Positive effects of broadleaf tree introduction on soil biodiversity and function in degraded pine forests

Background and aims: Restoring degraded forests is essential for supporting soil biodiversity and multifunctionality by increasing tree diversity, yet the consequences for soil biodiversity and function remain poorly understood. Methods: Our study compared soil microbial communities (16S rRNA/ITS sequencing) in a pair of degraded and restored monoculture and mixed-species pine (Pinus spp.) plantations, restoration initiated in 1981 and 2000 in southern China. This 20-year age difference allowed us to assess how long-term restoration drives the cumulative complementarity effects of tree species mixing, and to examine the role of microbial communities in mediating soil multifunctionality. We analyzed their diversity patterns and functional roles in soil multifunctionality, which encompasses carbon (C), nitrogen (N), and phosphorus (P) cycling, as well as substrate quality—an index of microbial degradability, where lower values indicate a greater susceptibility to microbial decomposition. Results: We found that mixed-species plantations (conifers and broadleaf trees), harbor higher soil biodiversity and multifunctionality than monocultures. Specifically, mixed plantations demonstrated stronger correlations between soil N/P cycling, substrate quality, and bacterial/fungal richness. Microbial diversity appeared to drive functional recovery through complementary associations with microbial functional groups (as indicated by the Shannon diversity of pathogenic fungi). Multiple regression models further suggested that soil multifunctionality in monocultures was primarily governed by litter carbon-to-nitrogen (C:N) ratios, whereas it was dually regulated by both soil C:N ratios and microbial diversity in mixed plantations. Together, our findings highlighted the importance of mixed-species plantations in enhancing soil multifunctionality, which should be considered in future sustainable forest restoration practices.