Sulfur Species Formed in the Seed Row of Sulfur-Fertilized Soils as Revealed by K-Edge X-ray Absorption Near-Edge Structure Spectroscopy

Knowledge of S fertilizer reaction products following application is needed to predict the ability of different fertilizer forms to provide sufficient plant-available S to crops. K-edge X-ray absorption near-edge structure (XANES) spectroscopy and wet chemical analysis techniques were used to identify the soil S species formed week to months after fertilization in the field on Brown Chemozem and Black Chernozem soils in Saskatchewan, Canada. The S fertilizer forms were ammonium sulfate, liquid ammonium thiosulfate, gypsum, elemental S, and a zero-S control treatment. Treatments were applied at seeding at 20 kg S ha(-1) Sulfur as ammonium sulfate, ammonium thiosulfate, and gypsum increased the extractable SO4-S in the seed row 1 wk after application; thereafter, it decreased through plant uptake and runoff. For all fertilizers, total S concentration in the seed row increased after application and decreased over time. The XANES results showed that the highly oxidized sulfate form of S was dominant (45-55%) in ammonium sulfate- and gypsum-treated soils 1 wk following application. Thereafter, the proportion as sulfate decreased over time while the reduced and intermediate S forms increased. The proportion of reduced S was highest in the elemental S and ammonium thiosulfate treatments and the proportion in oxidized forms increased from Week 1 to Week 8. Overall, the highly oxidized fertilizer S forms applied (sulfates) transformed to intermediate and reduced S species via immobilization and reduction processes, whereas the highly reduced forms (e.g., elemental S) underwent some oxidation as reduced S decreased and highly oxidized sulfate increased.