Recovery of organic waste from a wastewater treatment plant, improved with plant growth promoting bacteria: model of Quercus suber L.

Cork oaks ( Quercus suber L.) are key tree species in Mediterranean ecosystems, playing a crucial role in fire mitigation due to their thick, fire‑resistant bark, while also contributing to biodiversity conservation and soil stability. Integrating waste valorization strategies with biofertilizers based on plant growth‑promoting bacteria (PGPB) may enhance reforestation efficiency. This study evaluated different irrigation regimes under controlled phytotron conditions, including water, organic fertilizer derived from a wastewater treatment plant (WWTP), and sterilized WWTP fertilizer, combined with Bacillus pretiosus CECT30673 T and Pseudomonas agronomica CECT30673 T . Microbial functional diversity (Shannon index), antibiotic resistance profiles, and rhizosphere community structure were assessed using 16S rRNA‑based metagenomic analyses, including taxonomic composition, beta diversity, and genus‑level relative abundances. Plant performance was evaluated through biomass production, stem length, and nutritional parameters, including protein composition, sugar content, and fatty acid profile. The application of PGPBs together with WWTP‑derived fertilizers resulted in a significant increase in plant biomass and stem length compared to traditional water irrigation. Nutritional quality was also significantly improved, with higher protein, sugar, and fatty acid contents. Additionally, the combined treatments reduced minimum inhibitory concentrations (MICs) within the rhizosphere microbial community while maintaining its functional and structural stability. These results demonstrate that combining PGPBs with WWTP‑derived matrices enhances cork oak growth and nutritional quality without disrupting native soil microbiomes, supporting their potential as sustainable tools for Mediterranean reforestation.