An integrated modeling approach to global carbon and nitrogen cycles: Balancing their budgets

A modeling attempt is presented to describe the global carbon and nitrogen cycle: where it originates, where it stays, and how the global carbon and nitrogen budgets can be balanced. We used integrated but simple box models: a coupled global carbon-nitrogen cycle and climate model. The global carbon-nitrogen cycle model describes on a global scale the exchange fluxes between the compartments and their main internal processes and different biogeochemical feedbacks (e.g., for CO2 and N fertilization and for temperature). The coupled model also includes an energy balance climate model, using as input the induced radiative forcing of greenhouse gases and sulfate aerosols. Simulation experiments performed with the coupled model showed that by incorporating only the CO2 fertilization and temperature-related feedbacks a balanced past carbon budget could not be created. Therefore the N fertilization of the anthropogenic nitrogen depositions for temperate forests, which depends on the fate of the anthropogenic nitrogen inputs, must be included. By varying the N fluxes by river transport, and deposition and denitrification, a balanced past nitrogen budget was obtained. This resulted in an anthropogenic deposition of 40 Tg N yr(-1) for the temperate forests and a N fertilization which constitutes 20-50% of the missing carbon sink. Finally, the future fate of anthropogenic emissions of the C and mobilization of the N compounds were projected given the condition of balanced carbon and nitrogen budgets.