Water content and communities of sulfur-oxidizing bacteria affect elemental sulfur oxidation in silty and sandy loam soils

Sulfur (S)-oxidizing microorganisms regulate S biogeochemistry in soil ecosystems, affecting ecosystem productivity and food quality. Elemental sulfur (S-0) supplies SO42- to plants after oxidation by S-oxidizing microorganisms, and soil pH, organic carbon, water content, and temperature affect the oxidative process. However, how soil water content and texture interact to affect S-oxidizing microbial communities and S-0 oxidation is unclear. In this study, S-0 oxidation rate and abundance and composition of microbial communities were examined in silty loam and sandy loam soils with different levels of water holding capacity (WHC) (20%, 40%, 60%). Total bacteria were analyzed using the 16S rRNA gene, S-oxidizing bacteria with soxB, and fungi with the ITS gene. The oxidation rate increased significantly with increases in WHC in the sandy loam soil but not in the silty loam soil. The difference might be because the interaction between water content and texture affected the area of contact between S-0 and S-oxidizers. At 40% and 60% WHC, the oxidation rate was significantly higher in sandy loam soil than in silty loam soil. The higher oxidation rate was attributed to high ratios of soxB to 16S rRNA gene abundances and key oxidizers in the sandy loam soil. The ratio of soxB to 16S rRNA was significantly higher in the sandy loam soil than in the silty loam soil, irrespective of WHC. The sandy loam soil also contained abundant and diverse typical S-oxidizing bacteria and unknown soxB-containing bacteria. Differences in abundance and composition of S-oxidizing bacterial communities between the two soils might be caused by different cropping histories.