Interactions between arbuscular mycorrhizal fungi and other microbial inoculants (Azospirillum, Pseudomonas, Trichoderma) and their effects on microbial population and enzyme activities in the rhizosphere of maize plants

Arbuscular mycorrhizal (AM) fungi as well as microbial-free inoculants used as phytostimulators (Azospirillum) or as biological control agents of fungi (Pseudomonas and Trichoderma) have shown beneficial effects on plant growth and health. The study of plants inoculated with biological control agents and AMF requires special attention because of the possibility that these fungal antagonists could also interfere with AM fungi. Our study was performed to test the effects of these inoculants upon mycorrhizal colonization in maize plants inoculated with Glomus mosseae, Glomus deserticola and natural AMF from the test soil. Populations of culturable bacteria and fungi in the rhizosphere soil were also examined since inoculation with AM fungi and other soil microorganisms can affect both quantitatively and qualitatively the microbial communities in the plant rhizosphere. Enzyme activities (esterase, phosphatase, trehalase and chitinase) were used as an index to detect changes in the microbial functioning in soil, as affected by mycorrhizal and other inoculation treatments. None of the microbial inoculants used, even those biocontrol agents of fungi, showed negative effects on AM establishment. Mycorrhizal colonization induced qualitative changes in the bacterial population depending on the inoculant combination involved. Esterase activity was particularly increased by G. mosseae (256%), phosphatase activity by natural AMF (166%), chitinase by G. mosseae (197%), G. deserticola (152%) and natural AMF (151%), and trehalase by G. deserticola (444%). As a result of mycorrhizal colonization and microbial inoculation, modifications of the microbial community structure and ecology were found. An understanding of these effects as part of ecosystem processes is essential for obtaining the maximum benefit for plant growth and health in the context of soil-plant system sustainability. (C) 2000 Elsevier Science B.V. All rights reserved.