Interrelations between Azospirillum and Rhizobium nitrogen-fixers and arbuscular mycorrhizal fungi in the rhizosphere of alfalfa in sterile, AMF-free or normal soil conditions

Co-inoculations of the alfalfa (Medicago sativa L.) plants with the associativeand/or the obligate nitrogen-fixing bacteria (Azospirillum brasilense, S; Rhizobium meliloti, R) and/or the vesicular arbuscular mycorrhiza fungus (Glomus fasciculatum, M) were evaluated in a pot experiment under controlled conditions. The effect of these beneficial microbes, as single(M, R, S), dual(MR, MS) or multilevel (MRS) inoculation-treatments were assessed in a calcareous loamy chernozem soil, originating from a grass-type natural ecosystem. A range of substrates were used to separate the influence of the indigenous microbes: C, untreated original soil (i.e. including all of the usual microflora); G, gamma-sterilised soil (no competitive microbes); GB, sterile soil (re-suspension of a mycorrhiza-free soil extract). The weight of the host, nodule-number, macroand microelement contents and the colonisation by the inoculated bacterial and fungal microsymbionts were recorded. In the gamma-sterilised substrate all of the mono(M), dual(MR, MS) or multilevel (MRS) co-inoculations with the selected, Glomus fasciculatum M 107 strain were effective in improving plant growth, nutrient-uptake and abundance of the microsymbionts in the rhizosphere of alfalfa. In contrast a competition from the indigenous microflora in the non-sterilised soil, greatly reduced the functioning of the applied mycorrhizal inoculum. Although the associative Azospirillum bacteria (MS) slightly reduced effects relative to single mycorrhizal inoculation (M), the multilevel treatments, with both of the diazotrophs (MRS), showed a further enhancement (a synergistic effect) for almost all of the tested parameters and substrates. The functional compatibility of the obligateand associative diazotrophs in the mycorhizosphere are discussed. © 2000 Elsevier Science B.V. All rights reserved.