Soil microorganisms play an important role in the functional connection between above- and below-ground ecosystems during vegetation restoration. However, the influencing patterns of plants and soils on the microbial properties in the process remain unclear, especially in ecologically fragile zones. Here, we investigated soil bacterial and fungal communities at twelve Pinus tabulaeformis stands on Ziwuling Mountain in Loess Hilly Region, along an age gradient from 15- to 45-year-old stands, with respect to abundance and diversity using Illumina sequencing of the 16S rRNA gene and ITS gene, respectively. Soil microbial biomass, enzyme activity, vegetation characteristics (i.e., coverage, fine root biomass, and the composition and diversity) and soil properties (i.e., bulk density, organic C, total N, and available N) were also determined. The results showed that the microbial biomass increased significantly along the restoration chronosequence, being consistently the highest at the 45-y site. However, the saccharase, urease, catalase, and alkaline phosphatase that regulate soil organic matter and nutrient transformation were less impacted by the restoration time. Bacterial alpha diversity gradually increased with time whereas fungal alpha diversity was highest at the 30-y site, which was related to increasing levels of NO3-, coverage, and fine root biomass. The response of soil bacterial beta diversity to the age gradients was larger than that of fungal beta diversity, although both groups were significantly affected by vegetation restoration. Moreover, bacterial communities were dominated by oligotrophic Actinobacteria at 15-y and 25-y sites and transformed into copiotrophic Proteobacteria-dominant at 30-y and 45-y sites. However, no significant variation in the fungal dominant phyla (i.e., Ascomycota, Basidiomycota, and Zygomycota) were detected with restoration. Furthermore, the dominant species [Pinus tabulaeformis (tree) and Carex tristachya (herb)] affected the microbial biomass, diversity, and abundance of bacterial dominant phyla significantly, but affected the fungal community composition much less. Vegetation diversity has no significant effect on microbial diversities (alpha and beta diversity). Soil physicochemical factors could widely explain the variations in microbial diversity and community composition, enzyme activity, and microbial biomass in restoration processes, especially bulk density and NO3-. Overall, our results demonstrate that soil bacteria rather than fungi can reflect the changes in plant and soil properties with increasing stand age, and the shifts in the predominant bacterial and fungal characteristics along the age gradient were mainly the result of differences in soil physicochemical factors prevailing at each restoration stage.
