One predicted positive feedback of increasing temperatures in the boreal region is carbon (C) loss through enhanced microbial decomposition of soil organic matter (SOM). The degree to which temperature sensitivity for decomposition varies across a range of C-substrates remains uncertain. Using incubations, we tested whether microorganisms shift to more recalcitrant substrates (with longer turnover times) at higher temperatures at low or increased soil moisture. We measured the radiocarbon (Delta C-14) and stable isotope (delta C-13) signature of CO2 respired from organic soils from six black spruce forests (0 to 150 years since fire). We identified major C substrates contributing to decomposition by comparing Delta C-14 of CO2 to Delta C-14 of roots, mosses, needles, and wood separated from bulk SOM. The Delta C-14 signatures of these components allow an estimation of their turnover times, further constraining their relative contribution to respiration. Fastest turnover rates were observed for herbaceous litter and needles ( annual to < decadal), the longest (> decadal) for mosses, with intermediate turnover times for roots. Dominant microbial C sources in 5 to 40 year old stands were fire remnants and litter of early succession species, while substrates with longer turnover times accounted for a larger proportion of CO2 in mature stands. At both low and increased moisture levels, the increase in CO2 efflux at higher temperatures was accompanied by a decline in delta(CO2)-C-13, but no shift in Delta(CO2)-C-14. This suggests that temperature sensitivity is not greater for recalcitrant C and that changes in delta C-13 likely reflect temperature dependence of microbial fractionation processes rather than a substrate shift.