Are native phosphate-solubilizing bacteria a relevant alternative to mineral fertilizations for crops? Part II: PSB inoculation enables a halving of P input and improves the microbial community in the rapeseed rhizosphere

In this study, three PSB strains belonging to the Pseudomonas genus, isolated from soil harvested under rapeseed (Pnapus), winter pea (Pisum sativum, Ppea), and faba bean (Vicia faba, Pfaba), were tested on rapeseed growth. This test was performed in a greenhouse experiment with four plant replicates in pots per modality during the vegetative stage of rapeseed (10 weeks), in combination with a gradient of P fertilization (P100, P50, P25 and P0 corresponding to 125, 62.5, 31.5 and 0 mg P2O5 per plant, respectively). Contrasted results were observed for the three tested strains. PSB from rapeseed (Pnapus) showed no effect on plant biomass or P status. Ppea and Pfaba increased the biomass and P amount in roots when P fertilization was decreased to 50%, but winter pea PSB (Ppea) dramatically decreased plant biomass below this P input threshold. For fertilization at P50 with Ppea and Pfaba strains, an increase in alkaline phosphatase activity was concomitantly observed in the rhizosphere. The structure and diversity of the soil microbial community was then assessed to explore possible indirect effects of Pfaba application. Relative abundance of several bacterial genera potentially involved in biological control against phytopathogens were decreased (Sphingomonas, Acidobacteria Gp6, Lysobacter, Thermomonas, and a genus belonging to the Spartobacteria class), whereas relative abundance of other genera involved in the nitrogen and sulphur cycles were increased (Ramlibacter, Nocardioides, Inhella, Bradyrhizobium, and Thiobacillus), possibly sustaining even more plant access to essential nutrients.