Sulphur isotope fractionation during sulphur mineralization: results of an incubation–extraction experiment with a Black Forest soil

Laboratory incubation–extraction experiments were used to study sulphur isotope fractionation during sulphur mineralization in Oh and Ah horizons of a Black Forest soil. Changes in δ34S values for sulphate extracted every three days with deionized-distilled water over a three week incubation period were small (<1.5‰). A second experiment used the addition of dilute ammonium sulphate solution, enriched in 34S relative to soil sulphur, to demonstrate unequivocally that sulphate adsorption and desorption during the incubation were negligible. Sulphur isotope fractionation during mineralization of carbon-bonded sulphur was shown to be a two-step process. The first and slower step was the formation of soluble organic sulphate from carbon-bonded sulphur, accompanied by a kinetic isotope effect, k32/k34=1.0040±0.0008. The faster step, identified with the hydrolysis of organic sulphate favored 34S in the product with a k32/k34=0.9967±0.0003. Leaching the soils led to a loss of isotopically light organic sulphate from the organic sulphur pool and is likely the process responsible for progressively heavier δ34S values for organic sulphur with depth in undisturbed forest soils in the Black Forest region.