Abstract Dark Septate Endophytic (DSE) fungi can benefit plants by optimizing nutrient uptake, biosynthesis of phytohormones-like compounds, and stress relief such as toxic metals. The objective was to characterize in vitro 57 strains of the DSE Periconia macrospinosa isolated from sugarcane roots, indicating the most promising in solubilizing phosphate sources, growing in different metal and vinasse contents, and producing molecules related to indoleacetic acid (IAA). The strains were from the Embrapa Agrobiology Fungi Collection. Over 35% of the strains solubilize calcium phosphate, highlighting A356 and A155. No strain solubilizes aluminum phosphate. Three strains did not grow in the presence of Cd (A333, A334, A163), but A226, A332, and A423 stand out showing high growth with Cd. All strains grew in the presence of Cu, Zn, and Vinasse. A163, A164, and A328 were even stimulated by Cu addition. A331 and A335 showed a marked growth decrease with Zn. All strains are highly adapted to grow in vinasse presence. A257 and A226 produced the highest amount of IAA. The most biotechnological potential strains are indicated by presenting high growth under Cd, Cu, Zn, and vinasse stress, associated with a high tolerance index to these pollutants, further calcium phosphate solubilization, or IAA production. (DSE uptake phytohormoneslike phytohormones like compounds metals 5 roots sources contents . (IAA) Collection 35 A A35 A155 A333, A333 (A333 A334 A163 , A163) A332 A42 Zn Vinasse A164 A32 addition A33 A25 A22 pollutants solubilization production (IAA 3 A3 A15 (A33 A16 A4 A2 A1 (A3 (A

Resumo: A interação simbiótica entre fungos micorrízicos arbusculares (FMA) e 70-90% das espécies de plantas é amplamente estudada, mas fungos ectomicorrízicos, Piriformospora indica, Trichoderma sp. e “dark septate” endofíticos (DSE), também estabelecem interações benéficas com as plantas. Apesar disso, a discussão conjunta dos aspectos bioquímicos, fisiológicos e moleculares da transferência de nutrientes, principalmente do micélio extrarradicular (ERM) para a planta, ainda é limitada. O objetivo deste artigo de revisão é apresentar abordagens bioquímicas, fisiológicas e moleculares da interação planta-FMA, bem como analisar a resposta da planta à inoculação com diferentes fungos promotores de crescimento. Aqui se destacam as características de H+-ATPases e de transportadores de NH4+ e H2PO4- envolvidos com a absorção de fósforo e nitrogênio pelo solo através do ERM de FMA, assim como os aspectos bioquímicos do metabolismo de ambos os nutrientes no ERM e as suas translocações do ERM para o micélio intrarradicular e para a planta hospedeira. Finalmente, apresenta-se a eficiência de aproveitamento de fertilizantes nitrogenados em plantas com inoculação de FMA, Trichoderma sp., P. indica e fungos DSE. Ao se examinar, conjuntamente, os aspectos bioquímicos, fisiológicos e moleculares da interação planta-FMA e a eficiência de aproveitamento de fertilizantes nitrogenados em plantas inoculadas, é possível concluir que a agricultura de baixo uso de insumos poderia ser alcançada com o emprego desses fungos nos agrossistemas.

Abstract: The symbiotic interaction between arbuscular mycorrhizal fungi (AMF) and 70-90% of the plant species is largely studied, but ectomycorrhizal fungi, Piriformospora indica, Trichoderma sp., and dark septate endophytes (DSE), also establish beneficial interactions with plants. Despite this, the joint discussion on the biochemical, physiological, and molecular aspects of nutrient transfer, mainly from the extraradical mycelium (ERM) to the plant, is still limited. The objective of this review is to present biochemical, physiological, and molecular approaches to the plant-AMF interaction, as well as to analyze the response of plants to inoculations with different growth-promoting fungi. Here, are highlighted the characteristics of the H+-ATPases and of the transporters of NH4+ and H2PO4- involved in the absorption of phosphorus and nitrogen by the soil through the ERM of the AMF, besides the biochemical aspects of the metabolism of both nutrients in the ERM and their translocations from the ERM to the intraradical mycelium and to the host plant. Finally, the nitrogen fertilizer recovery efficiency in plants inoculated with AMF, Trichoderma sp., P.indica, and DSE fungi is presented. By examining, together, the biochemical, physiological, and molecular aspects of the plant-AMF interaction and the nitrogen fertilizer recovery efficiency in inoculated plants, it is possible to conclude that a low-input agriculture could be achieved with the use of these fungi in agrosystems.

