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

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Abstract

[1] The LPJ terrestrial carbon isotope model, which includes isotopic fractionation of 13C 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 δ13C for the years 1901 to 1998. A transient, spatially explicit data set of C4 crops and tropical C4 pastures has been compiled. In combination with a land use scheme this allows the analysis of the impact of land use conversion of C3 ecosystems to C4 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‰ and 17.0‰ depending on the chosen land use scheme and also the year of the simulation. Results from the simulation experiment prescribing the conversion of C3 ecosystems into C4 crops and C4 pastures show the lowest leaf discrimination. Modeled values of isotopic disequilibrium flux, caused by the δ13C difference between fixed CO2 and released CO2, similarly depend on the amount of prescribed C4 vegetation and vary between 37.9 Pg C‰ yr−1 and 23.9 Pg C‰ 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 ≈10 Pg C‰ 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 C4 cultivation and the experiment including C4 crops and pastures could account for a shift of about 1 Pg C yr−1 from the inferred terrestrial sources to the ocean fluxes.

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