Objective This study aimed to investigate changes in soil multifunctionality and to identify its key driving factors following the conversion of monoculture Chinese fir (Cunninghamia lanceolata) plantations into mixed, uneven-aged, two-layered plantations composed of Chinese fir and Phoebe chekiangensis (hereinafter referred to as two-layered plantations). The objective was to provide a scientific basis for evaluating the ecological benefits of close-to-nature plantation management.

Methods Based on Chinese fir monocultures and two-layered plantations, changes in topsoil (0~20 cm) properties of the two stand types, including soil nutrients, enzyme activities related to carbon (C), nitrogen (N), and phosphorus (P) cycling, soil fungal diversity, community composition, and function, as well as multifunctionality indices related to C, N, and P cycling were analyzed. Variance partitioning analysis was applied to quantify the relative contributions of fungal communities and vegetation factors to soil multifunctionality.

Results Compared with monocultures, the contents of soil organic carbon, total N, total P, and available nutrient contents in two-layered plantations increased by 17.6%~91.1%. The activity of β-1,4-glucosidase significantly increased, whereas the activities of β-1,4-N-acetylglucosaminidase and acid phosphatase significantly decreased. The soil fungal α-diversity and the relative abundances of Basidiomycota, Rozellomycota, Mortierellomycota and Mucoromycota in two-layered plantations increased significantly compared with monocultured plantations. In addition, the relative abundances of saprotrophic, saprotrophic–symbiotic, pathogenic–symbiotic, and pathogenic–saprotrophic–symbiotic fungi were also significantly higher in two-layered plantations. The functional indices of soil C, N, and P cycling and overall multifunctionality in two-layered plantations were 5.4, 1.7, 1.8, and 2.3 times higher than those in monocultures plantations, respectively. Soil fungal communities (α-diversity, composition, and function) and vegetation factors (litter and fine root biomass) explained 92.3% of the variation in soil multifunctionality, with fungal communities contributing a larger proportion than vegetation factors.

Conclusion The conversion of monoculture Chinese fir plantations into two-layered plantations can significantly improve soil multifunctionality, particularly carbon cycling functions. These improvements are primarily derived by shifts in soil multifunctionality diversity, composition, and fungal structure. These findings deepen the understanding of how forest structure regulation maintains soil fertility and provide theoretical support for the close-to-nature transformation of subtropical plantations.