Sulphur speciation and availability in long‐term fertilized soil: evidence from chemical fractionation and S K ‐edge XANES spectroscopy

Knowledge of sulphur ( S ) speciation in soil following applications of chemical and organic fertilizers is important for understanding the availability and fate of S in agricultural ecosystems. Sulphur speciation of the soil in three fields, which are part of the N ational L ong‐term M onitoring N etwork of S oil F ertility and F ertilizer E ffects of C hina, was investigated with a combination of a wet‐chemical fractionation scheme and S K ‐edge X ‐ray absorption near edge structure ( XANES ) spectroscopy. Treatments of the soil included no fertilizers (control), regular applications of nitrogen, phosphorus and potassium fertilizers ( NPK ), or of NPK plus organic manures ( MNPK ) since 1990. Soil samples were taken from the topsoil (0–20 cm) and subsoil (20–40 cm) horizons in 2011. Chemical fractionation showed that available sulphate was the predominant inorganic S species in the E utric C ambisol, whereas HCl ‐extractable sulphate was the main inorganic S species in the H aplic P haeozem and C alcaric C ambisol. Organic S accounted for up to 77% of total S in all soil samples. Results from the fractionation (inorganic S plus ester‐ S ) and XANES analysis (oxidized S ) were correlated ( r = 0.585, P < 0.01), which confirmed the presence of HCl ‐extractable sulphate and the speciation of residual S in soil. Long‐term application of organic manures facilitated significantly the accumulation of intermediate and reduced S species and increased total and organic S in the soil ( P < 0.01). In contrast, the application of chemical fertilizers ( NPK ) had little effect on the distribution of organic S species compared with the control. Atmospheric deposition was identified as an important source of S . The combination of chemical fractionation and XANES spectroscopy provided complementary information on the availability and speciation of S in soil.HighlightsAvailability and species of S are characterized with chemical fractionation and XANES spectroscopy. Insoluble sulphate is identified as an important form of S in the soil regardless of soil pH . Organic manure facilitates the accumulation of organic S in long‐term fertilized soil. Atmospheric deposition contributes importantly to the input and cycling of S in soil.