ABSTRACT The leguminous inoculation with nodule-inducing bacteria that perform biological nitrogen fixation is a good example of an “eco-friendly agricultural practice”. Bradyrhizobium strains BR 3267 and BR 3262 are recommended for cowpea (Vigna unguiculata) inoculation in Brazil and showed remarkable responses; nevertheless neither strain was characterized at species level, which is our goal in the present work using a polyphasic approach. The strains presented the typical phenotype of Bradyrhizobium with a slow growth and a white colony on yeast extract-mannitol medium. Strain BR 3267 was more versatile in its use of carbon sources compared to BR 3262. The fatty acid composition of BR 3267 was similar to the type strain of Bradyrhizobium yuanmingense; while BR 3262 was similar to Bradyrhizobium elkanii and Bradyrhizobium pachyrhizi. Phylogenetic analyses based on 16S rRNA and three housekeeping genes placed both strains within the genus Bradyrhizobium: strain BR 3267 was closest to B. yuanmingense and BR 3262 to B. pachyrhizi. Genome average nucleotide identity and DNA–DNA reassociation confirmed the genomic identification of B. yuanmingense BR 3267 and B. pachyrhizi BR 3262. The nodC and nifH gene analyses showed that strains BR 3267 and BR 3262 hold divergent symbiotic genes. In summary, the results indicate that cowpea can establish effective symbiosis with divergent bradyrhizobia isolated from Brazilian soils.

Leguminosas forrageiras são importantes recursos na região semi-árida, devido a sua capacidade de adaptação a solos de baixa fertilidade e fixação de nitrogênio quando associados a bactérias diazotróficas. Aqui, aplicamos uma abordagem polifásica com o objetivo de caracterizar bactérias isoladas de nódulos obtidos de Clitoria ternatea e Stylosanthes capitata cultivadas em solos do semiárido brasileiro. O isolamento das bactérias produziu uma coleção de 126 isolados: 45 isolados de C. ternatea e 81 de S. capitata. Os testes de nodulação resultaram em apenas dez isolados que nodularam seu hospedeiro original: seis de C. ternatea e quatro de S. capitata. Esses dez isolados de nódulos de leguminosas tiveram suas características fenotípicas e genotípicos avaliadas. Todos os isolados cresceram em frutose, glicose, glutamato de sódio, maltose, xilose e sacarose. Testes metabólicos mostraram uma relação entre a tolerância ao sal e altas temperaturas; onde o isolado que tolerou a maior concentração de sal também tolerou a temperatura mais alta. Três isolados apresentaram atividade amilolítica e quatro foram capazes de produzir carboximetil celulolítico. A estreptomicina foi o antibiótico mais limitante para o crescimento bacteriano, e o ácido nalidíxico foi o menos limitante. Sete dos dez isolados eram produtores de IAA. O sequenciamento parcial do gene 16S rRNA permitiu identificar isolados como membros dos gêneros Bacillus (1), Bradyrhizobium (4), Leifsonia (3), Microvirga (1) e Rhizobium (1). Esses dados revelam diversidade fenotípicas e genotípicas de bactérias que habitam nódulos das forrageiras C. ternatea e S. capitata, e representam uma importante fonte microbiana para a prospecção de novos produtos biotecnológicos para promover melhor desenvolvimento das leguminosas forrageiras na região semiárida.

