BACTERIA FROM RHIZOSPHERE AND HYPHOSPHERE SOILS OF DIFFERENT ARBUSCULAR-MYCORRHIZAL FUNGI

Effects of roots and of arbuscular-mycorrhizal (AM) fungi on the compo sition of soil bacterial colonies and the combined effects of AM fungu s-rhizobacterium associations on plant and soil development are little -known. We grew sorghum (Sorghum bicolor L.) either nonsymbiotically o r colonized by one of two isolates of the AM fungi Glomus etunicatum, Glomus intraradices, or Glomus mosseae. The isolates were either exoti c or native to the test soil. Soils adhering (rhizosphere) or not adhe ring (hyphosphere) to the roots were sampled 45 days after planting. T otal populations of bacteria were estimated by counting colony-forming units on a nonselective medium and grouped by colony and cell morphol ogy. Rhizosphere populations of fluorescent pseudomonads were determin ed on P1 medium. Visually distinct isolates were selected for identifi cation 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 fre quent in hyphosphere and Pseudomonas in rhizosphere soils. More bacter ial species were encountered in hyphosphere than in rhizosphere soil, and bacterial communities varied within and among AM treatments. The d evelopment of the AM mycelium in soil had little influence on the comp osition of the microflora in the hyphosphere, while AM root colonizati on was positively related with bacterial numbers in the hyphosphere an d with the presence of Pseudomonas in the rhizosphere. The results sug gest that qualitative effects of the AM fungal taxon on the hyphospher e, such as the nature of exudates, are more important to composition a nd proliferation of rhizobacteria than the quantitative development of AM soil mycelia.