Managers increasingly seek to increase forest soil carbon but long-term controls on soil organic matter (SOM) sources and stability are weakly understood. We used a 30-year detrital input/removal treatment experiment in a deciduous forest to evaluate the importance of root and leaf litter to SOM. Inputs were assessed by excluding roots and leaves (litter) or by doubling litter inputs. In mineral soil, %SOM differed only at 0–10 cm (p = 0.11), with concentrations in the no litter and no roots treatments of 26% and 9%, respectively, lower than the controls. Cessation of litter inputs had a stronger effect on SOM than cessation of root inputs, but root litter inputs may contribute stable SOM. Doubled litter increased mineral SOM.