INFLUENCE OF CROP RHIZOSPHERES ON POPULATIONS AND ACTIVITY OF HETEROTROPHIC SULFUR-OXIDIZING MICROORGANISMS

Heterotrophic microorganisms are the most abundant S-oxidizers in Saskatchewan soils. This study investigated the S-oxidizing activity of heterotrophic microorganisms as influenced by crop rhizospheres. Spring wheat (Triticum aestivum L. var. Neepawa) and canola (Brassica napus L. var Regent) were grown in four different Saskatchewan soils under growth chamber conditions. Populations of total and S-oxidizing heterotrophs were enumerated using the most-probable-number method with selective media. The S-oxidizing potential of the soils was followed using a standard 6 day incubation assay. In all four soils the number of total heterotrophic organisms was greater in rhizospheres than in the non-rhizosphere controls, and the largest increase in their numbers was 11.0 and 14.3% in wheat and canola rhizosphere soils, respectively. Similarly, the numbers of S-oxidizing heterotrophs in rhizosphere soils was greater than in non-rhizosphere soils. Thiosulfate producing heterotrophs increased by up to 10.4 and 8.8%, and sulfale-producing heterotrophs by up to 32.4 and 18.7% in wheat and canola rhizosphere soils, respectively. In general, S-oxidation was significantly greater in all the rhizosphere soils. These results demonstrate the stimulatory nature of the rhizosphere on heterotrophic S-oxidizing activity, and suggest that S° fertilizer efficiency will be affected by the type of crop. © 1990.