Arbuscular mycorrhizal infection changes the bacterial 16 S rDNA community composition in the rhizosphere of maize

Mycorrhizal and non-mycorrhizal (NM) maize plants were grown for 4 or 7 weeks in an autoclaved quartz sand-soil mix. Half of the NM plants were supplied with soluble P (NM-HP) while the other half (NM-LP), like the mycorrhizal plants, received poorly soluble Fe and Al phosphate. The mycorrhizal plants were inoculated with Glomus mosseae or G. intraradices. Soil bacteria and those associated with the mycorrhizal inoculum were reintroduced by adding a filtrate of a low P soil and of the inocula. At 4 and 7 weeks, plants were harvested and root samples were taken from the root tip (0-1 cm), the subapical zone (1-2 cm) and the mature root zone at the site of lateral root emergence. DNA was extracted from the roots with adhering soil. At both harvests, the NM-HP plants had higher shoot dry weight than the plants grown on poorly soluble P. Mycorrhizal infection of both fungi ranged between 78% and 93% and had no effect on shoot growth or shoot P content. Eubacterial community compositions were examined by polymerase chain reaction-denaturing gradient gel electrophoresis of 16 S rDNA, digitisation of the band patterns and multivariate analysis. The community composition changed with time and was root zone specific. The differences in bacterial community composition in the rhizosphere between the NM plants and the mycorrhizal plants were greater at 7 than at 4 weeks. The two fungi had similar bacterial communities after 4 weeks, but these differed after 7 weeks. The observed differences are probably due to changes in substrate composition and amount in the rhizosphere.