The effect of slope aspect on the phylogenetic structure of arbuscular mycorrhizal fungal communities in an alpine ecosystem

Slope aspect not only modifies microclimate and soil-forming processes, but also significantly influences biogeographical patterns of species and multiple ecosystem functions across terrestrial landscapes. However, the effect of slope aspect on arbuscular mycorrhizal (AM) fungi, a group of functionally important root-associated microorganisms, remains poorly understood. In this study, we investigated AM fungal communities in an alpine ecosystem on a northwest-facing slope (NS) and a southeast-facing slope (SS) along a steep environmental gradient. Ecological processes structuring the fungal communities were inferred according to phylogenetic patterns and species abundance distributions. Thirty-two distinct AM fungal virtual taxa (VTs) were identified, predominantly belonging to the genus Glomus. The change of slope aspect from SS to NS recorded a significant decline in AM fungal richness, resulting in a marked loss of Glomus VTs and the dominance of Rhizophagus. Moreover, both taxonomic and phylogenetic composition of AM fungal communities significantly varied between the two slope aspects, this being largely driven by niche partitioning based on microclimate, soil pH and plant species composition. The influence of geographical distance was negligible. AM fungal communities were phylogenetically clustered on SS and random on NS, suggesting that the central process structuring communities shifted from a niche-dominated filter to the combination of competitive exclusion and a niche filter. Similarly, AM fungal species abundance distribution showed the best fit with the Lognormal model on SS and the Zipf-Mandelbrot model on NS, indicating that community dynamics on both slopes were also in accordance with niche-based mechanisms.