Tree mycorrhizal type couples soil microbial N mobilization to foliar N pools

Tree species differ in how they acquire nutrients through arbuscular mycorrhizal fungi (AMF) and ectomycorrhizal fungi (EMF) symbioses, yet how these contrasting mycorrhizal strategies regulate soil microbial nutrient mobilization and its coupling to tree nutrient status remains poorly resolved. Here, using a 10-year temperate tree diversity experiment, we quantified soil microbial community structure, phospholipid fatty acid (PLFA)-derived carbon (C) sources tracing, ecoenzymatic stoichiometry, and foliar nutrient pools to test how tree diversity and mycorrhizal type control the linkage between soil microbial nutrient acquisition and foliar nutrient pools. We found that the tree mycorrhizal type exerted a dominant control on microbial community structure. AMF-associated tree mixtures (plots dominated by AMF tree species) were characterized by saprotrophic Ascomycota, which exhibited a higher contribution of root-derived C to fungal biomass, whereas EMF-associated mixtures were dominated by symbiotrophic Basidiomycota that relied more strongly on detritus-derived C. Ecoenzymatic strategies diverged consistently with these contrasting C acquisition pathways. AMF-associated soils exhibited higher C-acquiring enzyme activity and a greater vector length (an index of microbial C acquisition investment), indicating stronger microbial investment in C acquisition from soil organic matter. In contrast, EMF-associated soils exhibited lower vector angles (indicating relative microbial investment in nitrogen versus phosphorus acquisition) and significantly higher nitrogen (N)-acquiring enzyme activity, reflecting enhanced microbial N acquisition. Across all tree mixtures, fungal community composition was tightly linked to ecoenzymatic stoichiometry, and both were significantly associated with foliar N pools. Partial least squares path modelling revealed that mycorrhizal type influenced foliar N pools primarily through indirect pathways contributed by fungal community structure and microbial N-acquisition strategy. Together, these results demonstrate that mycorrhizal type governs how soil microbes channel C from the tree into N mobilization pathways, thereby regulating the strength of belowground–aboveground N coupling. Our findings reveal a mechanism by which mycorrhizal associations, rather than tree diversity alone, shape soil microbe–tree interactions in temperate forest ecosystems.