Root-associated nitrogen fixing bacteria and arbuscular mycorrhizal (AM) fungi play key role in the optimal nutrition of soybean. Host’s water and nutrient supply and stress resistance can be considerably improved by the application of various inocula, based on selected, compatible microsymbiont strains.

A pot experiment was designed to investigate the infectivity and the efficiency of two commercially available AM fungal and two Bradyrhizobium japonicum inocula on three Hungarian registered soybean cultivars (Aliz, Emese, Sponsor). Microbial inocula were used both separately and in combination. Plants were grown for eight weeks in pumice medium, originally free of infective agents. The photosynthetic efficiency (Fv/Fm) and the root electrical capacitance (EC), as an indicator to root system activity were measured during the cultivation. At the end of experiment, we determined the plant biomass and leaf area, the symbiotic infectivity (root nodulation, AM fungal colonization) and functionality (acetylene reduction assay) as well as the shoot nitrogen and phosphorus content.

Bradyrhizobium treatment had a significant effect on root nodulation, acetylene reduction (with differences between the two inoculum combinations) and the shoot nitrogen content. Both the AM fungal and the combined inoculations increased significantly the host’s biomass production. Nevertheless, aforementioned observations were not confirmed by the low root colonisation of AM fungi. The Fv/Fm parameter and the magnitude of root EC proved to be appropriate for characterising the soybean cultivars.

Keywords: soybean, microsymbionts, arbuscular mycorrhizal fungi (AMF), Bradyrhizobium inocula