Diversity and Abundance of Bacterial Sulfur-Oxidizers in Australian Cropping Soils, and Their Role in Oxidizing Elemental Sulfur.

Elemental sulfur (ES) oxidation is thought to largely depend on the abundance and diversity of S-oxidizers in soil. However, attempts to correlate ES oxidation with culturable S-oxidizer populations from soil have been unsuccessful due to the fact that over 90% of soil microorganisms are unculturable. In this study, an assay of quantitative PCR of soxBgenes has been developed to quantify S-oxidizing bacterial populations in 10 Australian cropping soils with diverse physico-chemical properties.

Soils amended with ES (1000 µg/g) and controls were incubated for 96 days under controlled environmental conditions. The oxidation rate of ES varied from 5.1 to 51.7 µg/cm²/d. soxB gene copies ranged from 2.8×10⁶ to 1.1×10⁸ copies at Day 8 in controls, and there was no significant change in soxB gene copies across incubation in controls. The abundance of soxB genes in amended soils at Day 48 or 96 were more than those at Day 8, though the increase was not always consistent across all soils.  The results from cloning and sequencing showed that most of colonies were affiliated with α- and β-proteobacteria. The most significant correlation was observed between ES oxidation rate and log transformed soxB gene copy numbers in control soils at Day 8 (R²=0.73, P<0.01, n=10), indicating that the population size of S-oxidizers affects ES oxidation rates in soil. Multiple regression analysis showed that soil pH (water) and the log transformed soxB gene numbers explained 82% of the variation of ES oxidation rate (P<0.01, n=10). These results suggest that a combination of soil biological and chemical properties provides a promising predictive estimate of oxidation potential of ES in agricultural soils.