Composition and Assembly Mechanisms of Ectomycorrhizal Fungal Communities Associated with Ammopiptanthus mongolicus Under the Dual Drivers of Geographic Space and Host Decline

Abstract

Ammopiptanthus mongolicus relies on symbiotic ectomycorrhizal fungi (EMF) to adapt to extreme environmental conditions. However, the impacts of geographical spatial heterogeneity and host decline on the community composition and assembly mechanisms of EMF remain poorly understood. In this study, A. mongolicus individuals from different habitats and decline grades were selected as research materials. Multiple approaches, including soil physicochemical property determination, rhizosphere DNA extraction, high-throughput sequencing and multivariate statistical analysis, were integrated to explore fungal community composition, key driving factors of community differentiation, and ecological assembly processes. Fungal species richness and evenness did not differ significantly across habitats or host decline gradients (p > 0.05), whereas community composition and structure varied markedly. Tricholoma dominated spatial sites, with community variation primarily associated with altitude, soil water content and longitude; Tomentella prevailed at declining host sites, where community variation was mainly related to available potassium and local microhabitat. Mechanistic analysis indicated that there was no significant linear correlation in the abundance–occupancy relationship across the two environmental dimensions (habitat type: R = −0.22, p = 0.48; decline gradient: R = −0.025, p = 0.94). The neutral model displayed higher explanatory power for fungal communities along the host decline gradient (R2 = 0.51) than for spatial sampling groups (R2 = 0.204), suggesting that neutral processes may contribute substantially to community assembly. βNTI and RCbray analyses collectively suggested that community assembly may be primarily governed by stochastic processes, with homogenizing dispersal potentially representing a key ecological process; this pattern appeared to be highly consistent across spatial and host decline dimensions, and was consistent with the neutral model. It should be noted that these approaches provide indirect lines of evidence and cannot directly confirm fungal dispersal events. In addition, rarefaction curves did not reach complete saturation, indicating that some rare taxa may not have been fully captured, and the completeness of community characterization should therefore be interpreted with caution. These findings provide insights into the differentiation patterns and assembly mechanisms of ECM fungal communities under spatial heterogeneity and host decline, and may offer support for A. mongolicus conservation and desert ecosystem stability.