Enumeration of sulfur-oxidizing populations in Saskatchewan agricultural soils

Abstract

Heterotrophic and autotrophic sulfur-oxidizing populations in 35 Saskatchewan agricultural soils were enumerated. These populations included heterotrophs that produce thiosulfate and or sulfate during elemental sulfur (S°) oxidation, heterotrophic thiosulfate oxidizers, and autotrophic thiosulfate oxidizers. Populations of Thiobacillus thiooxidans and T. ferrooxidans were not detected in any of the soils tested. Heterotrophs that oxidized S° to thiosulfate as the major oxyanion were the most abundant oxidizers enumerated (107–108 cells g−1) and were found in all soils. Autotrophic thiosulfate-oxidizers were detected in 10 of the soils surveyed. Heterotrophic S° and thiosulfate-oxidizing populations exhibited positive trends with soil pH, total-S, hydriodic reducible-S, and clay content, whereas populations of autotrophic thiosulfate oxidizers were negatively correlated with these factors and positively related to sand content and increasing C:S ratios. In soils containing autotrophic thiosulfate oxidizers the amount of thiosulfate relative to sulfate detected was reduced although no effect on S° oxidation rate was detected. Amendment of 15 selected agricultural soils with 0.5% S° significantly reduced total heterotrophic populations, whereas autotrophic thiosulfate oxidizers increased from undetectable levels to 104 cells g−1. Therefore most Saskatchewan soils contain abundant populations of heterotrophic S° oxidizers, and populations of autotrophs that respond to S° applications. Key words: Sulfur oxidation, autotrophic sulfur oxidizers, heterotrophic sulfur oxidizers, soil properties