Effect of Repeated Applications of Elemental Sulfur on Microbial Population, Sulfate Concentration, and pH in Soils

Abstract

Though there exists a wide spectrum of sulfur‐oxidizing microorganisms in soils, the oxidation rate of soil‐applied elemental sulfur (S0) is regularly limited because of a restricted population size. An incubation experiment was conducted to determine the effect of repeated S0 applications on different microbial populations, sulphate (SO4 2−)‐S concentration, and soil pH. Elemental sulfur was applied repeatedly at a rate of 15 mg S g−1 soil in a 15‐day interval cycle of 7 times. After each cycle, 7.5 mg lime (CaCO3) g−1 soil was applied to adjust the soil pH to an optimum range. Soil pH and 0.025 M potassium chloride (KCl)–extractable SO4 2−‐S were determined every 3 days. The population of Thiobacillus spp. and aerobic heterotrophic sulfur‐oxidizing bacteria were counted 3 and 15 days after each S0 application. The results showed that the soil pH decreased rapidly from an initial value of 7.6 to 5.3, 15 days after the first S0 application. Lime applications successfully counterbalanced the acidifying effect of S0 oxidation, and soil pH values were maintained in the optimum range with a pH of about 6.4. The 0.025 M KCl–extractable SO4 2−‐S content increased with repeated applications of S0, showing a maximum value of 3,800 mg S kg−1 soil after the sixth S0 application. Thereafter, the SO4 2−‐S concentration decreased significantly. The Thiobacillus spp.count increased consistently with repeated S0 applications. The number of Thiobacillus spp. at the first application of S0 was significantly lower than the count after all other applications. A maximum Thiobacillus spp. count of 1.0 · 108 g−1 soil was observed after the seventh application of S0. The fastest S0 oxidation rate was found after the second application of S0. The population of aerobic heterotrophic sulfur‐oxidizing bacteria increased also with repeated S0 applications, showing a maximum count of 5.0 · 104 g−1 soil after the fourth S0 application. Thereafter, the population declined steadily. Significant relationships between SO4 2−‐S concentration and count of Thiobacillus spp. (R2=0.85, p<0.01) and aerobic heterotrophic sulfur‐oxidizing bacteria (R2=0.63, p<0.01) were found. Based on these results, it may be concluded that repeated S0 applications decrease soil pH, increase Thiobacillus spp. counts, and thus increase extractable SO4 2−‐S concentration in soils. The results further suggest that soils that receive regular S0 applications have a higher Thiobacillus spp. count and thus have conjecturally a higher S0 oxidation potential than soils that have never received S0. This again indicates a priming effect of S0 oxidation by Thiobacillus spp., which needs to be confirmed under field conditions.