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 changes in tropical temperature over geologic time and the significant responses of tropical ecosystems to shorter-term variations such as El Nino/La Nina argue for a robust understanding of the temperature sensitivity of tropical decomposition. To examine the responsiveness of heterotrophic respiration to temperature, we measured rates of heterotrophic respiration from a wide range of tropical soils in a series ol laboratory incubations. Under conditions of optimal soil water and nonlimiting substrate availability, heterotrophic respiration rose exponentially with rising temperature. The mean Q(10) measured across all temperature ranges in these short-term incubations was 2.37, but there was significant variation in eros across sites. The source of this variation could not be explained by soil carbon or nitrogen content, soil texture, site climate, or lignin to nitrogen ratio. At the beginning 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 response declined above 35 degreesC. The documented changes in temperature sensitivity with substrate availability argue for using temperature relationships developed under optimal conditions of substrate availability for models which include temperature regulation of heterotrophic respiration. To evaluate the significance of this natural variation in temperature control over decomposition, we used the Century ecosystem model gridded for the areas between the tropics of Cancer and Capricorn. These simulations used the mean and upper and lower confidence limits of the normalized exponential temperature 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.
Biospheric trace gas fluxes and their control over tropospheric chemistry
Abstract Terrestrial and marine ecosystems function as sources and sinks for reactive trace gases, and in doing so, profoundly influence the oxidative photochemistry in the troposphere. Principal biogenic processes include microbial methane production and oxidation, the emission of volatile organic compounds from forest ecosystems, the emission of nitric oxide from soils, the emission of reactive sulfur compounds and carbon monoxide from marine ecosystems, control over the production of hydroxyl radical concentration by regional hydrologic processes, and deposition of ozone and nitrogen oxides to ecosystems. The combined influence of these processes is to affect the tropospheric concentrations of ozone, hydroxyl radicals, reactive nitrogen oxides, carbon monoxide, and inorganic acids, all of which constitute fundamental components of oxidative photochemistry. In this review we discuss the recent literature related to the primary controls over the biosphere-atmosphere exchange of reactive trace gases, and also to efforts to model the dominant biospheric influences on oxidative dynamics of the troposphere. These studies provide strong support for the paradigm that biospheric processes exert the dominant control over oxidative chemistry in the lower atmosphere. Improvements in our ability to model biospheric influences on tropospheric chemistry, and its susceptibility to global change, will come from inclusion of more explicit information on the processes that control the emission and uptake of reactive trace gases and the impact of changes in ecosystem cover and land-use change.
Simulation of carbon and nitrogen cycling in an Alpine tundra
Simulations of an alpine tundra ecosystem using the CENTURY ecosystem model were conducted to test model descriptions of carbon and nitrogen cycling and to explore the alpine ecosystem response to physical and chemical components of global change. The parameterization of the alpine tundra for CENTURY was updated to reflect current knowledge of the site, and sensitivity analyses were conducted. Verification of results from a 6-yr fertilization experiment in the alpine tested the predictive capabilities of the parameterization. Simulations with increased winter precipitation and with the climate predicted under doubled atmospheric carbon dioxide concentrations were then conducted.
Modifications to the parameterization necessary to describe carbon and nitrogen cycling included decreasing the C:N ratios of plant tissues, increasing the amount of nitrogen retranslocated at the end of the growing season, extending the length of the growing season, and lowering the rate of decomposition. The updated parameterization requires 30% greater than observed inputs of net primary productivity to simulate observed levels of total soil carbon suggesting that soil carbon sequestration is not well represented in the model. Carbon and nitrogen cycling showed greatest sensitivity to the length of the growing season and to the temperature regulation of decomposition. Simulation of the nitrogen fertilization experiment resulted in 11% greater productivity than observed empirically, a reasonable verification of the updated parameterization. The major impact from increasing winter precipitation was a 30% increase in the amount of nitrogen in stream flow. Simulation with the climate predicted for a doubling of current carbon dioxide levels reduced production 10% while total soil carbon remained constant. This response was largely controlled by reduced soil moisture during the growing season.