Diameter Class-Dependent Species-Specific Tree–Soil Feedback Linked to Soil Quality Between Cunninghamia lanceolata (Lamb.) Hook. and Quercus fabri Hance in Subtropical Forests

The coupling between tree biomass and soil microhabitats is central to subtropical forest soil functioning, yet species- and stage-specific tree–soil interactions remain understudied. This study quantified these interactions in two dominant species—Cunninghamia lanceolata (Lamb.) Hook. (C. lanceolata) and Quercus fabri Hance (Q. fabri)—across five diameter at breast height (DBH) classes (5–10, 10–15, 15–20, 20–25, 25–30 cm). Soil quality was characterized via the Soil Quality Index (SQI) based on 16 physicochemical and enzyme activity parameters, while random forest models identified biomass importance. Soil properties and enzyme activities varied with diameter class (p < 0.05): C. lanceolata showed a unimodal pattern (minimum at 15–20 cm DBH), whereas Q. fabri increased consistently (peaking at 20–30 cm DBH). The diameter class × species interaction significantly influenced SQI (p < 0.01): Q. fabri showed higher SQI than C. lanceolata at larger DBH, and vice versa at smaller DBH. Aboveground biomass dominated SQI variation in C. lanceolata (weight = 0.57), whereas belowground biomass dominated in Q. fabri (weight = 0.52; model R2 > 0.75). These findings demonstrate that DBH size and species identity jointly shape soil microenvironments, providing a mechanistic basis for informed subtropical forest management.