Sulfur Speciation in an Arable Soil as Affected by Sample Pretreatments and Sewage Sludge Application

Sulfur speciation under the influence of sample pretreatment and sewage sludge (SS) application in a continuously SS‐amended soil was assessed by wet‐chemical fractionation and S K ‐edge XANES spectroscopy. Distinct differences in S speciation among the following pretreatments including fresh, air‐drying, anoxic freeze‐drying, oven‐drying, and conditioning (incubation at room temperature and 75% of water holding capacity for 2 wk) were detected in both topsoils and subsoils. Air‐drying significantly increased available S content but decreased C‐bonded S content in the soils compared with anoxic freeze‐drying. Oven‐drying and conditioning pretreatments showed a similar effect on S fractions to air‐drying in the soils but to a lesser extent. X‐ray adsorption near‐edge structure analysis showed that both reduced and intermediate organic S tended to decrease while oxidized S obviously increased in the air‐dried samples. In contrast, the conditioning pretreatment facilitated the transformation of intermediate S into reduced organic S. Continuous application of SS significantly increased total soil S, C‐bonded S, and residual S contents but decreased available S content in the topsoil. Meanwhile, XANES analysis indicated that reduced organic S was markedly increased (over 50% of total S) in the SS‐amended topsoil. No obvious effect of the application of SS on S speciation in the subsoil was observed except for the facilitated transportation of S into lower depth. Overall, our results suggest that anoxic atmospheres during the entire pretreatment process from sampling to drying, grinding, and analysis is crucial for the accurate speciation analysis of S in soils, and the anoxic freeze‐drying is, therefore, highly recommended.