Plant roots and their associated mycorrhizal fungi critically mediate decomposition of soil organic carbon (C), but the general patterns of their impacts over a broad geographical range and the primary mediating factors remain unclear. Based on a synthesis of 596 paired observations from both field and greenhouse experiments, we found that living roots and/or mycorrhizal fungi increased organic C decomposition by 30.9%, but low soil N availability (i.e., high soil C:N ratio) critically mitigated this promotion effect. Also, the positive effect of living roots and/or mycorrhizal fungi on organic C decomposition was higher under herbaceous and leguminous plants than under woody and non-leguminous plants, respectively. Surprisingly, there was no significant difference between arbuscular mycorrhizal fungi and ectomycorrhizal fungi in their effects on organic C decomposition. Further, roots and/or mycorrhizal fungi significantly enhanced decomposition of leaf litter, but not root litter. Together, these results advance our understanding of how roots and their symbiotic fungi modulate soil C dynamics in the rhizosphere or mycorrhizosphere and may help improve predictions of soil global C balance under a changing climate.