Dynamic microbial community composition, co-occurrence pattern and assembly in rhizosphere and bulk soils along a coniferous plantation chronosequence

During plantation development, soil microbial communities in different soil compartments (e.g., rhizosphere and bulk soils) is critical for the establishment of plant diversity and multiple ecosystem functions. The assembly process shaped microbial communities. However, the assembly mechanisms of soil microbial communities have rarely been studied. Therefore, we investigated the bacterial and fungal community compositions, species cooccurrence, and assembly processes in rhizosphere and bulk soils along a Pinus tabulaeformis plantation chronosequence (15, 30 and 60 years old). Results showed that plantation development altered the soil properties and microbial community structures in the rhizosphere and bulk soils. The effect of stand age on the soil properties and microbial community structures was stronger than the effect of the soil compartment. Higher turnover rates of soil microbial communities were observed in rhizosphere soil than in bulk soil. Dispersal limitation governed the bacterial and fungal community assembly throughout plantation chronosequence, while the fungal community was more susceptible to dispersal limitation. The bacterial and fungal keystone species compositions in the rhizosphere had significant positive correlations with the soil total phosphorus and nitrite nitrogen and total nitrogen and total phosphorus, respectively, indicating their importance in soil nitrogen and phosphorus cycling. The complexity of bacterial networks increased along the chronosequence, whereas fungal network complexity did not show a clear age-related trend but higher in the rhizosphere than in bulk soils. These results indicated that during Pinus tabulaeformis plantation development, soil microbial assembly was less environmentally constrained due to an increase in resource availability.