Effects of Stand Spatial Structure on Soil Enzyme Activities in Chinese Fir Plantations in the Mid-Subtropical Zone of China.

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 (rs = -0.92 to -0.82, p < 0.01), whereas the competition index was positively associated with cellobiohydrolase activity (rs = 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.