Roots promote the formation of slow‐cycling soil carbon (C), yet we have a limited understanding of the magnitude and controls on this flux. We hypothesised arbuscular mycorrhizal (AM)‐ and ectomycorrhizal (ECM)‐associated trees would exhibit differences in root‐derived C accumulation in the soil, and that much of this C would be transferred into mineral‐associated pools. We installed δ13C‐enriched ingrowth cores across mycorrhizal gradients in six Eastern U.S. forests (n = 54 plots). Overall, root‐derived C was 54% greater in AM versus ECM‐dominated plots. This resulted in nearly twice as much root‐derived C in putatively slow‐cycling mineral‐associated pools in AM compared to ECM plots. Given that our estimates of root‐derived inputs were often equal to or greater than leaf litter inputs, our results suggest that variation in root‐derived soil C accumulation due to tree mycorrhizal dominance may be a key control of soil C dynamics in forests. Plant carbon (C) inputs to soils represent a major ecosystem C flux, with belowground inputs disproportionately important for soil C accumulation. Here we demonstrate both the impressive magnitude of root‐derived C accumulation in soils as well as how plant mycorrhizal association can drive variation in soil C dynamics through differences in belowground C supply. Across six temperate U.S. forests, we find greater root‐derived C accumulation in forest plots dominated by arbuscular mycorrhizal (AM)‐associated trees compared to plots dominated by ectomycorrhizal (ECM)‐associated trees.