Forage legumes are important resources in semiarid regions due to their abilities to adapt to soils with low fertility and fix nitrogen when associated with diazotrophic bacteria. Here, we applied a polyphasic approach to characterize a set of legume nodule isolates obtained from Clitoria ternatea and Stylosanthes capitata cultivated in the soils of a semiarid region of Brazil. A total of 126 bacterial isolates were obtained: 45 isolates from C. ternatea and 81 isolates from S. capitata. Nodulation tests revealed only ten isolates that nodulated their original host: six isolates from C. ternatea and four isolates from S. capitate. These ten legume nodule isolates were phenotypically and genotypically characterized. All isolates grew in fructose, glucose, sodium glutamate, maltose, xylose, and sucrose. Metabolic tests showed a relationship between tolerance to salt and high temperatures, where isolates that tolerated the highest salt concentration also tolerated the highest temperature. Three isolates showed amylolytic activity, and four isolates showed carboxymethyl cellulolytic activity. Streptomycin was the most limiting and nalidixic acid was the least limiting antibiotic to bacterial growth. Seven out of ten isolates were indol-acetic acid producers. Additionally, 16S rRNA gene partial sequencing enabled the identification of members of the genera Bacillus (1), Bradyrhizobium (4), Leifsonia (3), Microvirga (1), and Rhizobium (1). These data reveal phenotypically and genotypically diverse bacteria inhabiting the nodules of the forage legumes C. ternatea and S. capitata represent an important microbial source to protect new biotechnological products and improve forage legumes in semiarid regions.

ABSTRACT The use of dark septate fungi (DSE) to promote plant growth can be beneficial to agriculture, and these organisms are important allies in the search for sustainable agriculture practices. This study investigates the contribution of dark septate fungi to the absorption of nutrients by rice plants and their ensuing growth. Four dark septate fungi isolates that were identified by Internal transcribed spacer phylogeny were inoculated in rice seeds (Cv. Piauí). The resulting root colonization was estimated and the kinetic parameters Vmax and Km were calculated from the nitrate contents of the nutrient solution. The macronutrient levels in the shoots, and the NO3--N, NH4+-N, free amino-N and soluble sugars in the roots, sheathes and leaves were measured. The rice roots were significantly colonized by all of the fungi, but in particular, isolate A103 increased the fresh and dry biomass of the shoots and the number of tillers per plant, amino-N, and soluble sugars as well as the N, P, K, Mg and S contents in comparison with the control treatment. When inoculated with isolates A103 and A101, the plants presented lower Km values, indicating affinity increases for NO3--N absorption. Therefore, the A103 Pleosporales fungus presented the highest potential for the promotion of rice plant growth, increasing the tillering and nutrients uptake, especially N (due to an enhanced affinity for N uptake) and P.

ABSTRACT The strain BR 3351T (Bradyrhizobium manausense) was obtained from nodules of cowpea (Vigna unguiculata L. Walp) growing in soil collected from Amazon rainforest. Furthermore, it was observed that the strain has high capacity to fix nitrogen symbiotically in symbioses with cowpea. We report here the draft genome sequence of strain BR 3351T. The information presented will be important for comparative analysis of nodulation and nitrogen fixation for diazotrophic bacteria. A draft genome with 9,145,311 bp and 62.9% of GC content was assembled in 127 scaffolds using 100 bp pair-end Illumina MiSeq system. The RAST annotation identified 8603 coding sequences, 51 RNAs genes, classified in 504 subsystems.

Abstract Abiotic stress is one of the major limiting factors for plant development and productivity, which makes it important to identify microorganisms capable of increasing plant tolerance to stress. Dark septate endophytes can be symbionts of plants. In the present study, we evaluated the ability of dark septate endophytes isolates to reduce the effects of water stress in the rice varieties Nipponbare and Piauí. The experiments were performed under gnotobiotic conditions, and the water stress was induced with PEG. Four dark septate endophytes were isolated from the roots of wild rice (Oryza glumaepatula) collected from the Brazilian Amazon. Plant height as well as shoot and root fresh and dry matter were measured. Leaf protein concentrations and antioxidant enzyme activity were also estimated. The dark septate endophytes were grown in vitro in Petri dishes containing culture medium; they exhibited different levels of tolerance to salinity and water stress. The two rice varieties tested responded differently to inoculation with dark septate endophytes. Endophytes promoted rice plant growth both in the presence and in the absence of a water deficit. Decreased oxidative stress in plants in response to inoculation was observed in nearly all inoculated treatments, as indicated by the decrease in antioxidant enzyme activity. Dark septate endophytes fungi were shown to increase the tolerance of rice plants to stress caused by water deficiency.

