EQUILIBRIUM RESPONSES OF GLOBAL NET PRIMARY PRODUCTION AND CARBON STORAGE TO DOUBLED ATMOSPHERIC CARBON-DIOXIDE - SENSITIVITY TO CHANGES INVEGETATION NITROGEN CONCENTRATION

We ran the terrestrial ecosystem model (TEM) for the globe at 0.5 degr ees resolution for atmospheric CO2 concentrations of 340 and 680 parts per million by volume (ppmv) to evaluate global and regional response s of net primary production (NPP) and carbon storage to elevated CO2 f or their sensitivity to changes in vegetation nitrogen concentration. At 340 ppmv, TEM estimated global NPP of 49.0 10(15) g (Pg) C yr(-1) a nd global total carbon storage of 1701.8 Pg C; the estimate of total c arbon storage does not include the carbon content of inert soil organi c matter. For the reference simulation in which doubled atmospheric CO 2 was accompanied with no change in vegetation nitrogen concentration, global NPP increased 4.1 Pg C yr(-1) (8.3%), and global total carbon storage increased 114.2 Pg C. To examine sensitivity in the global res ponses of NPP and carbon storage to decreases in the nitrogen concentr ation of vegetation, we compared doubled CO2 responses of the referenc e TEM to simulations in which the vegetation nitrogen concentration wa s reduced without influencing decomposition dynamics (''lower N'' simu lations) and to simulations in which reductions in vegetation nitrogen concentration influence decomposition dynamics (''lower N+D'' simulat ions). We conducted three lower N simulations and three lower N+D simu lations in which we reduced the nitrogen concentration of vegetation b y 7.5, 15.0, and 22.5%. In the lower N simulations, the response of gl obal NPP to doubled atmospheric CO2 increased approximately 2 Pg C yr( -1) for each incremental 7.5% reduction in vegetation nitrogen concent ration, and vegetation carbon increased approximately an additional 40 Pg C, and soil carbon increased an additional 30 Pg C, for a total ca rbon storage increase of approximately 70 Pg C. In the lower N+D simul ations, the responses of NPP and vegetation carbon storage were relati vely insensitive to differences in the reduction of nitrogen concentra tion, but soil carbon storage showed a large change. The insensitivity of NPP in the N+D simulations occurred because potential enhancements in NPP associated with reduced vegetation nitrogen concentration were approximately offset by lower nitrogen availability associated with t he decomposition dynamics of reduced litter nitrogen concentration. Fo r each 7.5% reduction in vegetation nitrogen concentration, soil carbo n increased approximately an additional 60 Pg C, while vegetation carb on storage increased by only approximately 5 Pg C. As the reduction in vegetation nitrogen concentration gets greater in the lower N+D simul ations, more of the additional carbon storage tends to become concentr ated in the north temperate-boreal region in comparison to the tropics . Other studies with TEM show that elevated CO2 more than offsets the effects of climate change to cause increased carbon storage. The resul ts of this study indicate that carbon storage would be enhanced by the influence of changes in plant nitrogen concentration on carbon assimi lation and decomposition rates. Thus changes in vegetation nitrogen co ncentration may have important implications for the ability of the ter restrial biosphere to mitigate increases in the atmospheric concentrat ion of CO2 and climate changes associated with the increases.