Regional distribution and pH sensitivity ofAzospirillum associated with wheat roots in Eastern Australia

In a survey ofAzospirillum spp. on the roots and associated soil of wheat grown in eastern Australia, azospirilla were isolated from approximately 40% of samples from areas of soil pH between 5.0 and 6.6. However, azospirilla isolates were rare in soil between pH 4.5 and 5.0 and absent below pH 4.5. Of 25 independent isolates, 17 wereA. brasiliense and eight wereA. lipoferum. No selection forA. brasiliense Nir^– strains by wheat roots was observed. Only one of six endorhizosphere isolates wereA. brasiliense Nir^–, compared with three of nine from unsterilized roots plus associated soil, and three of eight from soil. With a medium buffered with 0.05 M malate and 0.05 M phosphate, it was found that allAzospirillum isolates had a lower minimum pH for growth when supplied with fixed nitrogen than when grown under nitrogen-fixing conditions. Strains isolated from soils had a minimum pH for growth that was less than the pH of the soil from which they were isolated. However, a significant proportion of strains isolated from roots had a minimum pH for growth that was higher than the pH of the associated soil suggesting that the wheat roots provided an ecological niche protecting against soil acidity.