Genetic diversity of rhizobia associated with common bean (Phaseolus vulgaris L.) grown under no-tillage and conventional systems in Southern Brazil

Brazil is the largest producer and consumer of the common bean (Phaseolus vulgaris L.), but yields are often low and may be improved by a higher N supply through symbiosis with rhizobia. One main limitation to the N-2-fixation process is the susceptibility of the symbiosis to environmental stresses frequent in the tropics, such as high soil temperatures and low soil moisture contents. Among other benefits, the no-tillage (NT) system reduces those stresses resulting in higher N, fixation rates and yields; however, the effects of NT on rhizobial diversity are poorly understood. This study evaluated the diversity of rhizobia compatible with common bean in cropping areas under the NT or the conventional tillage (CT) systems in Ponta Grossa, State of Parana, Southern Brazil. Genetic diversity was assessed by DNA analyses using the methodologies of BOX-PCR and RFLP-PCR of the 16S rDNA region. A high level of diversity was observed among the strains and the DNA profiles from the CT system were quite different from those from the NT system. Twenty-three RFLP-PCR profiles were obtained, indicating that many tropical rhizobial species remain to be described. Strain differentiation was achieved in the BOX-PCR analysis; diversity was slightly higher under the NT when compared with the CT system. Surprisingly, the rhizobial grouping based on cluster analysis of the RFLP-PCR of the 16S rDNA region indicated a higher diversity of species under the CT. It could be that the environmental stability offered by the NT system has led to a decrease in the number of species, with the predominance of the most successful ones, although genetic diversity within each species has increased. The results obtained in this study show that we still understand poorly the relation between microbial diversity and soil sustainability and that the complexity of the ecosystems require the evaluation of several parameters to define and monitor soil quality. (c) 2005 Elsevier B.V. All rights reserved.