Northern forest soils represent globally important stores of carbon (C), yet there is no consensus about how they are altered by the widespread practice of harvesting that dominates many forested landscapes. Here we present the first study to systematically investigate the utility of δ ¹³C and C content depth profiles to infer temporal changes in belowground carbon cycling processes following disturbance in a pure C₃ ecosystem. We document carbon concentration and δ ¹³C depth profile enrichment trends consistent with a kinetic fractionation arising from soil organic carbon (SOC) humification across a northern forest chronosequence (1, 15, 45, 80 and 125+ yrs). Reduced soil C storage that coincided with observed soil profile δ ¹³C-enrichment patterns which intensified following clearcut harvesting, pointed to losses of SOC in the deeper (>20 cm) mineral soil. This study suggests the δ ¹³C approach may assist in identifying mechanisms responsible for soil C storage changes in disturbed C₃ forest ecosystems.