Postfire changes in the local energy balance and soil chemistry may significantly alter rates of carbon turnover in organic-rich soils of boreal forests. This study combines field measurements of soil carbon uptake and emission along a 140-year chronosequence of burned black spruce stands to evaluate the timescales over which these disturbance effects operate. Soil CO2 efflux increased as a function of stand age at a mean rate of 0.12 Mg C ha-1 yr-2 up to a maximum of 2.2 Mg C ha-1 yr-1 in the 140-year-old stand. During this same time period, organic soil horizons sequestered carbon and nitrogen at rates of 0.28--0.54 and 0.0076 Mg N ha-1 yr-1, respectively. A mass balance model based on field measurements suggests that postfire changes in root and microbial respiration caused these soils to function as a net source of carbon for 7--15 years after fire, releasing between 1.8 and 11.0 Mg C ha-1 to the atmosphere (12.4--12.6% of total soil organic matter). These estimates are on the same order of magnitude as carbon losses during combustion and suggest that current models may underestimate the effect of fire on carbon emissions by a factor of 2.