Root-associated and soil-dwelling fungi in alpine meadows show differential shifts in community assembly mechanisms and interspecific interactions along an elevation gradient.

Soil fungi, as key players in maintaining ecological functioning and stability, have been widely studied in alpine ecosystems. However, prior studies focused mainly on their spatial patterns and temporal dynamics, as well as their driving factors. In-depth research on community assembly mechanisms, particularly how biotic interactions influence this, is lacking. In this study, we collected root samples of an ectomycorrhizal plant, Bistorta macrophylla, and bulk soil around them, along a 4300-4750 m gradient in alpine meadows of Baima Snow Mountain, northwestern Yunnan, China, and obtained ITS2 sequences using high-throughput sequencing, which were subjected to bioinformatic processing and statistical analyses, including differential abundance analyses, inference of community assembly mechanisms and interpretation of co-occurrence networks. Our results reveal that fungal community assembly in soil is influenced more by stochastic processes with the increase of elevation, but homogeneous selection consistently acts as the predominant process in shaping root-associated communities. This helps keep a stable core mycobiota dominated by Cenococcum and Phialocephala, both being melanized fungi, in the root systems of B. macrophylla. Nevertheless, members of the order Helotiales and certain EcM genera consistently act as key nodes in fungal co-occurring networks in both soil and root samples. Further, we find that the elevational change trend of positive correlations between ectomycorrhizal and saprotrophic fungi matches with the theoretical expectation by the stress gradient hypothesis. Our results emphasize the pivotal role of compartment filtering by plant roots in selecting symbiotic partners and shaping fungal correlation networks, and highlight that the stress gradient hypothesis could be applicable in harsh alpine environments.