Effects of different mixed tree species on the concentration of organic acids in the rhizosphere of Pinus massoniana

Through analyzing the effects of different mixed tree species on the concentration of organic acids and phosphorus availability in the rhizosphere soil of Pinus massoniana, this study provides a basis for selecting suitable mixed species for P. massoniana. A pot experiment was conducted, involving five native tree species from Guizhou province and one widely planted Robinia pseudoacacia mixed with P. massoniana at a 1:1 ratio, with monoculture P. massoniana as the control. The study examined the concentrations of 10 common organic acids, phosphorus content in the rhizosphere soil, and phosphorus variations in plant tissues under seven different treatments. The results showed significant differences in total organic acids among treatments (P<0.05). The Schima superba × Pinus massoniana (SS × PM) treatment had the highest total organic acid concentration (129.26 mg/kg), followed by the Liquidambar formosana × Pinus massoniana (LF × PM) treatment (59.93 mg/kg). Except for the LF × PM treatment, the total and available phosphorus content in the rhizosphere soil of P. massoniana increased compared to the control. Phosphorus content in the roots was higher in all mixed treatments than in the control (except SS × PM), with the LF × PM treatment showing the highest increase (0.74 mg/kg higher than the control). Correlation analysis revealed a highly significant positive relationship between total organic acid concentration and total phosphorus content in the rhizosphere soil (P< 0.01), while tartaric acid and propionic acid concentrations were significantly negatively correlated with available phosphorus (P<0.01). Stepwise regression analysis indicated a highly significant linear relationship (R²>0.97) between soil/plant phosphorus content and organic acid concentrations. In conclusion, mixed planting of P. massoniana with the six tree species enhanced phosphorus content in both the rhizosphere soil and roots of P. massoniana. Specifically, mixing with Schima superba and Liquidambar formosana significantly increased total organic acids and formic acid concentrations in the rhizosphere, thereby influencing phosphorus availability in the soil.