In the previous half-century, natural rock phosphates (PN) have been a valuable alternative for phosphorus (P) fertilizer for sustainable agriculture; furthermore, phosphogypsum (PG) has been widely used as a soil amendment fertilizer since it improves some soil properties, increases crop yields, and represents an environmental concern that can make a good economic profit; this research aimed to study the effects of microbial consortia of phosphate-solubilizing microorganisms (PSM) on the solubilization of PN and PG in the soil, and their effects on promoting plant growth and nutrient assimilation using ryegrass as a plant model. Local supply of PG with Pseudomonas fluorescens (MW165744) significantly increases root proliferation and plant biomass dry weight compared to other isolates, as well as improves total P uptake, with a maximum value of 62.31 mg/pot. The opposite occurred in mixing inoculation with Pseudomonas fluorescens, Pantoea agglomerans (MW165752) and Stenotrophomonas maltophilia (MW221274), with a negligible total P assimilation of 5.39 mg/pot. Whereas the addition of Pseudomonas agglomerans with PG gave outstanding total P absorption of 57.05 mg/pot when compared with PN input of 38.06 mg/pot. Finally, the results prove that the co-inoculation of Pseudomonas fluorescens with PG could be a promising and alternative option to use it as a source of P fertilizer for plants and to maintain a high level of nutrients in the soil.

Soil microorganisms play an important role in maintaining natural ecological balance through active participation in carbon, nitrogen, sulfur, and phosphorous cycles. Phosphate-solubilizing bacteria (PSB) are of high importance in the rhizosphere, enhancing the solubilization of inorganic phosphorus complexes into soluble forms available for plant nutrition. The investigation of this species of bacteria is of major interest in agriculture, as they can be used as biofertilizers for crops. In the present study, 28 isolates of PSB were obtained after the phosphate enrichment of soil samples from five Tunisian regions. Five PSB species were identified by 16S rRNA gene sequencing including Pseudomonas fluorescens, P. putida, and P. taiwanensis, Stenotrophomonas maltophilia, and Pantoea agglomerans. Solid and liquid Pikovskaya’s (PVK) and National Botanical Research Institute’s (NBRIP) media containing insoluble tricalcium phosphate were used for the evaluation of the phosphate solubilization ability of the bacterial isolates by two methods: visual evaluation of the solubilization zone around colonies (halo) and determination of solubilized phosphates in liquid medium by the colorimetric method of the vanado-molybdate yellow. Based on the results of the halo method, the isolate of each species that showed the higher phosphate solubilization index was selected for evaluation of phosphate solubilization by the colorimetric method. In the liquid media, the bacterial isolates showed phosphate solubilization ranging from 535.70 to 618.57 µg mL−1 in the NBRIP medium, and 374.20 to 544.28 µg mL−1 in the PVK medium, with the highest values produced by P. fluorescens. The best phosphate solubilization ability and higher reduction in broth pH, which indicates higher organic acid production, were achieved in NBRIP broth for most of the PSB. Strong correlations were observed between the average capability of PSB to solubilize phosphates and both the pH and total phosphorous content in the soil. The production of the hormone indole acetic acid (IAA), which can promote plant growth, was observed for all five PSB species. Among them, P. fluorescens obtained from the forest soil of northern Tunisia showed the highest production of IAA (50.4 ± 0.9 µg mL−1).