To assess the alterations in soil properties resulting from the interplanting of broad-leaved tree species within coniferous forests, we conducted an investigation into soil quality in a mixed Chinese fir and broad-leaved forest, as well as in a Chinese fir pure forest (used as a control) in subtropical China. A total of 15 soil physicochemical properties were assessed across three soil depths—0–15 cm, 15–30 cm, and 30–45 cm—for the two forest types in the experimental study. Principal component analysis in conjunction with the Norm value was employed to create a minimal data set (MDS) for assessing six indicators, including bulk density (BD), total nitrogen (TN), total phosphate (TP), available potassium (AK), soil pH, and catalase (CAT). The soil quality index (SQI) was calculated for both forest types. The results demonstrated that following the interplanting of broad-leaved tree species in the Chinese fir forest, all soil physicochemical indicators were significantly improved compared to the control, and significant differences were also observed in the 0–15 cm and 15–30 cm soil layers (p < 0.05). The overall average of the SQI of the mixed forest (0.8523, 0.6636) was significantly higher than that of the control (0.4477, 0.3823) (p < 0.05) in the 0–15 cm and 15–30 cm soil layers, respectively. However, there was no significant difference in the SQI in the 30–45 cm soil layer (p > 0.05) between the two forest types. The results indicated that the SQI based on the minimal dataset (MDS) can reflect the SQI of the total dataset (TDS) when assessing soil quality in forests. Our research provides valuable scientific insights into soil science and an understanding of the relationships between soil properties, forest structure, and species composition in sustainable forest management.

Soil enzyme activities regulate organic matter decomposition and nutrient cycling and are widely used as indicators of microbial functioning and ecosystem processes. However, the mechanisms underlying their spatial heterogeneity, particularly the role of forest stand spatial structure, remain poorly understood. This study investigated subtropical Chinese fir (Cunninghamia lanceolata) plantations in southern China across different stand developmental stages. Relationships between stand spatial structure attributes and soil enzyme activities were evaluated using Spearman rank correlation analysis, redundancy analysis, and response surface methodology. Soil enzyme activities exhibited distinct stage-dependent variation, with generally lower activities in middle-aged stands and partial recovery in mature stands. The first two axes of redundancy analysis explained 62.96% of the variation in soil enzyme activities, while Spearman analysis showed significant negative associations between the complete mingling index and urease, catalase, and polyphenol oxidase activities (r_s = −0.92 to −0.82, p < 0.01), whereas the competition index was positively associated with cellobiohydrolase activity (r_s = 0.77, p < 0.05). Moreover, mingling and competition exhibited nonlinear and interactive effects on certain enzyme activities. These findings highlight the importance of stand spatial structure in regulating soil biochemical processes and provide ecological support for structure-oriented management of subtropical Chinese fir plantations.