ABSTRACT Inga edulis Mart. is a leguminous tree adapted to acidic and low-fertility soils that establishes symbioses with nitrogen (N)-fixing bacteria. The identification of effective bacteria in biological N fixation may bolster the use of I. edulis in degraded or modified areas and agroforestry systems. Therefore, the aims of this study were evaluation of the symbiotic effectiveness of eight strains of the Bradyrhizobium genus native to Roraima in Inga edulis plants, and in vitro evaluation of the ability of the eight strains of Bradyrhizobium to develop plant growth-promoting characteristics. Determination of symbiotic effectiveness was carried out via three experiments: the first in a greenhouse in pots with a sterile substrate; the second in a greenhouse in pots containing non-sterile soil; and the third in a nursery in bags with a non-sterile substrate. Twelve treatments were evaluated: inoculation with eight strains of Bradyrhizobium ingae (ERR 490, ERR 492, ERR 493, ERR 494T, ERR 496, ERR 497, ERR 498, and ERR 569); inoculation with two strains indicated for Inga marginata, BR 6609 and BR 6610 (positive controls); no inoculation but with mineral N; and neither inoculation nor mineral N. All of the experiments were conducted in a completely randomized design with four replicates. The first experiment was conducted for 60 days, and the other experiments were conducted for 100 days. For all of the experiments, the number of nodules, nodule dry matter, root dry matter, shoot dry matter, number of leaflets, plant height, stem diameter, total N in the shoots, root/shoot dry matter ratio, Dickson’s quality index, relative effectiveness, and the Pearson correlation between the variables under study were evaluated. The strains were also evaluated by their ability to solubilize calcium and aluminum phosphates and to produce indolic compounds. The results showed that B. ingae strains were effective in biological N fixation, especially the ERR 493, ERR 498, and ERR 569 strains. These strains increased the production of shoot dry matter and total N and exhibited relative effectiveness higher than 100 % in all of the experiments. The B. ingae strains were also able to solubilize calcium and aluminum phosphates, despite their synthesis of indolic compounds. Thus, the strains of B. ingae can be used for inoculation in the production of I. edulis seedlings.

Abstract The strain BR 3267 is a nitrogen-fixing symbiotic bacteria isolated from soil of semi-arid area of Brazilian Northeast using cowpea as the trap plant. This strain is used as commercial inoculant for cowpea and presents high efficient in nitrogen fixation as consequence of its adaptation potential to semi-arid conditions. We report here the draft genome sequence of Bradyrhizobium sp. strain BR 3267, an elite bacterium used as inoculant for cowpea. Whole genome sequencing of BR 3267 using Illumina MiSeq sequencing technology has 55 scaffolds with a total genome size of 7,904,309 bp and C+G 63%. Annotation was added by the RAST prokaryotic genome annotation service and has shown 7314 coding sequences and 52 RNA genes.

Abstract The strain BR 3262 was isolated from nodule of cowpea (Vigna unguiculata L. Walp) growing in soil of the Atlantic Forest area in Brazil and it is reported as an efficient nitrogen fixing bacterium associated to cowpea. Firstly, this strain was assigned as Bradyrhizobium elkanii, however, recently a more detailed genetic and molecular characterization has indicated it could be a Bradyrhizobium pachyrhizi species. We report here the draft genome sequence of B. pachyrhizi strain BR 3262, an elite bacterium used as inoculant for cowpea. The whole genome with 116 scaffolds, 8,965,178 bp and 63.8% of C+G content for BR 3262 was obtained using Illumina MiSeq sequencing technology. Annotation was added by the RAST prokaryotic genome annotation service and shown 8369 coding sequences, 52 RNAs genes, classified in 504 subsystems.

