EFFECT OF A VESICULAR ARBUSCULAR MYCORRHIZAL FUNGUS AND RHIZOSPHERE MICROORGANISMS ON MANGANESE REDUCTION IN THE RHIZOSPHERE AND MANGANESE CONCENTRATIONS IN MAIZE (ZEA-MAYS L)

Maize (Zea mays L. cv. Tau) plants were grown in a calcareous soil for six weeks in pots having separate compartments for growth of roots and vesicular-arbuscular (VA) mycorrhizal fungal hyphae. Soil was sterilized and either left non-inoculated (sterile treatment), or was inoculated with rhizosphere micro-organisms only (MO-VA) or with rhizosphere micro-organisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol. & Gerd.) Gerdemann & Trappe (MO + VA)]. Concentrations of Mn in roots and shoots, as well as exchangeable Mn in rhizosphere soil, decreased in the order MO-VA > MO + VA > sterile treatment. In all treatments, the concentration of exchangeable Mn was lower in the rhizosphere soil (0-5 mm distance from the root surface) than in the bulk soil (5-15 or 15-25 mm distance from the root surface). In the rhizosphere soil, the total microbial population was similar in mycorrhizal (MO + VA) and non-mycorrhizal (MO - VA) treatments, but the proportion of Fe- or Mn-reducers was 20- to 30-fold higher in the non-mycorrhizal treatment, suggesting substantial qualitative changes in rhizosphere microbial populations upon root infection with the mycorrhizal fungi. The Mn+IV-reducing potential (net balance between reduction and oxidation) in the rhizosphere soil was also distinctly lower in mycorrhizal treatment compared to non-mycorrhizal treatment. In the sterile treatment, low Mn+IV-reducing potential and correspondingly low concentration of exchangeable Mn in soil, compared to the other treatments, indicates the importance of micro-organisms in Mn reduction in soil and acquisition of Mn by plants. Therefore, the lower Mn concentrations in mycorrhizal plants are most probably caused by a shift in composition and activity of rhizosphere micro-organisms. As a side effect of the treatments, improved soil aggregation, as indicated by soil adhering to the nylon net (facing hyphal compartments) after plant harvest, occurred in non-mycorrhizal and sterile treatments but not in the mycorrhizal treatment.