Soil fungi are crucial microorganisms in the functioning of ecosystems. They shape the soil properties, facilitate nutrient circulation, and assist with plant growth. However, their biogeography and distribution studies are limited compared to other groups of organisms. This review aims to provide an overview of the main factors shaping the spatial distribution of soil fungi (with a special focus on mycorrhizal fungi). The review also tries to identify the field frontier where further studies are needed. The main drivers of soil fungal distribution were classified and reviewed into three groups: soil properties, plant interactions, and dispersal vectors. It was apparent that ectomycorrhizal and arbuscular fungi are relatively overrepresented in the body of research, while the other mycorrhiza types and endophytes were grossly omitted. Notwithstanding, soil pH and the share of ectomycorrhizal plants in the plant coverage were repeatedly reported as strong predictors of mycorrhizal fungal distribution. Dispersal potential and vector preferences show more variation among fungi, especially when considering long-distance dispersal. Additionally, special attention was given to the applications of the island biogeography theory to soil fungal assemblages. This theory proves to be a very efficient framework for analyzing and understanding not only the soil fungal communities of real islands but even more effective islands, i.e., isolated habitats, such as patches of trees discontinuous from more enormous forests.

Mycorrhiza is a mutualistic symbiosis between some plant species and a diverse group of soil fungi. Mycorrhizal symbiosis is categorized into discrete types based on the structural differences and the taxonomic position of involved plants and fungi. The main types of mycorrhiza include arbuscular, ericoid, orchid, monotropoid and ectomycorrhiza, the latter being the main focus of this review. Identifying ectomycorrhizal fungi (EMF) is crucial for understanding the ecology and preservation of rare and endangered plants, fungi and habitats. Thus, since the discovery of ectomycorrhiza, new identification methods have been constantly developed. This review tracks the history of EMF identification methods, presents their limitations and aims to discern the frontier of the field. The early methods were based on the morphology and anatomy of EMF, which allowed ectomycorrhizae formed by different EMF to be distinguished. Recognizing the EMF species that formed them, however, was difficult. The introduction of molecular methods that analysed taxon-specific DNA barcodes was a breakthrough in EMF identification, and the internal transcribed spacer gene was adopted as a routine fungal barcoding sequence. Identifying EMF from randomly sampled roots by morphology or polymerase chain reaction (PCR)-based methods results in rare EMF taxa being missed and underreported. This is addressed by bulk sampling combined with environmental DNA metabarcoding methods, which, in turn, yield a lower resolution of taxonomic identification. Most EMF identification methods incorporate PCR reactions, which are biased against some groups of EMF; developing PCR-free molecular methods would constitute a significant advancement. As the traditional concept of species is challenging for fungi, studying EMF functional rather than taxonomic diversity appears to be promising.