Diffusive gradients in thin films measurement of sulfur stable isotope variations in labile soil sulfate

Abstract

A diffusive gradient in thin films (DGT) technique, based on a strongly basic anion exchange resin (Amberlite IRA-400), was successfully tested for ³⁴S/³²S analysis in labile soil sulfate. Separation of matrix elements (Na, K, and Ca) that potentially cause non-spectral interferences in ³⁴S/³²S analysis by MC ICP-MS (multi-collector inductively coupled plasma-mass spectrometry) during sampling of sulfate was demonstrated. No isotopic fractionation caused by diffusion or elution of sulfate was observed below a resin gel disc loading of ≤79 μg S. Above this threshold, fractionation towards ³⁴S was observed. The method was applied to 11 different topsoils and one mineral soil profile (0-100 cm depth) and compared with soil sulfate extraction by water. The S amount and isotopic ratio in DGT-S and water-extractable sulfate correlated significantly (r² = 0.89 and r² = 0.74 for the 11 topsoils, respectively). The systematically lower ³⁴S/³²S isotope ratios of the DGT-S were ascribed to mineralization of organic S.