The Phosphate-solubilizing Mechanism of Three Phosphate-solubilizing Bacteria and Their Promoting Effect on Cunninghamia lanceolata

To elucidate the phosphate-solubilizing mechanisms of phosphate-solubilizing bacteria (PSB) in the rhizosphere of Cunninghamia lanceolata in the red soil region of southern China and validate their growth potential, this study aims to provide data support for the development and utilization of these strains. Three PSB strains, namely W1 (Acinetobacter sp., A), W10 (Acinetobacter sp., B), and Y9 (Klebsiella sp., C), isolated from the rhizosphere of Cunninghamia lanceolata, were employed as test strains. The soluble phosphorus content was determined for inorganic PSB strains under various insoluble inorganic phosphorus sources Ca₃(PO₄)₂, FePO₄, AlPO₄ and for organic PSB strains under different insoluble organic phosphorus sources (calcium phytate, lecithin). A preliminary analysis of the PSB strains' mechanisms was conducted by measuring D₆₀₀, soluble phosphorus content, pH, types and contents of organic acids, and enzymatic activity during the phosphate-solubilization process. Additionally, a pot experiment with Cunninghamia lanceolata was performed to elucidate the effects of different strain combinations (A, B, C, AB, AC, BC, ABC) on the growth index, nutritional index, and soil nutrient content of Cunninghamia lanceolata seedlings, thereby verifying their growth-promoting abilities. The key findings are as follows: (1) The inorganic PSB strains W1 and W10 exhibited the optimal dissolution effect on Ca₃(PO₄)₂, primarily secreting organic acids to solubilize unavailable phosphorus. The pH of the bacterial solution was significantly negatively correlated with available phosphorus content (P < 0.01). In contrast, the organic PSB strain Y9 primarily relied on acid phosphatase and alkaline phosphatase for hydrolysis. (2) The seedling height and ground diameter growth in the late stages of the ABC treatment with compound bacterial fertilizer were significantly higher than those of other treatments (P < 0.05). Compared to the control (CK), the growth increasing rates were 20.5% (seedling height) and 33.2% (ground diameter), respectively. (3) The aboveground biomass of the ABC treatment was 49.4% higher than that of CK (P < 0.05), whereas the underground part growth was less than that of the BC treatment. (4) Under the mixed bacteria ABC treatment, plant nutrient content (total nitrogen, total phosphorus, total potassium, total organic carbon) and soil nutrient content (soil available phosphorus, available potassium, hydrolyzed nitrogen) significantly increased compared to CK. Each strain demonstrated varying abilities to solubilize insoluble phosphorus in different forms. During the hydrolysis of inorganic phosphorus, W1 and W10 primarily secreted organic acids, whereas Y9 primarily relied on enzymatic secretion for organic phosphorus solubilization. Regarding growth promotion, the three strains all positively influenced plant growth and soil nutrient content to varying degrees, with the combined benefit of strains being significantly superior to that of a single strain. The results of this study provide data support and a research basis for improving the available phosphorus content of Cunninghamia lanceolata soil and plant growth index through the application of microbial fertilizer. Furthermore, the results of this study preliminarily validate the feasibility of enhancing the nutritional environment of forest trees by using functional bacterial fertilizer.