Forest Age and Soil Depth Mediate the Effects of Soil and Root Traits on Soil Microbial Community in Plantations

The soil microbial community composition and Alpha diversity serve as key indicators of soil quality changes driven by forest development. We explored the variations in soil properties, root traits, microbial communities, and their interrelationships across forest age and soil depth in Populus tomentosa, Platycladus orientalis, and Styphnolobium japonicum plantations. The results showed that the Chao, Shannon, and Pielou_e indices of the soil microbial community increased with forest age. Soil properties and root traits had a stronger influence on the composition of soil bacteria (41.4%) compared to fungi (28.8%). In comparison to root traits (7%–10%), soil properties had a more significant influence (23%–26%) on microbial composition. Soil clay, water content, and conductivity showed positive effects on bacterial diversity and composition, while fungi were mainly affected by soil total phosphorus and soil pH. The influence of root traits on bacterial diversity declined with forest age, whereas the effect of soil properties increased. Fungal diversity was jointly shaped by soil properties and root traits in 13–19a plantations, but mainly by soil properties in 9–12a and 16–36a plantations. With increasing soil depth, the impact of roots on bacterial diversity grew while on fungal diversity diminished. The results highlight the need to account for forest age and soil depth when revealing the association among soil microbial diversity, environmental variation, and root traits.