Isolation and characterization of azotobacteria from pine rhizosphere.

Plant growth-promoting rhizobacteria contain Nitrogen-fixing, Phosphorus- solubilizing, Potassiumsolubilizing and other bio-active bacteria. Utilization of microbial Nitrogen fixation, Phosphorus- and Potassium-solubilizing activities can facilitate plant growth and avoid the lack of soil nutrients. This study used the plate dilution method to screen and isolate pro-biotic bacteria from the rhizosphere of pine trees, growing on the outskirts of Nanning City, Guangxi Province China. 16S rDNA was sequenced for the bacteria with strong biological activities. A total of 23 Azotobacter strains were isolated from the rhizosphere of Pinus massoniana Lamb. All of them presented Nitrogenase activity. Particularly, strain N4 and N9 had a strong Nitrogenase activity of 260.975 nmol C2H4/h·L and 149.787 nmol C2H4/h·L, respectively. Six Phosphorus-solubilizing strains were isolated, among which P3 and P1 had a strong Phosphorus-solubilizing ability of 7.00 and 3.87 µg/ml, respectively. Among the 5 Potassium-solubilizing strains, K6 had a strong Potassium-solubilizing ability of 2.3 µg/ml. The ability of the strains to solubilize Phosphorus and Potassium was significantly correlated with the composition of the culture medium. All the Potassium-solubilizing strains belong to the genus of Burkholderia. The Phosphorus-solubilizing strains include Enterobacter, Pseudomonas and Serratia marcescens. Nitrogen-fixing strains include Enterobacter, Burkholderia, Rhizobium, Erwinia, Agrobacterium and Gamma proteobacterium.