Buchmann, N., J.R. Brooks, L.B. Flanagan, and J.R. Ehleringer. 1998. Carbon isotope discrimination of terrestrial ecosystems, p. 203-221. In H. Griffiths (ed.), Stable Isotopes Integration of Biological, Ecological, and Geochemical Processes. BIOS Scientific Publ., Oxford.

Carbon dynamics of terrestrial ecosystems play an important role for the global carbon budget. The need for detailed information about interactions between atmosphere, biosphere and pedosphere became even more crucial since carbon dioxide concentrations ([CO2]) in the atmosphere have been steadily increasing in the last decades (Komhyr et al., 1985; Conway et al., 1988; Conway et al., 1994). Stable isotopes proved to be useful indicators for ecophysiological parameters at the leaf-level such as ci/ca or water-use efficiency (Farquhar et al., 1989; Ehleringer et al., 1993;), and might also serve as valuable tools at the ecosystem level. Based on the fact that tropospheric and respired CO2 from terrestrial ecosystems have very different carbon isotope ratios (app. -7.8 permil and -27 permil, respectively), these CO2 sources can be identified, and the coupling of terrestrial and atmospheric carbon fluxes can be addressed (Keeling, 1958; Sternberg, 1989; Tans et al. 1990; Lloyd et al., 1996). Global circulation models indicated a large carbon sink on the Northern Hemisphere, either oceanic or terrestrial (Tans et al. 1990; Ciais et al., 1995; Denning et al., 1995; Francey et al., 1995; Bender et al., 1996; Keeling et al. 1996). Carbon isotope ratios (d13C) of the atmosphere have been used in these inverse global models (i.e., using a top-down approach) not only to determine the global distribution of carbon sinks, but also to quantify the relative contribution of oceans and terrestrial plants to the carbon removal from the atmosphere (Ciais et al., 1995). However, lacking sufficient real data, important input parameters such as the internal [CO2] in the mesophyll of plants (ci), and the carbon isotopic composition of respired CO2 were estimated using models for the biosphere and for soils. Further uncertainties arise due to regional and temporal variability of these estimates, and due to disequilibrium effects. Thus, detailed knowledge about the ecophysiology of terrestrial ecosystems and their interactions with the atmosphere is needed to evaluate model outputs, and to identify potential mechanisms for CO2 sequestration.