Roots promote the formation of slow-cycling soil carbon (C), yet we have limited understanding of the magnitude and controls on this flux. We hypothesized that root-derived inputs from ectomycorrhizal (ECM)-associated trees would be greater than those from arbuscular mycorrhizal (AM)-associated trees, and that soils receiving the greatest inputs would promote greater root-derived C accumulation in mineral-associated pools. We installed δ13C-enriched ingrowth cores across mycorrhizal gradients in six Eastern U.S. forests (n = 54 plots). Counter to our hypothesis, root-derived C was 54% greater in AM versus ECM-dominated plots, resulting in 175% more root-derived C in mineral-associated, slow-cycling pools in AM compared to ECM plots. Notably, root-derived soil C was comparable in magnitude to leaf litter inputs and aboveground net primary production. Our results suggest that variation in root-derived C inputs due to tree mycorrhizal dominance may be a key control of soil C dynamics in forests.