Oxidation and transformation of elemental sulphur in soils

The oxidation of ³⁵S-S⁰ (³⁵S-S⁰, sulphur flowers) added to four cultivated soils was well described using first-order kinetics. The half-life (t_l/2) of ³⁵S-S⁰ ranged from 12 days in a paddy soil to 176 days in an Ultisol. During the early phase of oxidation, a large proportion of the oxidised S was incorporated into organic fractions. After the first week any additional ³⁵S-S⁰ oxidised remained primarily as SO-S in most of the test soils. The amount of KH₂PO₄- extractable ³⁵S-SO-S as a percentage of total oxidised ³⁵S-S was at a maximum when approximately half of the added ³⁵S-S⁰ was oxidised. By the end of the 56-day incubation period, with the exception of the Ultisol, 54–70% of the oxidised ³⁵S-S in the test soils was present as ³⁵S-SO-S. The remaining oxidised S is presumed to have been converted into organic matter fractions. This ³⁵S-organic-S could be fractionated into the 0.1 M KH₂PO₄-extractable organic- S (5–13% of total oxidised ³⁵S-S) and the 0.01 M NaOH-extractable organic-S (14–23% of total oxidised ³⁵S-S). With the exception of the Ultisol where the undetermined organic S was about 36%, the amounts of unaccounted for S (not directly measured), in all soils were very small (typically less than 4%).