A global-scale simulation of the CO2 exchange between the atmosphere and the terrestrial biosphere with a mechanistic model including stable carbon isotopes, 1953–1999

Abstract

This paper presents the results of a simulation with a mechanistic terrestrial ecosystem model, focusing on the atmosphere—biosphere exchange and stable isotope composition of carbon. The simulation was performed from 1953 to 1999 on the basis of observed climate data and atmospheric carbon dioxide (CO2) concentration and stable carbon isotope ratio (δ13C). The model, termed Sim-CYCLE, captures carbon dynamics from photosynthetic assimilation to microbial decomposition, including seasonal and interannual variability. Photosynthetic discrimination effect on δ13C was considered at three levels: (1) leaf-level fractionation, (2) canopy-level CO2 recycling and (3) continent-level C3/C4 pattern. The 47-yr simulation estimated that the average gross CO2 flux was 121 Pg C yr−1, and that the average photosynthetic δ13C discrimination coefficient (Δ) was 18.2%. A sensitivity analysis indicated that the estimated Δ depends heavily on the parameterization of stomatal conductance and C3/C4 composition. In spite of their small biomass, C4 plants contributed considerably to the biospheric productivity and belowground carbon supply. The estimated net CO2and isotopic exchange of the terrestrial ecosystems corresponded, at least qualitatively, with observed atmospheric CO2 and its δ13C seasonal patterns in the Northern Hemisphere. The gross CO2 fluxes of photosynthesis and respiration indicated a wide range of interannual variability, which was in a sufficient magnitude to induce anomalies in the atmospheric CO2 growth rate. The estimated Δ showed a wide range of latitudinal and longitudinal variations and seasonal oscillation, but little interannual change. However, during the 47-yr period, the estimated δ13C of carbon pools decreased by 0.3%, while the δ13C of atmospheric CO2 decreased by 0.7%. These results carry implications for the application of a top-down approach, i.e. the double-deconvolution method, to inferring the global terrestrial CO2 budget.