Sulphur

Abstract

Sulphur occurs in the soil both in inorganic and organic forms but in most soils organically bound S provides the major S reservoir. In most instances in soils of humid and semi humid areas the range of total S is from 100 to 1000 mg S/kg, a range that is similar to that of total P (Syers et al. 1987). Organic S concentration generally decreases with soil depth and approaches zero in soil depth >1.5 m. In peat soils organic S may amount to almost 100% of the total S. The C:N:S ratio of soil organic matter is approximately 125:10:1.2. Sulphur in soil organic matter can be divided into 2 fractions, carbon bonded S, and non carbon bonded S. These fractions are distinguished by treatment with hydriodic acid since S which is not bonded to C can be reduced to H₂S. This includes compounds with a C-O-S linkage, an organic ester sulphate or a C-N-S linkage as sulphamate. Carbon bound-S is reduced and is present in cysteine and methionine in which the S atom has accepted an e⁻ from each of both binding partners. The sulphate esters can be readily hydrolyzed by soil sulphatases (Martens et al. 1992) and the S present therefore becomes rapidly available to plant uptake. By contrast, however, mineralization of reduced S in organic matter proceeds at a much slower rate (Maynard et al. 1985, Nguyen and Goh 1992). The reverse process of mineralization of organic S, that of immobilization of inorganic S, can also occur by microbial assimilation (Pirela and Tabatabai (1988). These authors found that under ryegrass the concentration of organic S increased. The biochemical pathway of assimilation involves the reduction of SO₄ ²⁻ to sulphide which combines with serine to form cysteine (Scott 1985) so that there are clear similarities with sulphate reduction by higher plants (see page 178).