Effect of arbuscular mycorrhizal fungi on growth, mineral nutrition and nitrogen fixation of three varieties of common beans (Phaseolus vulgaris)

A pot experiment was conducted under glasshouse conditions to investigate the effect of inoculation with four arbuscular mycorrhizal (AM) fungi (Glomus clarum, G. etunicatum, G. manihotis, Gigaspora margarita), either individually or in a mixture, and with Rhizobium leguminosarum biovar phaseoli, on the growth, mineral nutrition, and contribution of biologically fixed N-2, to three nodulating bean (Phaseolus vulgaris) varieties (Carioca, Negro Argel and Rio Tibagi). The N-15 isotope dilution technique and three control crops (non-nodulating bean, Panicum maximum and sorghum) were used to quantify the contribution of N-2 fixation. There were significant differences between the capacity of the different AM fungi to infect and colonize the roots of the different bean varieties. Inoculation with AM fungi significantly increased production of dry matter by 8-23% and the concentration of P in plants by 160-335%. A strongly positive correlation of % root colonization by AM fungi and plant P concentration and accumulation was observed which suggests that the observed inoculation responses were a result of improved P acquisition by the mycorrhizal roots. N derived from the atmosphere increased significantly with AM fungi inoculation, with Carioca showing the best response, followed by Negro Argel and Rio Tibagi, thereby showing the indirect and positive effect of AM fungi inoculation on the N-2-fixing association between Rhizobium and Phaseolus vulgaris.