Designing a multi-species inoculant of phosphate rock-solubilizing bacteria compatible with arbuscular mycorrhizae for plant growth promotion in low-P soil amended with PR

Abstract

Four phosphate rock-solubilizing bacteria (PRSB) (Asaia lannensis Vb1, Pseudomonas sp. Vr14, Rahnella sp. Sr24, and Pantoea sp. Vr25) isolated from the mycorrhizosphere of maize were evaluated as inoculants, individually or in different combinations, together with Rhizophagus irregularis DAOM 197198, under greenhouse conditions. To create an effective bacterial mix for P solubilization and growth promotion, parameters such as the ability of the strains (individually or in mixture) to mobilize phosphate rock (PR), their biocompatibility, and their capacity for biofilm formation over PR particles and root colonization were tested. Accompanying PR solubilization, the bacteria produced organic acids and reduced the pH of the medium, produced exopolysaccharides, and had variable capacity to colonize the roots of maize plantlets. In low-phosphorus soil amendment with PR, all strains, regardless of their capacity to solubilize PR, increased dry weight, nutrient (N, P, and K) uptake, and the percentage of indigenous arbuscular mycorrhizae (iAM) root colonization in maize plants, compared to the non-inoculated control. However, mixed inoculation of the strains showed significantly better results. Addition of Rhizophagus irregularis resulted in further growth stimulation. Overall results showed that two combinations—Rahnella sp. Sr24 + Pantoea sp. Vr25 + R. irregularis, and Pantoea sp. Vr25 + Pseudomonas sp. Vr14 + R. irregularis—were better inoculants. We concluded that an effective PRSB combination of biocompatible strains with capacity for PR solubilization, successful biofilm formation, effective root colonization, different growth promotion traits, and the addition of AM has potential as inoculants for more sustainable agriculture in low-phosphorus soils amended with low-reactivity PR.