Inoculation with Elite Strains of Phosphate-Solubilizing Bacteria Enhances the Effectiveness of Fertilization with Rock Phosphates

Eight bacterial strains expressing high-levels of rock phosphate solubilization were isolated from Tunisian agricultural soils. 16S rDNA sequencing assigned these strains to four genera, Pseudomonas, Bacillus, Serratia, and Burkholderia. Both qualitative and quantitative assays were performed to study their plant-growth promoting abilities. Five strains released inorganic phosphorus up to 600 mu g ml(-1) with rock phosphate as sole phosphorus source. They displayed additional capabilities of producing phytases (16.1-24.8 Uml(-1)), IAA (till 39.6 mu g ml(-1)) and siderophores (9-81.1%). When tested on Medicago truncatula on sterile stand, two strains were discarded since they did not show significant enhancement. The remaining bacterial strains, Pseudomonas corrugata SP77, Pseudomonas koreensis LT62, and Pseudomonas frederiksbergensis G62, were further tested on Medicago in two phosphorus-deficient soils amended with Tunisian rock phosphate. The three strains induced significant enhancement in shoot dry weight and nodule fresh weight; however, the best results were observed with SP77 and the consortium. The increase in shoot dry yield ranged from 40 to 134% and from 13 to 87% for soil 1 and soil 2, respectively. The biomineralization of rock phosphates by these elite strains will constitute an eco-friendly alternative to chemical fertilizer, mainly in neutral and alkaline soils.