RESUMO Plantas de milho podem estabelecer associações benéficas com bactérias promotoras do crescimento vegetal. No entanto, poucos estudos de caracterização e inoculação com essas bactérias foram realizados na região Amazônica. O objetivo deste estudo foi caracterizar bactérias endofíticas isoladas de milho na região Amazônica e avaliar sua capacidade de promover o crescimento. Cinquenta e cinco isolados bacterianos foram obtidos de plantas de milho cultivadas em diferentes tipos de ecossistemas, cerrado (savana) e área de floresta. Estes foram caracterizados pela presença do gene nifH, capacidade de sintetizar ácido indol-3-acético (AIA), solubilizar fosfato de cálcio (CaHPO4) e sequenciamento parcial do gene 16S rRNA. Vinte e quatro bactérias apresentaram o gene nifH. Destas, sete são oriundas de plantas cultivadas em uma área de cerrado e 17 em área de floresta. Quatorze bactérias demonstraram a capacidade de solubilizar AIA e quatro de solubilizar fosfato de cálcio. Entre estas bactérias, foram encontrados os seguintes gêneros: Pseudomonas; Acinetobacter; Enterobacter; Pantoea; Burkholderia e Bacillus. Oito isolados com capacidade de promoção do crescimento vegetal foram selecionados para experimento em casa de vegetação com a inoculação em dois diferentes genótipos de milho (um híbrido e uma variedade) em vasos com solo. Quanto à promoção de crescimento vegetal, a inoculação promoveu o desenvolvimento de plantas de milho, mas não foi encontrada interação significativa entre a cultivar de milho e a inoculação bacteriana. Na região Amazônica há uma diversidade de bactérias endofíticas e estas possuem potencial para promover o desenvolvimento de plantas de milho.

ABSTRACT Maize plants can establish beneficial associations with plant growth-promoting bacteria. However, few studies have been conducted on the characterization and inoculation of these bacteria in the Amazon region. This study aimed to characterize endophytic bacteria isolated from maize in the Amazon region and to assess their capacity to promote plant growth. Fifty-five bacterial isolates were obtained from maize grown in two types of ecosystems, i.e., a cerrado (savanna) and a forest area. The isolates were characterized by the presence of the nifH gene, their ability to synthesize indole-3-acetic acid (IAA) and solubilize calcium phosphate (CaHPO4), and 16S rRNA partial gene sequencing. Twenty-four bacteria contained the nifH gene, of which seven were isolated from maize plants cultivated in a cerrado area and seventeen from a forest area. Fourteen samples showed the capacity to synthesize IAA and only four solubilized calcium phosphate. The following genera were found among these isolates: Pseudomonas; Acinetobacter; Enterobacter; Pantoea; Burkholderia and Bacillus. In addition, eight isolates with plant growth-promoting capacity were selected for a glasshouse experiment involving the inoculation of two maize genotypes (a hybrid and a variety) grown in pots containing soil. Inoculation promoted the development of the maize plants but no significant interaction between maize cultivar and bacterial inoculation was found. A high diversity of endophytic bacteria is present in the Amazon region and these bacteria have potential to promote the development of maize plants.

As espécies vegetais que ocorrem naturalmente na Caatinga brasileira apresentam diversas adaptações às condições adversas que podem ser exploradas para aumentar a produção vegetal. Entre essas estratégias adaptativas para o enfrentamento da baixa disponibilidade hídrica, a tolerância à dessecação pode ser destacada. Até o momento, a associação entre essas espécies e os microrganismos benéficos do solo não é bem entendida. Os objetivos deste trabalho foram caracterizar bactérias diazotróficas de Tripogon spicatus e avaliar a capacidade de promoção do crescimento delas em plantas de arroz. Dezesseis isolados bacterianos foram estudados quanto a seu posicionamento taxonômico por meio do sequenciamento parcial do gene 16S rRNA, potencial diazotrófico, produção de ácido indol acético (AIA) e solubilização de fosfato de cálcio in vitro, metabolismo de nove diferentes fontes de C em meio semissólido, tolerância a diferentes concentrações de NaCl a pHs e resistência intrínseca a nove antibióticos. A habilidade em promover o crescimento vegetal foi avaliada utilizando o arroz (Oryza sativa) como planta-modelo. Dentre os 16 isolados avaliados, oito foram classificados como pertencentes à família Enterobacteriaceae, relacionados aos gêneros Enterobacter e Pantoea. Seis outros isolados foram relacionados a Bacillus e outros dois a Rhizobium e Stenotrophomonas. A avaliação da incorporação do N ao meio semissólido demonstrou o potencial diazotrófico de todos os isolados. Duas bactérias produziram mais AIA do que a estirpe BR 11175T de Herbaspirillum seropedicae. Isolados bacterianos foram capazes de formar película em meio semissólido suplementado com concentrações de NaCl de até 1.27 mol L-1. A resistência intrínseca a antibióticos e o metabolismo de diferentes fontes de C indicaram um perfil fisiológico muito variável. Sete isolados foram capazes de aumentar o crescimento das plantas de arroz, mais eficientemente do que a estirpe de referência Ab-V5 de Azospirillum brasilense. Os resultados indicaram o potencial de T. spicatus como uma espécie nativa fonte de bactérias promotoras de crescimento vegetal.

