Mineral type versus environmental filters: What shapes the composition and functions of fungal communities in the mineralosphere of forest soils?

Mineral surfaces in soil, where fungal communities contribute to the formation and turnover of mineral-associated OM (MAOM), are important interfaces for organic matter (OM) and nutrient cycling. The significance and contribution of secondary minerals such as goethite (iron oxide) or illite (clay mineral) to fungal community structure and functioning under natural conditions in forest soils remains elusive. We placed mineral containers filled with mixtures of secondary minerals (either goethite or illite) and quartz-sand in 30 forest topsoils across three regions in Germany for five years. The mineral samples in the containers were separated from the surrounding soil with 50-mu m mesh barriers. Mineral and surrounding soil samples were analyzed for abundance and community composition of saprotrophic and ectomycorrhizal fungi based on phospholipid fatty acid profiles and amplicon sequencing, and for enzyme activities (beta-glucosidase, beta-xylosidase, N-acetylglucosaminidase, and acid phosphatase). Compared to the surrounding soil, fungal communities in the carbon-poor mineral samples were less diverse and communities were distinct. The community composition of saprotrophic fungi was affected by mineral type, likely reflecting variability in the minerals' capacities to bind OM. Higher relative microbial enzymatic nutrient acquisition was associated with a shift from saprotrophic towards ectomycorrhizal fungi, indicating a tight link between trophic fungal groups and their distinct functional roles in the mineralosphere. At a larger scale, we observed strong site effects, suggesting that environmental filters such as physico-chemical soil properties, tree species, and region shape the structuring of fungal communities more strongly than the specific mineralosphere conditions.