Assembly processes, driving factors, and shifts in soil microbial communities across secondary forest succession

Secondary forest succession after clear-cutting is one of the foremost ecosystem restoration strategies, while soil microbes play essential roles in the processes by modulating nutrient cycling. However, the assembly processes and driving factors of soil microbial communities across secondary forest succession remain unclear. Here, we studied the assembly processes of soil microbial communities and examined shifts in soil microbial community-associated functional dynamics across secondary forest succession. Our results showed that the stochastic process was more important in shaping bacterial community assembly throughout the successional process [modified stochasticity ratio (MST) > 50%], while the fungal community assembly was initially governed by deterministic processes (MST < 50%), but there was a progressive increase in stochastic selection as succession proceeded. Soil organic carbon and pH were principal factors for the explanation of changes in the bacterial community structure (total explained 43% change), and tree richness and productivity were principal factors for the explanation of shifts in the fungal community structure (total explained 17% change). The relative abundance of nitrogen transformation and saprotroph functional groups increased gradually with succession, whereas ectomycorrhizal fungi significantly declined. The results suggested that microbial community succession might accelerate the soil carbon and nitrogen turnover rates. Used together, the mechanisms shaping fungal and bacterial community structure are different in secondary forest succession and highlight that those fungi and bacteria are primarily controlled by plant traits and soil properties, respectively. Variations in microbial functional groups provide new insight into the mechanisms underlying the soil microbe-driven soil nutrient cycles during secondary forest succession.