A Bacillus pretiosus Biofertilizer Enhances Early Growth, Nutritional Profile, and Rhizospheric Dynamics in Quercus Species for Forest Restoration

Plant growth-promoting bacteria (PGPBs) have emerged as essential biological inputs to improve the establishment, physiological performance and survival of forest species used in ecological restoration programs. The decline of Quercus forests throughout the Mediterranean basin requires innovative and sustainable strategies to improve early seedling establishment. Among possible PGBs, Bacillus pretiosus has attracted significant interest due to its ability to promote plant growth, modulate the nutrient status of the host, and safely interact with native rhizosphere microbiota. At the same time, evaluating urban wastewater waste using PGPB offers a promising circular economy approach. To provide an integrated functional and ecological assessment of B. pretiosus C1, this in silico study harmonizes and reinterprets experimental datasets through contrasting fertigation regimes—water (W), wastewater treatment plant effluent (EDAR) and electrochemically treated effluent (EDARST)—evaluating inoculation (C1) versus uninoculated controls (C0). Inoculation resulted in a consistent positive biometric response, significantly promoting outbreak length (up to +16.2% in EDARST). In addition, B. pretiosus C1 improved nitrogen leaf metabolism, raising crude protein levels (+11.8%), soluble protein and total amino acids (+15.4%). Sequencing and functional profiling of the high-yield 16S rRNA gene revealed that the introduction of strains did not significantly alter the alpha-diversity or overall beta structure of the native rhizosphere microbiome, demonstrating high ecological compatibility. Phenotypic profiling of the resistome by cenoantibiogram did not confirm an increase in resistance to antibiotics at the community level, instead showing a significant reduction in the minimum inhibitory concentration (MIC) for imipenem and amoxicillin/clavulanic acid under the fertigation of effluents. Together, these findings demonstrate that combining effluents valued with p > 0.05 B. pretiosus C1 shows a consistent functional profile with a high-value biotechnological agent, improving the biometric and nutritional status of seedlings without altering the stability of the soil microbial community. This supports its potential application as a safe biofertilizer candidate within forest restoration strategies and One Health, although more prospective field trials will be needed to validate each site.