Abstract:

Objective: The objectives of this study are to investigate the effects and regulatory mechanisms of roots and mycelium of Pinus massoniana and Castanopsis hystrix forests and their mixed forest on the content of different soil phosphorus fractions, so as to provide a theoretical basis for the species selection and nutrient management in the establishment of subtropical mixed-species plantations. Method: Sampling plots were set up in P. massoniana and C. hystrix forests and their mixed forest. Ingrowth bags with varying pore sizes (2 mm, 48 μm, and 1 μm) were used to physically distinguish the regulatory effects of roots and mycelium on the contents of different soil phosphorus fractions. Soil physicochemical properties such as phosphorus fraction contents, along with microbial biomass carbon, nitrogen, and phosphorus contents, and enzyme activities were measured. A systematic comparison was conducted on the effects of roots and mycelia on different soil phosphorus fractions across the three types of forests. Key regulatory factors were identified using correlation analysis, variance partitioning, and redundancy analysis. Result: 1) Compared to the pure P. massoniana stand, the mixed stand of P. massoniana and C. hystrix exhibited a significant increase in the positive root effect on the moderately active phosphorus fraction (NaOH-Po) and the negative root effect on the active phosphorus fraction (NaHCO₃-Po). Additionally, the mixed stand substantially elevated the positive mycelial effect on the active phosphorus fraction (NaHCO₃-Po) and the negative mycelial effect on the stable phosphorus fraction (HCl-Pi) (P<0.05). 2) On the one hand, mixed tree species significantly promoted the accumulation of moderately active phosphorus fractions (NaOH-Po) through root-mediated biological processes (inhibition of β-1,4-glucosidase activity) and abiotic processes (decreasing soil pH) (P<0.05), thereby promoting the transformation of active phosphorus fractions (NaHCO₃-Po) into moderately active phosphorus fractions. On the other hand, the mixed stand significantly enhanced the content of active phosphorus fractions (NaHCO₃-Po) through mycelium-mediated processes (increasing microbial biomass carbon content and microbial biomass nitrogen content) (P<0.05). This process also activated stable phosphorus components, facilitating their conversion into the active phosphorus fractions. 3) Variance decomposition, redundancy analysis, and correlation analysis consistently demonstrated that biotic factors were the primary determinants influencing the regulation of different soil phosphorus fraction contents by roots and mycelium. Conclusion: Tree species mixing primarily regulates the contents of different soil phosphorus fractions through root-mediated biotic and abiotic processes and mycelium-mediated biotic processes, with biological factors playing a central role. In the management of plantations, full consideration should be given to the ecological strategies of roots and mycelia of different tree species, and species configuration should be optimized to enhance soil phosphorus availability and plantation productivity.

Key words: plantation, root effect, mycelia effect, phosphorus fractions