We took advantage of the northern forest near Sudbury, Ontario, Canada where logging and metal-mining pollution had degraded the landscape prior to it being restored and reforested to investigate how soil organic matter is stabilized with stand age in highly ‘eroded’ sites with minimal residual soil and in ‘stable’ sites with residual soil. Soil organic matter in sandy, acidic soils of the northern forest is thought to be stabilized in organo-mineral particles (<20 μm in size); however, forest disturbance and soil erosion might help to promote less-decomposed plant detritus being stabilized within aggregate structures (53 μm in size). We use a combination of physical fractionation (size) and chemical analysis (carbon to nitrogen ratio, natural abundance of stable isotopes of carbon and nitrogen) of the fractions. The composition of bulk soils with young trees (15-yr old) was 25% microaggregates (53 to 250 μm in size) and 47% in a smaller silt + clay fraction (<53-μm in size). Bulk soil with older trees (28- and 30- to 32-yr old) had equal proportions of macroaggregates (>250 μm in size), microaggregates, and silt + clay fraction. The C density was much greater and less decomposed for macroaggregates than for the silt + clay fraction. Untreated sites, with trees but not restored with added lime, had a smaller proportion of macroaggregates in bulk soil. Even though aggregate structures typically are not associated with sandy, acidic forest soils, the results for these Regosols in the northern forest of the Greater Sudbury region show that stabilized soil C involves formation of macroaggregates along with microaggregates, both of which help to stabilize particulate organic matter.