Effect of repeated applications of elemental sulfur on microbial population, sulfate concentration, and pH in soils

Though there exists a wide spectrum of sulfur-oxidizing microorganisms in soils, the oxidation rate of soil-applied elemental sulfur (S-0) is regularly limited because of a restricted population size. An incubation experiment was conducted to determine the effect of repeated S-0 applications on different microbial populations, sulphate (SO42-)-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 SO42- -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 S-0 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 So application. Lime applications successfully counterbalanced the acidifying effect of S-0 oxidation, and soil pH values were maintained in the optimum range with a pH of about 6.4. The 0.025 M KCl-extractable SO42- content increased with repeated applications of S-0, showing a maximum value of 3,800 mg S kg(-1) soil after the SO42- sixth S-0 application. Thereafter, the SO42- concentration decreased significantly. The Thiobacillus spp.count increased consistently with repeated S-0 applications. The number of Thiobacillus spp. at the first application of S-0 was significantly lower than the count after all other applications. A maximum Thiobacillus spp. count of 1.0 center dot 10(8) g(-1) soil was observed after the seventh application of S-0. The fastest So oxidation rate was found after the second application of S-0. The population of aerobic heterotrophic sulfur-ox idizing bacteria increased also with repeated S-0 applications, showing a maximum count of 5.0 center dot 10(4) g(-1) soil after the fourth So application. Thereafter, the population declined steadily. Significant relationships between So(4)(2-)-S concentration and count of Thiobacillus spp. (R-2 = 0.85, 2 p < 0.01) and aerobic heterotrophic sulfur-oxidizing bacteria (R-2 = 0.63, p < 0.01) were found. Based on these results, it may be concluded that repeated S-0 applications decrease soil pH, increase Thiobacillus spp. counts, and thus increase extractable SO42--S concentration in soils. The results further suggest that soils that receive regular S-0 applications have a higher Thiobacillus spp. count and thus have conjecturally a higher S-0 oxidation potential than soils that have never received S-0. This again indicates a priming effect of S-0 oxidation by Thiobacillus spp., which needs to be confirmed under field conditions.