Positive Correlation Between Tree Species Richness and Soil Quality in Subtropical Natural Pinus massoniana Lamb. Forests

This study investigates the relationship between tree species diversity and soil quality in natural Pinus massoniana Lamb. forests, providing a scientific basis for the sustainable management of natural stands and the establishment of plantations. Located in Ganzhou City, Jiangxi Province, China, a subtropical region, the study focuses on natural P. massoniana forests. The Patrick richness index and Shannon–Wiener diversity index were each classified into three levels—low, medium, and high—to analyze how soil physicochemical indicators vary across these diversity levels. A minimum data set of key soil indicators was established to evaluate soil quality. Linear and nonlinear scoring functions were used to compare the relationships between the total data set and minimum data set-derived soil quality indices, and the most suitable scoring function for this study was selected. An obstacle factor diagnosis model was applied to identify the factors limiting soil quality improvement under different diversity levels. The results showed that (1) in the 20–40 cm soil layer, soil water content, maximum water-holding capacity, and total porosity significantly increased with rising Patrick richness and Shannon–Wiener diversity levels. (2) With increasing Patrick richness and Shannon–Wiener diversity levels, organic matter and total nitrogen in the 0–20 cm soil layer, as well as total potassium, available nitrogen, and pH in both the 0–20 cm and 20–40 cm soil layers, showed significant upward trends. In contrast, total phosphorus in both layers significantly decreased. Furthermore, available phosphorus in both layers significantly declined as Patrick richness increased. (3) The minimum data set based on the nonlinear scoring function can effectively substitute for the total data set (r = 0.791, R2 = 0.63), showing better applicability for assessing soil quality changes in natural P. massoniana forests. Additionally, the soil quality index in the 0–40 cm soil layer significantly increased with rising Patrick richness levels. (4) At different Patrick richness levels, bulk density had the greatest impact on soil quality in the 0–40 cm soil layer, while organic matter had the least. A comprehensive analysis shows that there is a positive correlation between tree species diversity and soil quality in natural P. massoniana forests. In P. massoniana plantation practices, a near-natural management approach should be adopted to create relatively species-rich and complex coniferous and broad-leaved mixed stands, which is conducive to improving soil quality.