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 fung i (Pseudomonas and Trichoderma) have shown beneficial effects on plant grow th and health. The study of plants inoculated with biological control agent s and AMF requires special attention because of the possibility that these fungal antagonists could also interfere with AM fungi. Our study was perfor med to test the effects of these inoculants upon mycorrhizal colonization i n maize plants inoculated with Glomus mosseae, Glomus deserticola and natur al 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 (est erase, phosphatase, trehalase and chitinase) were used as an index to detec t changes in the microbial functioning in soil, as affected by mycorrhizal and other inoculation treatments. None of the microbial inoculants used, ev en those biocontrol agents of fungi, showed negative effects on AM establis hment. Mycorrhizal colonization induced qualitative changes in the bacteria l population depending on the inoculant combination involved. Esterase acti vity 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 resu lt 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-plan t system sustainability. (C) 2000 Elsevier Science B.V. All rights reserved .