Root causes of below‐ground processes: Tree mycorrhizal associations outweigh tree richness

Abstract Despite decades of research, the associations between plants and their mycorrhizal fungi remain poorly understood. In ecosystems with a mix of arbuscular mycorrhizal (AM) and ectomycorrhizal (EM)‐associated trees, tree species richness and tree mycorrhizal association may shape mycorrhizal fungal communities and below‐ground processes. Utilizing a long‐term biodiversity–ecosystem function experiment at the Smithsonian Environmental Research Center (SERC), we investigated how tree species richness (1, 4, 12 species) and tree mycorrhizal association (AM, EM, mixed) influence the diversity and composition of the soil microbiome. We further assessed how above‐ and below‐ground taxonomic and functional diversity influence soil characteristics. We hypothesized that soil microbial communities would be more strongly driven by tree mycorrhizal association than tree species richness and that both tree mycorrhizal association and fungal diversity would affect soil characteristics. Tree mycorrhizal type influenced EM fungal richness, diversity and turnover more strongly than tree species richness, with tree mycorrhizal association effects on turnover 2.4 times greater. Conversely, tree species richness led to a significant increase in AM hyphal length, while AM fungal relative abundance was linked to mycorrhizal association. Neither tree species richness nor tree mycorrhizal association directly impacted soil characteristics. Instead, shifts in fungal diversity and composition affected soil characteristics. 3. AM fungal richness was positively associated with acid phosphatase (AP) extracellular enzyme activity (EEA), whereas AM fungal hyphal lengths were positively associated with N ‐acetyl‐glucosaminidase (NAG) EEA. The AP EEA was negatively correlated with inorganic phosphate (PO 4 3− ) concentrations in soils whereas NAG EEA was positively correlated with inorganic nitrate (NO 3 − ). 4. Shifts in the composition of saprotrophic and pathogenic fungi correlated with soil organic carbon (SOC) stocks, suggesting SOC decomposition rate, rather than accumulation, drove carbon storage in these plots. Synthesis . By jointly manipulating tree species richness and tree mycorrhizal association, we demonstrated that mycorrhizal associations drive EM fungal diversity and composition and AM fungal abundance while indirectly influencing soil characteristics via below‐ground microbiomes. As global change alters plant and fungal diversity and guild structure, understanding the universality of these above‐ and below‐ground linkages will be critical for predicting ecosystem‐level responses.