Worldwide tree diversity loss raises concerns about functional and energetic declines across trophic levels. In this study, we coupled 160 above- and belowground food webs, quantifying energy fluxes to microorganisms and invertebrates in a tree-mycorrhiza diversity experiment, to test how tree diversity affects fluxes of energy above and below the ground. The experiment differentiates three mycorrhizal type treatments: only AM tree species (with arbuscular mycorrhizae), only EcM tree species (with ectomycorrhizae; one, two, and four tree species), or mixtures of both AM and EcM tree species (AM+EcM; two and four tree species). Our results indicate that most energy initially flowed through belowground communities, with soil microorganisms contributing 97.7% of total energy and belowground fauna accounting for 60.9% of energy to animals. Consequently, belowground fauna fueled surface (62.3% of predation) and aboveground (30.5% of predation) predators. Tree diversity increased ecosystem multifunctionality (indicated by total and averaged energy fluxes) by ∼30% and energy across most trophic levels in EcM tree communities, while it shifted food webs from fast (such as bacterial-dominated) to slow (such as fungal-dominated) channels in AM tree communities. Tree diversity primarily impacted energy fluxes through belowground communities and strengthened the coupling of above- and belowground food webs, with increasing importance of belowground prey for predators at the soil surface and above the ground. These findings highlight that tree diversity and mycorrhizal types drive above- and belowground ecosystem functioning via belowground energy fluxes.