Microbial transformation of sulphur: An approach to combat the sulphur deficiencies in agricultural soils

Abstract

Sulphur, an essential component for plant as well as animals, is present in soils in both organic and inorganic forms, with organic form particularly sulphate esters and carbon-bonded sulphur contributing ~75-90% of the total. The major sources of sulphur in soils are sulphur-containing minerals, plant and elemental residue and external addition including atmospheric deposition. Sulphur deficiency in plants results in poor nitrogen metabolism thus protein biosynthesis, chlorosis, low oil percentage and ultimately low yield. The conversion of organic sulphur in organic matter to inorganic form and vice versa is dominantly a microbiological process. In well-aerated soil, organic sulphur is mineralized to sulphate and taken up by plants. Concurrently inorganic sulphur is immobilized to organic form and incorporated in microbial tissue. The rate of these processes obviously depends on soil reaction, temperature, moisture and addition of crop residue and many other factors that ultimately affect the activity of microorganism. Several enzymes in soil, viz. arylsulphatase, play a major role in sulphur mineralization process though very little information is available till now towards the pathway of decomposition. In addition to this process, inorganic sulphur in soil undergoes various oxidation and reduction process, modulated by microorganisms. Various reduced inorganic sulphur compounds are oxidized by a group of bacteria in suitable condition and utilize the energy. The wide range of stable redox states and their interconversion affect sulphur cycle, fate of applied fertilizer and ultimately its availability to plants and microbes. In this chapter we reviewed the sulphur cycle and its transformation by various microbial processes.