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

The symbiotic effectiveness of 30 fast-growing rhizobial strains (doubling times of 85-225 min and acid reaction in yeast mannitol medium) isolated fr om soybean nodules in Brazil and of Sinorhizobium fredii reference strains was evaluated under greenhouse and field conditions. Most Brazilian fast-gr owing strains were genetically related to the Rhizobium tropici-Rhizobium g enomic species Q-Agrobacterium spp. branch and five to the Bradyrhizobium j aponicum and B. elkanii species. Under axenic conditions, some of the fast- growing strains fixed as much N-2 as the B. japonicum/B. elkanii strains ca rried in Brazilian commercial inocula. However, in a co-inoculation experim ent, very few strains were able to compete against B. elkanii strain SE MIA 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. jap onicum/B. elkanii was low under acid conditions (pH 5.1 and pH 5.4), but in creased when the pH was raised to 6.8 and 7.9. Therefore, as the great majo rity of Brazilian soils are acidic and show a very high population of natur alized B. japonicum/B. elkanii, the low competitiveness of S. fredii and of the Brazilian rhizobial strains investigated in this study, under the give n conditions, limits, at this time, their recommendation for use in commerc ial inocula.