Forest conversion from pure to mixed Cunninghamia lanceolata plantations enhances soil multifunctionality, stochastic processes, and stability of bacterial networks in subtropical southern China

Aims Conversion from pure plantations to mixed plantations can significantly increase forest productivity and provide better ecosystem services, yet there is still a lack effective of assessment methods to determine how this conversion affects belowground biodiversity and ecological functions. Methods We conducted an in-situ experiment to investigate the impacts of forest conversion ( Cunninghamia lanceolata pure plantations vs. C. lanceolata - Betula luminifera mixed plantations) on soil multifunctionality, bacterial composition, network patterns and assembly mechanisms in southern subtropical China. Results The results showed that compared with monoculture plantations, most soil physicochemical properties and enzyme activities were higher in mixed plantations. The mixed plantations increased bacterial α-diversity, and community structure differed between the two forest types. Network analysis showed that the network structure of the mixed plantation was more complex and stable, and contained more keystone taxa. Furthermore, stochastic processes primarily governed the assembly of bacterial communities. Forest conversion increased habitat niche breadth and the importance of stochastic processes. Based on PICRUSt2, the mixed plantations significantly increased soil multifunctionality and bacterial functions (e.g., carbohydrate metabolism and energy metabolism). Moreover, variations in the bacterial community and functionality were highly correlated with soil pH and nutrients. Conclusions Our study showed that the conversion of monoculture plantations into mixed plantations enhances soil fertility and has more positive benefits. The changes in soil bacterial composition and function were mainly mediated by soil pH and nutrient increases caused by forest conversion, which contributes to assessing the eco-environmental effects of mixed planting in reforestation.