Optimizing Phosphorus Application Rate and the Mixed Inoculation of Arbuscular Mycorrhizal Fungi and Phosphate-Solubilizing Bacteria Can Improve the Phosphatase Activity and Organic Acid Content in Alfalfa Soil

Alfalfa ( Medicago sativa L.) is an important legume forage, and phosphorus is a significant nutrient element of alfalfa with high quality and high yield. However, the effect mechanism of different phosphorus application rates on soil bacteria, and the phosphorus efficiency of alfalfa are still unclear. Therefore, we present the results of a study in which alfalfa was inoculated with Funneliformis mosseae , Bacillus megaterium , double inoculated with Funneliformis mosseae and Bacillus megaterium, and given no inoculation bacteria. P 2 O 5 was applied under the condition of bacterial inoculation, and the contents were 0, 50, 100, and 150 mg kg −1 , respectively, to explore the effect of bacterial inoculation on alkaline phosphatase, organic acid, pH, organic matter, and the relationship between the indicators in alfalfa soil, under different phosphorus application rates. The effect of mixed inoculation was significantly higher than that of the non-inoculated control group ( p < 0.05). The organic matter content of rhizosphere soil was higher than that of non-rhizosphere soil. When the phosphorus application rate was 100 mg kg −1 , the content of alkaline phosphatase in the soil inoculated with Funneliformis mosseae and Bacillus megaterium was better than that in the single inoculation, and no inoculation. Principal component analysis showed that the top three treatments were: double inoculation bacteria and treatment group with phosphorus application rate of 100 mg kg −1 >; double inoculation bacteria and treatment group with phosphorus application rate of 50 mg kg −1 >; double inoculation bacteria and treatment group with phosphorus application rate of 150 mg kg −1 . In addition, when P 2 O 5 was 100 mg kg −1 , the addition of Funneliformis mosseae and Bacillus megaterium to alfalfa soil could increase the content of organic matter in the soil, promote the metabolism of alfalfa root exudates, and increase the organic acid of the rhizosphere soil, compared with the control without inoculation, and without phosphorus application. At the same time, the phosphatase activity in the soil had a significant positive correlation with malic acid, oxalic acid, acetic acid, total organic acid, and soil pH, thereby improving soil fertility and promoting phosphorus absorption by plants. These findings provide new insights into alfalfa root soils and the effects of Funneliformis mosseae and Bacillus megaterium additions on soil nutrients.