Resumo

FERNANDEZ, Leticia Andrea; ZALBA, Pablo; GOMEZ, Marisa Anahí  e  SAGARDOY, Marcelo Antonio. Phosphate inorganic solubilizing bacteria isolated from soybean region soils. Cienc. suelo [online]. 2005, vol.23, n.1, pp.31-37. ISSN 1850-2067.

The present research focuses on the study of the ability to solubilize inorganic phosphate of different bacterial groups as well as of Bradyrhizobium sp. strains isolated from soybean soils. Soil samples were collected and the microflora as well as the total number of bacteria, fungi, and solubilizers was assessed. The phosphate solubilization ability of the predominant bacterial groups, of 250 Bradyrhizobium sp. strains, and of 10 collection strains was tested in Petri dishes containing NBRIP supplemented with 5 g L-1 of tricalcium phosphate. The size of solubilizing halos was measured and those colonies showing halos larger than 4 mm were isolated. Solubilized phosphate was quantitavely estimated in broth with and without buffer in these strains and it was compared with a commercial strain. The total bacteria mean values reached 5.1 106 (0.06% solubilizers) while the total fungi reached 3.3 104 (9.70% solubilizers). No significant differences in the number of solubilizing bacteria were observed in the different soils. Fourteen solubilizing strains were obtained: 10 from microflora and 4 Bradyrhizobium sp. strains whose halos ranged from 4 to 15 mm. All collection strains except MSDJ G49 solubilized phosphate in the plate assay. The amounts of solubilized phosphate by the strains of microflora varied from 3% to 24.1% in a liquid medium without buffer while with buffer varied from 0.07% to 4.82%. In a liquid medium without buffer, the strains of Bradyrhizobium sp. solubilized in percentages ranging from 7.1% to 8.5% while with buffer, the percentages ranged from 0.1% to 0.16%. No significant differences were observed in the amounts of solubilized phosphate in a liquid medium with buffer between strains of Bradyrhizobium sp. and the majority of the microflora strains. The phosphate solubilization ability of the commercial strain studied was not as relevant as that of the other strains. Our results lead to conclude that there are bacterial strains with different abilities to solubilize inorganic phosphate and that the strains of Bradyrhizobium sp. show relevant solubilization ability with respect to other bacterial genera.

Palavras-chave : Phosphate solubilization; Bradyrhizobium; Soil bacteria.

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