AimsBelowground fungal communities mediate plant-soil feedbacks, yet their spatial distribution and assembly in monoculture afforestation remain unclear. This study explores how host selection and environmental filtering shape fungal community dynamics across introduction sites and niches.MethodsThis study investigated Mongolian pine plantations across an aridity gradient, from the Mu Us to the Hulunbuir and the Horqin Desert, to examine the assembly patterns, stability, and community composition of root-associated (RAF), rhizosphere soil (RSF), and bulk soil fungi (BSF).ResultsCommunity assembly was shaped by stronger homogeneous selection in RAF (18.07%) than in RSF and BSF, whereas dispersal limitation dominated in RSF (62.86%). RAF was dominated by symbiotrophs (e.g., Acephala and Geopora). RSF harbored abundant ectomycorrhizal fungi (e.g., Inocybe and Laccaria) and saprotrophs, while BSF contained more saprotrophs strongly correlated with soil nutrients. RSF and BSF showed higher diversity and network complexity than RAF. Community stability varied across introduction sites. In the least arid and nutrient-rich Horqin Desert, increased competition in RAF enhanced stability, whereas fungal networks in the Hulunbuir Desert remained relatively stable. In the most arid and nutrient-poor Mu Us Desert, low clustering and strong positive associations in RAF and BSF further reduced stability.ConclusionsNiche differentiation critically shapes fungal structure, diversity, and function, while habitat conditions further modulate fungal stability and taxonomic composition. The growth and health of plantations linked to the balance of fungal functional groups and the stability of belowground fungal communities.