Nearly 30a shrub introduction in desert steppes has led to an increase in saprotrophic fungi, accelerating the degradation of carbon compounds and nitrate reduction.

Clarifying how soil microbial communities respond to shrub introduction after overgrazing in desert steppe and their potential functions is crucial for understanding the biogeochemical processes involved in vegetation transformation and sustainability of desert grasslands. However, the dynamics of microbial communities remain poorly understood. We selected enclosed grasslands (20a), overgrazed grasslands, and shrublands (6a, 15a, and 25a) to explore how shrubs introduced influence soil microbial structure and functional groups over the long term after desert grassland degradation. The results showed that overgrazing and shrub introduction (Caragana korshinskii) had more significant impacts on microbial β diversity than on α diversity. Fungal communities were more sensitive to grassland degradation. In contrast, introducing shrubs affected both fungal and bacterial communities, and an increase in shrub age drove the synergistic effects of fungal species. Soil nitrate and microbial biomass nitrogen were the key factors affecting bacterial and fungal communities. Compared with enclosed grasslands, overgrazing and introducing shrubs significantly increased soil nitrate reduction, ectomycorrhizals, and endophytes. The introduction of shrubs after overgrazing in desert steppe further enhanced the decomposition of carbon compounds and reduced processes such as denitrification. During shrub introduction, total phosphorus, nitrate–nitrogen, and N-acetylglucosaminidase were key factors affecting carbon, nitrogen, and fungal functional groups. Variations in microbial diversity and functional groups, through their influence on extracellular enzymes activity and the availability of microbial biomass nutrients, ultimately explained 85%, 42%, and 34% the observed variations in soil carbon, nitrogen, and phosphorus content, respectively. This study aimed to investigate the long-term effects of anthropogenic shrub expansion on the structure and function of soil microbial communities in degraded desert steppes, provide a scientific basis for steppe restoration and management, and further understand their significance for ecosystem sustainability. • Shrub introduction has significantly altered fungal and bacterial communities. • Shrub age drive fungal community structure towards more synergistic directions. • The function of fungi is shifting from pathotroph to symbiotroph and saprotroph. • Shrubs enhances the degradation carbon compounds and improves nitrate reduction. • Soil nitrogen and extracellular enzymes primarily shape microbial community. [ABSTRACT FROM AUTHOR]