The Importance of Considering Levels of P and N Fertilization to Promote Beneficial Interaction between Rapeseed and Phosphate-Solubilizing Bacteria

Biointrants constitute a promising opportunity to lower mineral input on rapeseed, characterized by high nutrient requirements. As bio-inoculants, phosphate-solubilizing bacteria (PSB) could increase the amount of available P in a soil solution. However, the deployment of these bio-inoculants in fields is not always successful. Disentangling the factors conditioning their reliability is necessary. Because the activities of microorganisms are particularly subject to nutrient availability, the N fertilization level could represent a key factor for the success of PSB inoculation in the early stages of plant growth. In this study, Pfaba (<i>Pseudomonas</i> sp.), a promising plant growth-promoting rhizobacteria (PGPR) strain isolated from soil, was inoculated on rapeseed grown in rhizotrons under two N fertilization levels (N160 or N80) in P labile or P complexed conditions. Pfaba confirmed its PSB potential to solubilize recalcitrant P complexed forms for the benefit of plant growth, but only when the N supply is adequate (N80). In a P complexed environment, Pfaba tended to increase root and shoot biomass (respectively, from 2.17 ± 0.47 g for control modality to 2.88 ± 0.85 g, and from 6.06 ± 1.67 g for control modality to 8.33 ± 1.70 g), increase the P and N contents in roots (respectively, from 0.15 ± 0.09 mg for control modality to 0.70 ± 0.51 mg, and from 37.90 ± 11.09 mg for control modality to 41.34 ± 14.16 mg), and restore root length at a comparable level than plants supplemented with labile P. Conversely, these positive effects were inhibited with lower levels of N fertilization. Our results highlight the importance of nutrient availability to promote beneficial interaction between plants and microorganisms. These findings could also contribute to ensuring the successful deployment of microbial biointrants.