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

A diffusive gradient in thin films (DGT) technique, based on a strongly basic anion exchange resin (Amberlite IRA-400), was successfully tested for (34)S/(32)S analysis in labile soil sulfate. Separation of matrix elements (Na, K, and Ca) that potentially cause non-spectral interferences in (34)S/(32)S 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 (34)S 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 (r (2) = 0.89 and r (2) = 0.74 for the 11 topsoils, respectively). The systematically lower (34)S/(32)S isotope ratios of the DGT-S were ascribed to mineralization of organic S.