Background and Aims <p>Arbuscular mycorrhizal (AM) fungi, crucial for nutrient cycling and plant health in grasslands, are susceptible to soil pH. Soil properties (i.e., Habitat hypothesis) and plant communities (i.e., Passenger hypothesis) are recognized as key drivers of AM fungal communities, how soil and root niches distinctly mediate their responses to soil pH remains unclear.</p> Methods <p>Through a long-term field experiment with eight sulfur addition rates in a meadow steppe, we examined how root and soil niche constraints shape AM fungal community responses to sulfur-induced soil acidification.</p> Results <p>The alpha diversity (richness and evenness) and community composition of AM fungi exhibited niche-specific responses to sulfur addition. While species richness remained relatively stable, evenness declined significantly under high sulfur levels, with root-associated AM fungal evenness showing a threshold response—indicating greater sensitivity than richness. Regression analysis revealed that root-associated AM fungal evenness was influenced by plant richness and soil nutrients, whereas soil AM fungal evenness depended solely on soil nutrients. The composition of AM fungal communities, particularly non-dominant taxa (e.g., Archaeosporaceae and Diversisporaceae in soil; Claroideoglomeraceae in roots), was highly sensitive to sulfur levels, displaying strong correlations. Structural equation modeling revealed that sulfur addition indirectly affected root-associated AM fungal diversity by altering soil properties and plant richness, supporting both the Habitat hypothesis and the Passenger hypothesis. In contrast, soil AM fungal diversity was mainly shaped by soil pH, reinforcing the Habitat hypothesis.</p> Conclusion <p>Our results highlight niche-specific responses of AM fungal diversity to soil pH, driven by niche-based processes, with contrasting alpha- and beta- diversity response patterns.</p>