Understanding native communities is a crucial step for the management of biological nitrogen fixation, since they may be either a source of efficient strains or a limiting factor when efficient strains need to be introduced. This work aimed to evaluate the density, diversity and efficiency of Leguminosae nodulating bacterial (LNB) communities and their component strains in soils under various land use systems (LUSs): pristine forest, agriculture, pasture, agroforestry, young secondary forest, and old secondary forest,. The LNB communities were trapped from these soils by using the promiscuous host siratro under controlled conditions. We also studied their relationships with physical and chemical attributes of the soil. Agroforestry and agriculture soil samples induced the highest number of nodules in siratro, while forest soil samples induced the lowest number of nodules. No relationship was found between LNB and Leguminosae species diversity in the LUSs. The soil chemical variables that were most related to differences in nodule number and shoot dry matter weight of plants inoculated with soil suspensions of the LUSs were, respectively: Ca 2+ , Mg 2+ , base saturation, exchangeable bases and Cu 2+ ; and pH, cation exchange capacity, B, Cu 2+ and clay. Although, LNB communities from all LUSs were efficient under controlled and similar conditions, they were found to be composed of strains with variable efficiency: inefficient, efficient, highly efficient and superior efficiency. Efficient strains occurred at the highest frequency in all LUSs. The isolated strains presented similar and new sequences that were phylogenetically related to well known LNB genera in α-and β-Proteobacteria. Unusual genera in these branches, as well as in other branches, which are probably endophytic bacteria, were also isolated from nodules. These data support siratro as a useful trap species to study the LNB biodiversity of diverse ecosystems in tropical soils. The fact that the highest diversity and nodulation were seen in managed systems such as agriculture and agroforestry suggests a high resilience of LNB communities to changes in land use after deforestation in a region where large forest areas are still preserved and can be a source of propagules.

Non-symbiotic diazotrophic bacteria are amongst the most important functional groups of soildwelling microorganisms. These bacteria contribute to plant growth predominantly through biological N 2 fixation. Here, we evaluated the density and diversity of non-symbiotic diazotrophic bacteria in soils taken from diverse land use systems (LUS) in Amazonia using nitrogen-free media. A total of 30 soil samples were collected from the following LUS: pristine forest, young secondary forest, old secondary forest, agroforestry, agriculture and pasture. Bacterial density was evaluated by the most probable number (MPN) method utilizing N-free semi-solid media with varied compositions (JNFb, NFb, LGI and Fam). Individual isolates were characterized by colony and cellular morphology as well as total protein profiles and nitrogenase activity. Isolate genotypes were determined by partial 16S rDNA sequences. No typical diazotrophic growth in the JNFb medium was observed. Bacterial densities in the NFb medium were higher in the agriculture and agroforestry soil samples. In LGI and Fam media, bacterial densities were highest in the pasture soil samples. Overall, 22 isolates with high phenotypic diversity were obtained. Eleven isolates exhibited nitrogenase activity. Sequences of 16S rDNA genes of 14 out of 19 isolates had similarities below 100 % with known nitrogen-fixing species. Isolates were identified as belonging to the Burkholderia, Enterobacter, Serratia, Klebsiella, and Bacillus genera. A higher number of isolates from pasture soil samples were isolated, with the majority of these belonging to the Burkholderia and Bacillus genera. Among the isolates, unknown sequences were obtained, possibly indicating new species. Taken together, these data demonstrate that Fam, NFb, and LGI semi-solid media allowed the growth of diazotrophic bacteria belonging to different phylogenetic lines.

RESUMOA soja, atualmente, é a leguminosa de maior expressão econômica no Brasil. Em razão da demanda crescente no mercado internacional, a cultura encontra-se em larga expansão em todo território brasileiro. Em alguns ensaios realizados em solos ácidos brasileiros, sob sistema de plantio direto, essa cultura tem mostrado baixa resposta à aplicação de calcário. Porém, a acidez é um dos fatores que limitam a eficiência da simbiose rizóbio-leguminosas. Para contornar esses problemas, poderiam ser utilizadas estirpes tolerantes à acidez do solo ou induzir tolerância nas estirpes já utilizadas. Em alguns trabalhos, tem-se demonstrado indução de tolerância à acidez quando a bactéria é previamente exposta a pH ácido. Assim, objetivou-se com este trabalho verificar o crescimento de Bradyrhizobium elkanii estirpe Br 29 em meios de cultura sob diferentes condições de pH inicial. Foram realizados dois experimentos com dois meios de cultivo diferentes, em que se variou o pH inicial (6,8; 6,0; 5,5). Foram avaliados: o crescimento da estirpe, pela contagem do número de unidades formadoras de colônia (UFC) e por densidade ótica, assim como as mudanças do pH nos meios. As curvas de crescimento obtidas variaram de acordo com o pH inicial e com a composição do meio de cultura. Com base no número máximo de UFC.mL -1 , para o meio Lorda & Balatti modificado, o melhor pH foi 6,0 e, no meio Lopreto, o melhor pH foi 5,5. O melhor crescimento em pH mais baixo (5,5) está associado à maior produção de exopolissacarídeo. Termos para indexação:Rizóbio, acidez, tempo de geração, unidades formadoras de colônia, densidade ótica. ABSTRACTCurrently soybean is the most economically important legume crop in Brazil. Due to its profitability soybean is being widely spread out all over the national territory. Some assays have shown low response of this crop to liming in acid Brazilian soil, under no-till age systems. Acidity is one of the factors that limit the efficiency of the symbiosis rhizobia -legumes. This problem could be solved if acid tolerant strains were obtained or if tolerance to acidity could be induced. Some papers demonstrated induced tolerance to acidity when the bacterium is previously exposed to a slightly acid pH. The aim of this study was to verify the growth of Bradyrhizobium elkanii strain Br 29 in culture media with different pH values (6.8; 6.0; 5.5). Two experiments were carried out with two different culture media varying mainly regarding initial pH. Strain growth, by optical density and colony forming units (CFU) number, and pH medium modification were evaluated. Growth curves varied either in accordance with pH or culture media composition. Growth based on maximum number of CFU.mL -1 was better at pH 5.5 in Lopreto medium and at pH 6.0 at Lorda and Balatti medium. Better growth on lowest pH (5.5) was related to greater exopolyssacaride production.