Effect of a vesicular–arbuscular mycorrhizal fungus and rhizosphere micro‐organisms on manganese reduction in the rhizosphere and manganese concentrations in maize (Zea mays L.)

Abstract

summaryMaize (Zea mays L. ev. 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.