Organic sulphur fractions labelled by addition of 35S-sulphate to soil

Abstract

35S sodium sulphate was added to three soils before incubation at 100 cm moisture tension at 30°C for periods up to 24 weeks. The nature of organic sulphur fractions labelled during incubation was investigated. Labelled sulphur was incorporated into both hydriodic acid reducible and carbon-bonded sulphur, and the hydriodic acid reducible sulphur had a greater specific activity than the carbon bonded sulphur. Some of the labelled organic sulphur was soluble in potassium phosphate solution at pH 7 and most of this was reducible with hydriodic acid. Sodium hydroxide extracted labelled and unlabelled sulphur in the same proportions as that in which they occurred in the organic matter but sodium bicarbonate preferentially extracted labelled sulphur. In both extracts over 90 per cent of the labelled sulphur extracted was reducible with hydriodic acid. In the bicarbonate extracts approximately 75 per cent of the labelled sulphur was present in the fulvic acid fraction.