Uncertainties in the temperature sensitivity of decomposition in tropical and subtropical ecosystems: Implications for models

Tropical ecosystems play a central role in the global carbon cycle. Large c hanges in tropical temperature over geologic time and the significant respo nses of tropical ecosystems to shorter-term variations such as El Nino/La N ina argue for a robust understanding of the temperature sensitivity of trop ical decomposition. To examine the responsiveness of heterotrophic respirat ion to temperature, we measured rates of heterotrophic respiration from a w ide range of tropical soils in a series ol laboratory incubations. Under co nditions of optimal soil water and nonlimiting substrate availability, hete rotrophic respiration rose exponentially with rising temperature. The mean Q(10) measured across all temperature ranges in these short-term incubation s was 2.37, but there was significant variation in eros across sites. The s ource of this variation could not be explained by soil carbon or nitrogen c ontent, soil texture, site climate, or lignin to nitrogen ratio. At the beg inning of the incubation, heterotrophic respiration increased exponentially with temperature for all sites, despite the fact that the fluxes differed by an order of magnitude. When substrate availability became limiting later in the incubation, the temperature response changed, and heterotrophic res ponse declined above 35 degreesC. The documented changes in temperature sen sitivity with substrate availability argue for using temperature relationsh ips developed under optimal conditions of substrate availability for models which include temperature regulation of heterotrophic respiration. To eval uate the significance of this natural variation in temperature control over decomposition, we used the Century ecosystem model gridded for the areas b etween the tropics of Cancer and Capricorn. These simulations used the mean and upper and lower confidence limits of the normalized exponential temper ature response of our experimental studies. We found that systems with the lowest temperature sensitivity accumulated a total of 70 Pg more carbon in soil organic carbon and respired 5.5 Pg yr(-1) less carbon compared to the systems with the highest sensitivity.