Plant species that naturally occur in the Brazilian Caatinga(xeric shrubland) adapt in several ways to these harsh conditions, and that can be exploited to increase crop production. Among the strategic adaptations to confront low water availability, desiccation tolerance stands out. Up to now, the association of those species with beneficial soil microorganisms is not well understood. The aim of this study was to characterize Tripogon spicatusdiazotrophic bacterial isolates from the Caatingabiome and evaluate their ability to promote plant growth in rice. Sixteen bacterial isolates were studied in regard to their taxonomic position by partial sequencing of the 16S rRNA gene, putative diazotrophic capacity, in vitro indole-acetic acid (IAA) production and calcium phosphate solubilization, metabolism of nine different C sources in semi-solid media, tolerance to different concentrations of NaCl to pHs and intrinsic resistance to nine antibiotics. Finally, the ability of the bacterial isolates to promote plant growth was evaluated using rice (Oryza sativa) as a model plant. Among the 16 isolates evaluated, eight of them were classified as Enterobacteriaceae members, related to Enterobacter andPantoeagenera. Six other bacteria were related toBacillus, and the remaining two were related toRhizobiumand Stenotrophomonas.The evaluation of total N incorporation into the semi-solid medium indicated that all the bacteria studied have putative diazotrophic capacity. Two bacteria were able to produce more IAA than that observed for the strain BR 11175Tof Herbaspirillum seropedicae.Bacterial isolates were also able to form a microaerophilic pellicle in a semi-solid medium supplemented with different NaCl concentrations up to 1.27 mol L-1. Intrinsic resistance to antibiotics and the metabolism of different C sources indicated a great variation in physiological profile. Seven isolates were able to promote rice growth, and two bacteria were more efficient than the reference strainAzospirillum brasilense, Ab-V5. The results indicate the potential of T. spicatus as native plant source of plant growth promoting bacteria.

O objetivo deste trabalho foi avaliar a contribuição de estirpes de rizóbio, eficientes na fixação de nitrogênio, sobre a produção de grãos de novas cultivares de feijão-caupi, indicadas para cultivo no Semiárido brasileiro, no Submédio do Vale do Rio São Francisco. Dois experimentos foram implantados nos perímetros irrigados de Mandacaru (Juazeiro, BA) e Bebedouro (Petrolina, PE). Os tratamentos consistiram da inoculação isolada de cinco estirpes de rizóbio - BR 3267, BR 3262, INPA 03-11B, UFLA 03-84 (Bradyrhizobiumsp.) e BR 3299T (Microvirga vignae) -, além de um tratamento com nitrogênio e de um controle sem inoculação ou aplicação de N. As seguintes cultivares de feijão-caupi foram avaliadas: BRS Pujante, BRS Tapaihum, BRS Carijó e BRS Acauã. Utilizou-se o delineamento experimental de blocos ao acaso, com quatro repetições. As plantas inoculadas apresentaram produtividade de grãos similar à observada em plantas adubadas com 80 kg ha-1 de N. As cultivares BRS Tapaihum e BRS Pujante destacaram-se quanto à produtividade e ao teor de proteínas nos grãos, quando inoculadas, o que mostra seu potencial para cultivo na região do Submédio do Vale do São Francisco.

