nitrates.

Members of the genus Thiobacillus are the main orga­nisms involved in the oxidation of elemental sulphur.The ability to oxidize sulphur is not restricted to only the genus Thiobacillus. Heterotrophic bacteria, actinomycetes and fungi are also reported to oxidize sulphur compounds. For example species of Bacillus, Pseudomonas, Arthrobacter and Flavobacterium are known to oxidize elemental sulphur or thiosulphate to sulphate.sulhur is first converted enzymatically to sulphite which is then oxidized to sulphate.

It is believed that some of the sulphite from the first reaction reacts with sulphur to yield thiosulphate which is then either cleaved to sulphur and sulphite or converted into tetrathionite. The latter is then metabolized to sulphur or sulphite which  are then oxidized to sulphate.under anaerobic conditions, sulphate is first reduced to H2S by sulphate reducing microorganisms, mostly the bacteria. Many bacteria including species of Bacillus and Pseudomonas are known to reduce sulphur or sulphate to H2S but among these, Desulfovibrio desulfuricans seems to be the most important.