| Summary: | The bacterial genus <i>Pantoea</i> has been widely evaluated as promising bacteria to increase phosphorus (P) availability in soil. The aim of this study was to characterize the phosphate solubilizing (PS) activity of a <i>Pantoea agglomerans</i> strain and to evaluate the impact of its application in a semi-arid soil on phosphate availability and structure of the bacterial communities as a whole. An incubation experiment under close-to-natural soil environmental conditions was conducted for 15 days at 30 °C. High-throughput sequencing of the bacterial 16S rRNA gene was used to characterize and to compare the bacterial community structure of <i>P. agglomerans</i>-inoculated soil with non-inoculated control. Furthermore, a qPCR-based method was developed for detection and quantification of the functional genes related to the expression of mineral phosphate solubilization (MPS) phenotype in <i>P. agglomerans</i>. The results showed that in vitro solubilization of Ca<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub> by <i>P. agglomerans</i> strain was very efficient (980 mg/L), and it was associated with a drop in pH due to the secretion of gluconic acid; these changes were concomitant with the detection of <i>gdh</i> and <i>pqqC</i> genes. Moreover, <i>P. agglomerans</i> inoculum application significantly increased the content of available P in semi-arid soil by 69%. Metagenomic analyses showed that <i>P. agglomerans</i> treatment modified the overall edaphic bacterial community, significantly impacting its structure and composition. In particular, during <i>P. agglomerans</i> inoculation the relative abundance of bacteria belonging to Firmicutes (mainly <i>Bacilli</i> class) significantly increased, whereas the abundance of Actinobacteria together with Acidobacteria and Chloroflexi phyla decreased. Furthermore, genera known for their phosphate solubilizing activity, such as <i>Aneurinibacillus</i>, <i>Lysinibacillus</i>, <i>Enterococcus</i>, and <i>Pontibacter,</i> were exclusively detected in <i>P. agglomerans</i>-treated soil. Pearson’s correlation analysis revealed that changes in soil bacterial community composition were closely affected by soil characteristics, such as pH and available P. This study explores the effect of the inoculation of <i>P. agglomerans</i> on the bacterial community structure of a semi-arid soil. The effectiveness in improving the phosphate availability and modification in soil bacterial community suggested that <i>P. agglomerans</i> represent a promising environmental-friendly biofertilizer in arid and semi-arid ecosystems.
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