FixaÃÃo biolÃgica de nitrogÃnio e solubilizaÃÃo de fosfatos por bactÃrias isoladas de nÃdulos de caupi [Vigna unguiculata (L.) Walp.] / Biological nitrogen fixation and phosphate solubilization by bacteria isolated from cowpea nodules [Vigna unguiculata (L.) Walp.]




The biological nitrogen fixation and solubilization of insoluble inorganic phosphates are processes mediated by soil microorganisms which can increase crop productivity. The objective of this study was to isolate and select bacteria nodulating cowpea and efficient in biological nitrogen fixation and solubilization of insoluble phosphates and, to identify and quantify the organic acids produced, possibly related to the solubilizing capacity. These bacteria were isolated from two soils containing high concentrations of insoluble P, from the cities of Lavras and Patos de Minas, by using cowpea [Vigna unguiculata (L.) Walp.] as trap species. 78 isolates were obtained which were characterized according to their cultural characteristics in the 79 culture medium together with strains UFLA 03-84, INPA 03-11B and BR3267 (approved by the MinistÃrio da Agricultura PecuÃria e Abastecimento â MAPA, for the production of cowpea inoculants) and also strain Burkholderia cepacia LMG1222, used as positive control in the solubilization of phosphates. To symbiotic efficiency (biological nitrogen fixation - BNF) was evaluated in the greenhouse and efficiency in the solubilization of insoluble phosphates linked to calcium (CaHPO4), aluminum (AlH6O12P3) and iron (FePO4.2H2O) on solid and liquid media. Strains with high efficiency in phosphate solubilization were also evaluated regarding the production of organic acids: citric, gluconic, lactic, oxalic, propionic and succinic. It was found that communities of bacteria isolated from cowpea nodules in the two soils are different and showed great variability in the efficiency of BNF and solubilization of insoluble inorganic phosphates. The inoculation with isolates L14, P6, P10, P20, P31 and P40 provided greater plant growth, higher nitrogen nutrition and good nodulation compared to the strains approved by MAPA and to the nitrogen control. Larger numbers of isolates solubilized phosphates in liquid than in solid medium and their solubilization capacity decreased in the following order: Ca-P >Fe-P >Al-P. The solubilization of Ca-P and Fe-P was related to the pH decrease in liquid medium. Isolates P31 and P40 stand out in the solubilization of P-Ca, while the isolates P22 and P29 in the solubilization of Al-P and isolates P28 and P39 in solubilization of Fe-P, together with these is the strain LMG1222, which also showed high solubility in the various sources of phosphates. The main organic acids produced by the isolates were propionic, succinic + lactic, gluconic, oxalic and citric. However there was no quantitative correlation of these with the solubilization of phosphates. Most organic acids produced by isolates capable of solubilizing Ca-P acted acidifying the culture medium, while in medium with Fe-P all isolates quelated the Fe3+ ion. So the good performance shown by some isolates evaluated in the experiments make them promising for future experiments on agronomic efficiency in cowpea.


phosphate solubilizing bacteria organic acids. diversidade fenotÃpica bactÃrias solubilizadoras de fosfatos Ãcidos orgÃnicos. phenotypic diversity fosfatos inorgÃnicos insolÃveis insoluble inorganic phosphates ciencia do solo

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