Aims <p>Shrub expansion is a major strategy for vegetation restoration in desertified regions and has been shown to enhance understorey herbaceous diversity. However, the role of soil microorganisms in this process remains unclear. Investigating the role of soil microorganisms in shrub expansion-driven changes in plant diversity will therefore improve our understanding of the underlying mechanisms.</p> Methods <p>This study focused on three communities in the Ulan Buh Desert with differing degrees of shrub expansion (mixed shrub forest, pure shrub forest, and pure herbaceous communities) to investigate the effects of shrub expansion on plant and microbial communities and to quantify the role of soil microbial communities in mediating the impact of shrub expansion on plant diversity.</p> Results <p>The soil bacterial diversity in mixed shrub forests was significantly higher than that in pure herbaceous communities, with a more complex microbial network structure. Both soil bacterial and fungal diversity were significantly and positively correlated with plant diversity, mainly driven by pathotrophic, saprotrophic, ectomycorrhizal fungi, as well as anaerobic chemoheterotrophic and nitrate-reducing bacteria. Artificial shrub expansion may indirectly promote plant diversity by increasing soil bacterial diversity, accounting for 19% of the variation in plant diversity, second only to the direct promotion effect of artificial shrubs (25%).</p> Conclusions <p>This study suggests that mixed shrub forests may enhance understorey herbaceous diversity, potentially through increases in soil bacterial diversity. Accordingly, establishing mixed shrub forests during the early stages of vegetation restoration in desertified regions may benefit understorey herbaceous diversity and regional vegetation stability.</p>