Biogeographic patterns of soil fungal diversity have been well documented in forest ecosystems, but the underlying mechanisms and processes that shape these patterns remain relatively unknown. This study took soil samples from 300 forest plots spanning six forest types along a latitudinal gradient in eastern China, which ranges from tropical rainforest to boreal forest ecosystems. A null-model analysis was used to compare the observed soil fungal beta diversity (β-diversity) with the β-diversity expected from random sampling of each local species pool. We also compared the relative importance of environmental and spatial variables on soil fungal β-diversity among forest types along the latitudinal gradient. Our results found that observed β-diversity was greater than expected β-diversity in all six forest types, which means that species tend to be more aggregated than expected. We determined that this species aggregation resulted from both environmental filtering and species dispersal limitations. Further, environmental variables had stronger influences on β-diversity than spatial dispersions. Additionally, the co-occurrence network showed that more species interactions occurred in the mid-latitude forests which lead to decreased soil fungal β-diversity and low interpretations of environmental and spatial variables. Study of these processes in different forest types along latitudinal gradients will provide important insights that local differences in the relative importance of different community assembly processes creates different gradients in global biodiversity.

Monocultures and mixed plantations have long been applied in forestry and landscape restoration to maximize the benefits of plantations. These different plantations can have unpredictable effects on the forest ecosystem. Monocultures and mixed plantations may influence soil bacterial communities, yet the underlying mechanisms of the soil bacterial community similarity response to monocultures and mixed plantations are still unknown. This study aimed to unravel how the community assembly processes and their associated factors mediate soil bacterial community similarity among monocultures and mixed plantations. We present a conceptual model to understand the mechanisms controlling soil bacterial community similarity among monocultures and mixed plantations. We tested these conceptual model hypotheses and the underlying mechanisms by conducting experiments in three plantation forest regions in southern China. We found that different monocultures led to a highly dissimilar environment, which increased heterogeneous selection and resulted in a high dissimilarity of soil bacterial communities among monocultures. Compared with monocultures, mixed plantations afford more similar environmental conditions for soil bacterial communities and decrease the heterogeneous selection process, leading to a higher soil bacterial similarity among mixed plantations. In addition, we demonstrate that stochastic processes are also the dominant driver in determining the soil bacterial community similarity among mixed plantations. Overall, the conversion from monocultures to mixed plantations affects the community assembly process by altering environmental similarity and edaphic factors, subsequently determining the similarity of soil bacterial communities. Our study can provide scientific guidance for exploring the role of mixed plantations in forest management.