Arbuscular Mycorrhizal Fungi Orchestrate Soil Microbial Community Assembly Along a <i>Salix cupularis</i> Restoration Chronosequence in a Desertified Alpine Grassland

Belowground microbes are emerging targets for ecosystem restoration. Understanding the assembly mechanisms of these microbial communities is critical for predicting ecosystem trajectories and optimizing restoration interventions. Arbuscular mycorrhizal fungi (AMF) are hypothesized to be key drivers of these eco-evolutionary dynamics as a crucial and unique functional group associating with approximately 80% of terrestrial plant species. However, relatively little empirical information is available on the role of AMF in the soil microbial community assembly. Here, we used <i>Salix cupularis</i>, a native pioneer shrub species of desertified alpine meadows, to investigate the temporal dynamics of soil rhizosphere microbial communities across a restoration chronosequence (5, 10, and 20 years), with a particular focus on the AMF community. The results showed that minimal changes occurred in bacterial community structure, whereas fungal community exhibited more pronounced shifts along the chronosequence. Bacterial community assembly was initially deterministic and then became stochastic, while fungal assembly was consistently stochastic. Shrub planting enhanced the complexity of both bacterial and fungal networks over time. Co-occurrence networks and Pearson correlation analysis revealed the "time-dependent" regulatory role of the AMF community in soil microbial assembly. AMF acted as an orchestrator in the 10th year after planting (the edge density of AMF peaking at 15.0) prior to the transition to a stable, ECM-dominated state in response to shifts in soil nutrient availability, particularly significant increases in MAOC and AP, as well as a decrease in DON. Our findings indicate that fungal communities exhibit higher sensitivity and highlight the dynamic regulatory function of AMF, especially under dual-mycorrhizal symbiosis. These results provide novel mechanistic insights into soil microbe trajectories, suggesting that targeted AMF inoculation is crucial for the early-to-mid establishment phase of restoring desertified alpine meadows.