Tree species composition and soil properties in pure and mixed beech-conifer stands drive soil fungal communities

Soil fungi, especially the functional guilds of saprotrophs and symbiotrophs, play a central role in ecosystem processes such as decomposition and plant nutrient acquisition. Fungal communities are influenced by soil properties and management strategies such as tree species selection. Yet, the implications of the enrichment of temperate forests consisting of tree species in their native range with non-native tree species on soil fungal diversity and their functional groups are unknown. Here, we studied fungal communities in 40 plots (2500 m2 size per plot) located in two regions differing in site conditions (nutrient content, soil moisture and climate) in forests composed of European beech, spruce and Douglas-fir (non-native) and mixtures of beech with either spruce or Douglas-fir. We hypothesized that fungal community structures are driven by soil properties and tree species composition, generally resulting in higher fungal diversity in mixed than in mono-specific forests. We further hypothesized that Douglas-fir has a negative effect on ectomycorrhizal fungal species richness compared to native species, whereas saprotrophic fungal richness is unaffected. Fungal DNA barcoding and dissimilarity analyses showed significant separation of fungal communities between nutrient-rich and nutrient-poor regions and among forest types. Mycorrhizal species richness did not vary with forest type, but the relative abundance of mycorrhizal species was lower in Douglas-fir and mixed beech-Douglas-fir forests than in spruce or beech-spruce mixture forests. Conifer forests contained higher relative abundances of saprotrophic fungi than mono-specific beech forests. Among 16 abundant fungal orders in the soil, two containing saprotrophic fungi (Tremellales and Hymenochaetales) were enriched in conifer forests, regardless of site conditions and tree species mixture. The other fungal orders, including those dominated by mycorrhizal fungi (Russulales, Boletales, Atheliales and Cantharellales) showed variable patterns depending on site conditions and tree species. Our results show the impact of tree species composition and soil properties on soil fungi. The response of fungal guilds and taxa to forest types and soil properties can potentially influence nutrient fluxes.