Mycorrhizal fungi serve as fundamental agents in forest establishment and progression, underpinning critical ecosystem functions through symbiotic root associations. Drylands, which cover nearly half of Earth's land, have limited forest establishment, and factors influencing mycorrhization in these stressful environments remain unclear. Here, we integrate large-scale field surveys along aridity gradients with greenhouse experiments and over 33,000 microscopic mycorrhizal observations, revealing that aridity significantly enhances mycorrhization. Mycorrhizal fungi undergo niche modification, whereby facilitative microbial interactions promote mycorrhization under aridity stress. We identify a core synthetic microbial community linked to mycorrhization and provide mechanistic evidence that this community facilitates mycorrhization through physical attachment to fungal hyphae and by alleviating soil metabolite inhibition that otherwise suppresses mycorrhization under arid conditions. In this work, our findings highlight the role of microbial interkingdom interactions in driving tree mycorrhizal colonization in arid regions, offering critical insights for guiding tree planting and restoration efforts in drylands.