Modeling terrestrial C-13 cycling: Climate, land use and fire

The LPJ terrestrial carbon isotope model, which includes isotopic fractionation of C-13 during assimilation and a full description of the isotopic terrestrial carbon cycle, has been used to calculate the atmosphere-biosphere exchange flux of CO2 and its delta C-13 for the years 1901 to 1998. A transient, spatially explicit data set of C-4 crops and tropical C-4 pastures has been compiled. In combination with a land use scheme this allows the analysis of the impact of land use conversion of C-3 ecosystems to C-4 cultivation, besides climate, fire disturbances, atmospheric CO2 and the isotope ratio of atmospheric CO2, on the terrestrial carbon stable isotope composition. Globally averaged values of modeled leaf discrimination vary between 17.9 parts per thousand and 17.0 parts per thousand depending on the chosen land use scheme and also the year of the simulation. Results from the simulation experiment prescribing the conversion of C-3 ecosystems into C-4 crops and C-4 pastures show the lowest leaf discrimination. Modeled values of isotopic disequilibrium flux, caused by the delta C-13 difference between fixed CO2 and released CO2, similarly depend on the amount of prescribed C-4 vegetation and vary between 37.9 Pg C parts per thousand yr(-1) and 23.9 Pg C parts per thousand yr(-1) averaged over the years 1985 to 1995. In addition, the effect of fire on the isotopic disequilibrium has been diagnosed; generally wildfires lead to a disequilibrium reduction of approximate to 10 Pg C parts per thousand yr(-1) because they shorten the turnover time of terrestrial carbon. If used in a global double deconvolution study, the differences in the results between the standard experiment without any C-4 cultivation and the experiment including C-4 crops and pastures could account for a shift of about 1 Pg C yr(-1) from the inferred terrestrial sources to the ocean fluxes