Radiocarbon signatures Delta 14C of carbon dioxide CO2 provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2-year period, we measured Delta 14C of soil respiration and soil CO2 in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand-replacing fire. Comparing bulk respiration Delta 14C with Delta 14C of CO2 evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Delta 14C of respired CO2 indicated marked variation in respiration sources in space and time. The 14C signature of respired CO2 respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had 14C greater (averaging ~120[per thousand]) than autotrophic respiration. The Delta 14C of autotrophically respired CO2 in young stands equaled those expected for recent photosynthetic products (70[per thousand] in 2003, 64[per thousand] in 2004). CO2 respired by black spruce roots in stands >40 years old had Delta 14C up to 30[per thousand] higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants. Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ~50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO2 had Delta 14C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Delta 14C of soil respiration in younger successional stands dropped below those of the atmospheric CO2.