Elemental Sulfur Oxidation by Thiobacillus spp. and Aerobic Heterotrophic Sulfur-Oxidizing Bacteria

The prediction of the oxidation rate of elemental sulfur (S 0 ) is a critical step in sulfur (S) fertilizer strategy to supply plant-available sulfur. An incubation experiment was conducted to determine the rate and amount of S 0 oxidation in relation to the contribution of Thiobacillus spp. and aerobic heterotrophic S-oxidizing bacteria. After 84 days, 16.3% and 22.4% of the total S 0 applied to the soil were oxidized at 20 and 30-C, respectively. The oxidation of S 0 proved to be a two-step process with a rapid oxidation during the first 28 days and a slow oxidation from then on. The highest oxidation rate of 12.8 μg S cm-2 d-1 was measured during the first two weeks at 30-C. At 20-C the highest oxidation rate of 10.2 μg S cm-2 d-1 was obtained from two to four weeks after start of the experiment. On an average the soil pH declined by 3.6 and 4.0 units after two weeks of experiment. At the same time the electric conductivity increased nine times. With the oxidation of S 0 the population of Thiobacillus spp. and aerobic heterotrophic S-oxidizing bacteria increased. The corresponding values for Thiobacillus spp. and aerobic heterotrophic S-oxidizing bacteria increased from 2.9 × 10 5 and 1.4 × 10 5 g-1 soil at the start of the experiment to 4 × 10 8 and 5.6 × 10 8 g-1 soil 14 days after S 0 application, respectively. No Thiobacillus spp. was present eight weeks after S 0 application. The results suggested that oxidation of residual S 0 completely relied on aerobic heterotrophic S-oxidizing bacteria.