ENUMERATION OF SULFUR-OXIDIZING POPULATIONS IN SASKATCHEWAN AGRICULTURAL SOILS

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 (S0) 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 S0 to thiosulfate as the major oxyanion were the most abundant oxidizers enumerated (10(7) - 10(8) cells g-1) and were found in all soils. Autotrophic thiosulfate-oxidizers were detected in 10 of the soils surveyed. Heterotrophic S0 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 S0 oxidation rate was detected. Amendment of 15 selected agricultural soils with 0.5% S0 significantly reduced total heterotrophic populations, whereas autotrophic thiosulfate oxidizers increased from undetectable levels to 10(4) cells g-1. Therefore most Saskatchewan soils contain abundant populations of heterotrophic S0 oxidizers, and populations of autotrophs that respond to S0 applications.