Recent studies have shown the significant effects of environmental selection and possible dispersal limitation on soil fungal communities. However, less is known about the role of soil depth in fungal community assemblages, especially under soil environments that are intensely cold, infertile and water-deficient. In Ngari drylands of the Asiatic Plateau, we studied fungal assemblages at two soil depths, using Illumina sequencing of the ITS2 region for fungal identification (0-15 cm as the surface soil and 15-30 cm as the subsurface soil). Fungal diversity in the surface soil was much higher than that in the subsurface soil (P<0.001), and communities differed significantly between the two layers (P=0.001). Neither soil properties nor dispersal limitation could explain variation in the surface-soil fungal community. For the subsurface, by contrast, soil, climate and space explained 27% of variation in fungal community. Collectively, these results point to high dispersal rates and absence of edaphic effects in the surface-soil fungal community assemblage in Ngari drylands. It also suggests that for soil fungi with highly effective dispersal, regional distributions may fit with Bass-Becking's paradigm that 'Everything is everywhere'.

Island biogeography theory (IBT) is one of the most fruitful paradigms in macroecology, positing positive species-area and negative species-isolation relationships for the distribution of organisms. Biotic interactions are also crucial for diversity maintenance on islands. In the context of a timberline tree species (Betula ermanii) as “virtual island”, we surveyed ectomycorrhizal (EcM) fungal diversity along a 430-m vertical gradient on the top of Changbai Mountain, China, sampling fine roots and neighboring soils of B. ermanii. Besides elevation, soil properties and plant functional traits, endophytic and saprotrophic fungal diversity were assessed as candidate predictors to construct integrative models. EcM fungal diversity decreased with increasing elevation, and exhibited positive diversity to diameter at breast height and negative diversity to distance from forest edge relationships in both roots and soils. Integrative models further showed that saprotrophic fungal diversity was the strongest predictor of EcM fungal diversity, directly enhancing EcM fungal diversity in roots and soils. Our study supports IBT as a basic framework to explain EcM fungal diversity. The diversity-begets-diversity hypothesis within the fungal kingdom is more predictive for EcM fungal diversity within the IBT framework, which reveals a tight association between saprotrophic and EcM fungal lineages in the timberline ecosystem.