Abstract The symbiotic effectiveness of 30 fast-grow- ing 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-Rhizo- bium genomic species Q-Agrobacterium spp. branch and five to the Bradyrhizobium japonicum and B. elkanii species. Under axenic conditions, some of the fast-grow- ing strains fixed as much N 2 as the B. japonicum/B. elk- anii strains carried in Brazilian commercial inocula. However, in a co-inoculation experiment, 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. 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 natural- ized B. japonicum/B. elkanii, the low competitiveness of S. fredii and of the Brazilian rhizobial strains investigat- ed in this study, under the given conditions, limits, at this time, their recommendation for use in commercial inocu- la. Keywords Agrobacterium · Bradyrhizobium · Competitiveness · Glycine max · Nitrogen fixation Introduction The main effective N 2 -fixing symbioses with soybean [Glycine max (L.) Merrill] are established with rhizobial strains of Bradyrhizobium japonicum and B. elkanii spe- cies, which are characterized by a slow growth rate and alkaline reaction in media containing mannitol as a C source (Jordan 1982; Kuykendall et al. 1992). However, the legume can also associate with fast-growing strains that were first isolated from soybean nodules and soil from the People’s Republic of China, within the centre of origin and diversity of this legume (Keyser et al. 1982), and later from other primary and secondary centres of origin of soybean (Xu and Ge 1984; Dowdle and Bohlool 1985; Young et al. 1988; Rodriguez-Navarro et al. 1996). These fast growers were classified as the new species Rhizobium fredii (Scholla and Elkan 1984), and later reclassified as Sinorhizobium fredii and S. xinjian- gensis (Chen et al. 1988). Soybean is an exotic plant in Brazil, but has been in- tensively cultivated since the 1960s. The first experi- ments, with disinfected seeds, showed that inoculation was needed, since native rhizobial strains were unable to effectively nodulate soybean (e.g. Lopes et al. 1976; Peres 1979; Vargas and Suhet 1980), and therefore, inoc- ulation with Bradyrhizobium has been mandatory. Al- though originally it was thought that S. fredii was specif- ic for Asian soybean lines (Keyser et al. 1982; Stowers and Eaglesham 1984; Devine 1985), more recently it was shown that several North American and Brazilian genotypes are able to form effective nodules with sino- rhizobia (Balatti and Pueppke 1992; Chueire and Hung- ria 1997). The use of these fast growers in commercial inocula would be of great interest, especially because on- ly half the time would be required to produce the 13 mil- lion doses annually sold in the country. However, a first attempt has shown that two S. fredii/S. xinjiangensis strains were not able to outcompete a B. elkanii Brazilian commercial strain (Chueire and Hungria 1997). In an attempt to isolate native sinorhizobia strains, a survey was performed in Brazil, and 30 fast-growing M. Hungria ( ✉ ) · R.J. Campo · L.M.O. Chueire Embrapa-Soja, Cx. Postal 231, 86001-970, Londrina, PR, Brazil e-mail: hungria@cnpso.embrapa.br Tel.: +55-43-3716206. Fax: +55-43-3716100 L. Grange Department of Genetics, Universidade Estadual de Londrina, Londrina, Brazil M. Megías Department of Microbiology and Parasitology, Facultad de Farmacia, University of Seville, Apdo. Postal 874, 41080, Seville, Spain Biol Fertil Soils (2001) 33:387–394 DOI 10.1007/s003740100338 ORIGINAL PAPER M. Hungria · R.J. Campo · L.M.O. Chueire L. Grange · M. Megías Symbiotic effectiveness of fast-growing rhizobial strains isolated from soybean nodules in Brazil Received: 15 September 2000 / Published online: 27 February 2001 © Springer-Verlag 2001