Mobilization of recently-formed soil organic sulphur

Abstract

Recently-formed soil organic sulphur was labelled with radioactive 35S by incubating samples of soil with carrier-free 35S-sulphate. Different amounts of 35S incorporation and differing proportions of 35S incorporation into hydriodic acid (HI)-reducible and carbon-bonded forms of S were obtained by varying the nature of the soil treatment (conditioned, air-dried, glucose treated) and duration of incorporation period. Mobilization of the recently-formed 35S-labelled organic S was measured using an open incubation system. There was a substantial difference between samples in the proportions of incorporated 35S mobilized during a 10 week incubation. Irrespective of the nature of the original soil treatment, the proportion of incorporated 35S mobilized during open incubation decreased with an increased duration of the incorporation period. Recently-formed organic S was much more readily mobilized than the bulk of the organic S present in the soil. For samples where incorporation of 35S had taken place in the absence of added glucose, mobilized 35S was derived from both HI-reducible and C-bonded forms of organic S. There appeared to be no difference in the ease of mobilization of S from these two forms. For samples where incorporation of 35S had taken place in the presence of glucose, mobilized 35S appeared to be derived exclusively from C-bonded forms of S and some small increases were observed in 35S-labelled HI-reducible S during the 10 week incubation. The observed decrease in mobilization of recently-formed organic S with increased duration of the incorporation period is intrpreted as a gradual conversion of incorporated S to less-easily mobilized forms.