Heterogeneity of soil organic carbon (SOC) has been used to explain variation in the temperature sensitivity of SOC fractions. However, controversy persists on how stable SOC will respond to changes in temperature. We conducted a series of long and short-term incubation experiments on soils with different initial SOC quality collected from an extended mean annual temperature (MAT) gradient in North America.  Our goal was to investigate the temperature responses of stable and labile soil C. By addition of labile substrate, we investigated how inputs of new labile C affect the temperature sensitivity of soils with different initial SOC quality. Soils depleted in labile C were obtained by incubating samples at a lab incubation temperature (LIT) of 30ºC for 300 days. Analysis of CO₂-C flux shows C loss to range between 7% and 18% across sites (mean = 13%, SE = 0.9, n= 10). LIT was set at 30ºC with short cycles of 25ºC and 35ºC. Decomposition in old soils depleted of labile C was significantly less sensitive to temperature than in new soils not depleted of labile C (Q₁₀ 1.25 ± 0.08, n = 10 versus Q₁₀ 2.69 ± 0.38, n =10, p<0.005) for LIT changes from 25ºC to 30ºC. Similar responses were observed between 30ºC and 35ºC (Q₁₀ 1.50 ± 0.12, n = 10 versus Q₁₀ 2.68 ± 0.31, n = 10, p< 0.01). Our results suggest that stable SOC fractions are less sensitive to temperature than labile C fractions and that availability of labile C determines decomposition rates. Research is needed on whether inputs of labile substrate enhance the decomposition of stable soil C.