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Understanding soil organic carbon (SOC) mineralization under different temperature regimes is critical for predicting SOC responses to climate change. Yet, the effects of altering temperature regimes on SOC mineralization remain poorly understood in forest plantations converted from natural forests. Forest conversion is extensive and could have major impact on SOC dynamics, so that this knowledge limits our ability of predicting the consequences of such land use change on carbon cycling. To fill this knowledge gap, we conducted a 360-day incubation experiment under constant and varying temperature regimes for soils of a natural forest and a Chinese fir (Cunninghamia lanceolata) plantation. Results showed that SOC mineralization was greater in the forest plantation soil than in the natural forest soil in both temperature treatments, possibly due to greater labile SOC in the forest plantation soil by 27-28%. The results suggested that replacing natural forests with forest plantations may increase CO2 emission via the mineralization of SOC. In the natural forest soil, SOC mineralization was greater in the varying temperature treatment relative to the constant temperature treatment but no difference was found in the forest plantation soil. Moreover, temperature sensitivity (Q(10)) of SOC mineralization was greater in the natural forest soil than the Chinese fir soil for the 0-180 day of the incubation. The difference in the response to the two temperature treatments between the two forest soils which was accompanied by difference in soil microbial communities. It was likely that soil microbes of the closed-canopy natural forest were less adapted to temperature fluctuations than soil microbes of the forest plantation soil as the canopy was rarely closed. Our results highlight that soil incubation experiments need to take temperature fluctuations into consideration to more accurately reflect SOC dynamics in the field, especially when evaluating the impacts of replacing natural forests with forest plantations on soil carbon dynamics.