The burning of fossil fuels in the last 50-100 years has greatly affected the distribution of sulphur with the deposition of S onto the land as sulphur dioxide. At about 100-500 mg kg-1 soil sulphur is a common element (Ehrlich, 1996) but with the withdrawal of S-containing fertilisers, increased crop yields and decrease in atmospheric pollution, sulphur deficiency in plants is becoming more prevalent (report from Rothamsted Research Station).
Sources of sulphur include rock mineral weathering and atmospheric deposition or from organically bound sulphur. About 90% of sulphur is contained in organic material but this is only released when the material decomposes. Sulphur is an essential element in the production of enzymes, proteins and is found in some organic compounds.
The cycling of sulphur (see opposite) is complex due to the many oxidation states of sulphur. Both aerobic and anaerobic bacteria as well as Archaea (Campbell, 1983, Ehrlich, 1996) are involved although plants can take in sulphur in the form of sulphate. The reduction of sulphur, being carried out by anaerobic bacteria, is restricted to water logged environments or where the redox potential is very negative and there is an adequate supply of sulphur and organic carbon sources which supply the necessary energy for the organisms (Killham, 1994). In some cases nitrate can be used thereby linking two nutrient cycles. Rice paddies provide an ideal environment for sulphate reduction being of low redox potential and the roots of the rice plants supplying carbon via root exudates. Any hydrogen sulphide, normally toxic to the plant, would then be removed by thiobacilli (Kimura et al., 1979). Soil acidity plays an important part in S-cycling and which organism can use a particular oxidation state of sulphur (Campbell, 1983). The application of elemental S has been used to artificially reduce soil pH by encouraging the activity of Thiobacillus. This has the effect of increasing the solubility of other elements although it may be detrimental to soil structure.
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