Soil C stocks are sensitive to changes in temperature and it is commonly believed that even minor increases in temperature may lead to large releases of C from soils to the atmosphere. Most of this thought is based on short-term incubation data and model output that implicitly assumes soil C pools are comprised of organic matter fractions with uniform temperature sensitivities. Some recent research suggests that labile and resistant soil C fractions may be equally sensitive to temperature, but other applicable research suggests that older, more resistant C fractions may be less temperature sensitive. We conducted three independent experiments to test the hypothesis that temperature sensitivity is greater for lower- than for higher-quality organic matter: parallel incubation with a newly derived method of data interpretation, temperature rises following by differing degrees of labile soil C depletion, and characterization of 13C derived from current and past vegetation. We found that that temperature sensitivity during the early stages of incubation is low (around 2) and increases significantly as more C is lost – we found this for a large cross-site litter decomposition experiment, for previously published soil C incubation data, and for data from our new soil C incubations. We also found that the response to a temperature rise early during soil incubation (when more labile C remained) tended to be less than that later during soil incubation (when labile C was depleted) and that older (forest-derived) soil C was significantly greater than that for younger (pasture-derived) soil C. Results from independent sources and different experiments agree that resistant soil C is more temperature-sensitive than labile soil C.
