Symbiotic effectiveness of fast-growing rhizobial strains isolated from soybean nodules in Brazil

Abstract.

The symbiotic effectiveness of 30 fast-growing rhizobial strains (doubling times of 85–225 min and acid reaction in yeast mannitol medium) isolated from soybean nodules in Brazil and of Sinorhizobium fredii reference strains was evaluated under greenhouse and field conditions. Most Brazilian fast-growing strains were genetically related to the Rhizobium tropici-Rhizobium genomic species Q-Agrobacterium spp. branch and five to the Bradyrhizobium japonicum and B. elkanii species. Under axenic conditions, some of the fast-growing strains fixed as much N₂ as the B. japonicum/B. elkanii strains carried in Brazilian commercial inocula. However, in a co-inoculation experiment, very few strains were able to compete against B. elkanii strain SEMIA 5019. Although isolated from acid soils (pH 3.0–5.1), the competitiveness of Brazilian fast growers and of S. fredii reference strains against B. japonicum/B. elkanii was low under acid conditions (pH 5.1 and pH 5.4), but increased when the pH was raised to 6.8 and 7.9. Therefore, as the great majority of Brazilian soils are acidic and show a very high population of naturalized B. japonicum/B. elkanii, the low competitiveness of S. fredii and of the Brazilian rhizobial strains investigated in this study, under the given conditions, limits, at this time, their recommendation for use in commercial inocula.