The objective of this work was to evaluate the contribution of efficient nitrogen-fixing rhizobial strains to grain yield of new cowpea cultivars, indicated for cultivation in the Brazilian Semiarid region, in the sub-medium of the São Francisco River Valley. Two experiments were set up at the irrigated perimeters of Mandacaru (Juazeiro, state of Bahia) and Bebedouro (Petrolina, state of Pernambuco). The treatments consisted of single inoculation of five rhizobial strains - BR 3267, BR 3262, INPA 03-11B, UFLA 03-84 (Bradyrhizobiumsp.), and BR 3299T(Microvirga vignae) -, besides a treatment with nitrogen and a control without inoculation or N application. The following cowpea cultivars were evaluated: BRS Pujante, BRS Tapaihum, BRS Carijó, and BRS Acauã. A randomized complete block design, with four replicates, was used. Inoculated plants showed similar grain yield to the one observed with plants fertilized with 80 kg ha-1 N. The cultivars BRS Tapaihum and BRS Pujante stood out in grain yield and protein contents when inoculated, showing their potential for cultivation in the sub-medium of the São Francisco River Valley.

A fixação biológica de nitrogênio que ocorre em leguminosas é realizada por um grupo de bactérias conhecidas como rizóbios. A sobrevivência destas bactérias no solo é influenciada por diversos fatores como a temperatura, umidade e fertilidade do solo. O objetivo deste trabalho foi avaliar a dinâmica da população de rizóbios em solo após o cultivo de soja, durante o período de estiagem no cerrado de Roraima. Foram amostradas três áreas: i) cerrado nativo como referência; ii) área cultivada uma vez com soja inoculada com rizóbio; iii) e área cultivada duas vezes com soja inoculada com rizóbio em anos consecutivos. O solo foi coletado na profundidade de 0-10 cm em cinco períodos a partir do inicio da estiagem no mês de setembro de 2006 coincidindo com a época de colheita da soja e prolongando-se até março de 2007 (0, 45, 90, 135 e 180 dias). A população de rizóbios no solo foi avaliada pela técnica do número mais provável (NMP) utilizando plantas de soja e de feijão-caupi como espécies isca. Foi observado que na área nativa praticamente não existiam bactérias nodulantes de soja, mas havia uma população capaz de nodular o feijão-caupi de até algumas centenas de rizóbios por gramas de solo. O cultivo da soja utilizando sementes inoculadas elevou a população de rizóbios no solo que foi constatada por ambas às espécies de plantas isca. Nas áreas cultivadas, constatou-se uma intensa redução da população de rizóbios no solo, em especial logo após a colheita da soja, continuando o decréscimo até o último período de avaliação. Conclui-se que o cultivo da soja inoculada com rizóbio eleva a densidade de rizóbios em solo do cerrado, mas durante a estiagem ocorre uma drástica redução dessa população, que pode chegar a mais de 99% considerando o início e final do período.

The biological nitrogen fixation in legumes is performed by a group of bacteria known as rhizobia. The survival of these bacteria in soils is affected by several factors, such as temperature, drought and soil fertility. This study was performed to evaluate the dynamics of rhizobia in the soil after soybean cultivation and during a dry season in the cerrado of Roraima. Three areas were sampled: i) native cerrado as reference; ii) an area previously cultivated with soybean for one season; and iii) another one cultivated for two seasons also with soybean. The soil was sampled at a depth of 0-10 cm in five times (0, 45, 90, 135 and 180 days) during the dry season (September 2006 to March 2007). The rhizobial density in the soil was evaluated by the most probable number method with infection of soybean and cowpea plants. It was observed very low number of soybean nodulating bacteria in the reference area, but a high density, of up to several hundred rhizobia capable to nodulate cowpea was measured in this same area. Cropping of soybean with inoculated seeds increased rhizobial density evaluated by both trapping hosts. In cropped areas, an intense reduction of rhizobium density was observed just after soybean harvest, and this reduction continued until the end of the period of evaluation. It was concluded that soybean cultivation increases the density of rhizobial in the cerrado soil; however, this density is drastically reduced, during the dry season, by 99% at the end of the dry period.