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12 páginas, 5 figuras, 2 tablas. ; It is well known that microbial-mediated soil respiration, the major source of CO2 from terrestrial ecosystems, is sensitive to temperature. Here, we hypothesize that some mechanisms, such as acclimation of microbial respiration to temperature and/or regulation by plant fresh C inputs of the temperature sensitivity of decomposition of soil organic matter (SOM), should be taken into account to predict soil respiration correctly. Specifically, two hypotheses were tested: (1) under warm conditions, temperature sensitivity (Q10 ) and basal rates of microbial-mediated soil respiration (Bs20, respiration at a given temperature) would be primarily subjected to presence/absence of plant fresh C inputs; and (2) under cold conditions, where labile C depletion occurred more slowly, microbial-mediated soil respiration could adjust its optimal temperatures to colder temperatures (acclimation), resulting in a net increase of respiration rates for a given temperature (Bs20). For this purpose, intact soil cores from an oak savanna ecosystem were incubated with sufficient water supply at two contrasting temperatures (10 and 30 C) during 140 days. To study temperature sensitivity of soil respiration, short-term temperature cycles (from 5 to 40 C at 8 h steps) were applied periodically to the soils. Our results confirmed both hypotheses. Under warm conditions ANCOVA and likelihood ratio tests confirmed that both Q10 and Bs20 decreased signifi- cantly during the incubation. Further addition of glucose at the end of the incubation period increased Bs20 and Q10 to initial values. The observed decrease in temperature sensitivity (Q10) in absence of labile C disagrees with the broadly accepted fact that temperature sensitivity of the process increases as quality of the substrate decreases. Our experiment also shows that after 2 months of incubation cold-incubated soils doubled the rates of respiration at cold temperatures causing a strong increase in basal respiration rates (Bs20). This suggest that microbial community may have up-regulated their metabolism at cold conditions (cold-acclimation), which also disagrees with most observations to date. The manuscript discusses those two apparent contradictions: the decrease in temperature sensitivity in absence of labile C and the increase in microbial-mediated soil respiration rates at cold temperatures. While this is only a case study, the trends observed could open the controversy over the validity of current soil respiration models. ; This research was supported by the Kearney Soil Science Foundation and the US Department of Energy's Terrestrial Carbon Program, grant No. DE-FG03-00ER63013. These sites are members of the AmeriFlux and Fluxnet networks. J Curiel Yuste received a Marie Curie Intra-European Fellowship (EIF) from de European Union for project MICROCARB (FP6- 2005-Mobility-5 # 041409-MICROCARB) while conducting this research. ; Peer reviewed