Bacteria from rhizosphere and hyphosphere soils of different arbuscular-mycorrhizal fungi

Effects of roots and of arbuscular-mycorrhizal (AM) fungi on the composition of soil bacterial colonies and the combined effects of AM fungus-rhizobacterium associations on plant and soil development are little-known. We grew sorghum (Sorghum bicolor L.) either nonsymbiotically or colonized by one of two isolates of the AM fungi Glomus etunicatum, Glomus intraradices, or Glomus mosseae. The isolates were either exotic or native to the test soil. Soils adhering (rhizosphere) or not adhering (hyphosphere) to the roots were sampled 45 days after planting. Total populations of bacteria were estimated by counting colony-forming units on a nonselective medium and grouped by colony and cell morphology. Rhizosphere populations of fluorescent pseudomonads were determined on P1 medium. Visually distinct isolates were selected for identification by Fatty-Acid-Methyl-Esther analysis; of these 25 were found to be separate species. Bacterial numbers were greater in rhizo- than in hyphosphere soil. Isolates of Bacillus and Arthrobacter were most frequent in hyphosphere and Pseudomonas in rhizosphere soils. More bacterial species were encountered in hyphosphere than in rhizosphere soil, and bacterial communities varied within and among AM treatments. The development of the AM mycelium in soil had little influence on the composition of the microflora in the hyphosphere, while AM root colonization was positively related with bacterial numbers in the hyphosphere and with the presence of Pseudomonas in the rhizosphere. The results suggest that qualitative effects of the AM fungal taxon on the hyphosphere, such as the nature of exudates, are more important to composition and proliferation of rhizobacteria than the quantitative development of AM soil mycelia.