Walnut growth and soil quality after inoculating soil containing rock phosphate with phosphate-solubilizing bacteria

Application of phosphate-solubilizing bacteria (PSB) has been shown to promote plant growth and development in the field and under controlled environmental conditions. The effects of PSB on microbial activity and soil quality are not well known. This study aims to determine the effect of inoculation with three PSB strains on the solubilization of rock phosphate (RP) in soil and growth promotion, nutrient uptake, photosynthetic capacity, and soil enzyme activities of 1-year-old walnut seedlings by using potted experiments with a completely randomized design under single or mixed inoculations. Inoculation with Pseudomonas aurantiaca or P. fluorescens significantly improved the plant height, shoot and root dry weight, phosphorus (P) and nitrogen (N) uptake, and net photosynthetic rate of walnut seedlings. Application of these two PSB strains also improved soil quality, as indicated by increased activities of dehydrogenase, neutral phosphatase, and urease in the soil. These increases were higher when PSB was applied together with RP. The most pronounced beneficial effects on plant growth and soil enzyme activities were observed when the three PSB strains were co-inoculated with RP. Bacillus cereus failed to promote plant growth. The results demonstrated that P. aurantiaca or P. fluorescens could be used as an environment-safe biofertilizer of walnut seedlings and other woody plants.