Soil functioning is intrinsically related to the structure of associated biological communities. This link is barely understood in the multispecies context of soil microbial communities, which often requires complex analytical approaches to discern structural and functional roles of microbial taxa inhabiting the soil. To investigate these ecological properties, we characterized the assembly and soil functioning contribution of ectomycorrhizal (ECM) fungal communities through co-occurrence network analysis. Co-occurrence networks were inferred from ECM root tips of Cistus albidus, Quercus faginea and Q. ilex on a regional scale, in Mediterranean mixed forests. Soil enzymatic activities related to carbon and nutrient cycling were also measured, and soil functionality outcomes related to ECM fungal network structure were evaluated on the community to taxon levels. Network complexity relied on habitat characteristics and seasonality, and it was linked to different dominant ECM fungal lineages across habitats. Soil enzymatic activities were habitat-dependent, driven by host plant identity and fungi with reduced structuring roles in the co-occurrence network (mainly within Thelephorales, Sebacinales and Pezizales). ECM fungal co-occurrence network structure and functioning were highly context-dependent, pointing to divergent regional fungal species pools according to their niche preferences. As increased network complexity was not related to greater soil functionality, functional redundancy might be operating in Mediterranean forest soils. The revealed differentiation between structural and functional roles of ECM fungi adds new insights into the understanding of soil fungal community assembly and its functionality in ecosystems.
