MODELING THE GLOBAL CARBON-CYCLE - NITROGEN-FERTILIZATION OF THE TERRESTRIAL BIOSPHERE AND THE MISSING CO2 SINK

The discrepancy between estimates of net terrestrial CO2 emissions der ived from (1) inverse modeling of the ocean/atmosphere system and (2) modeling of land use change, better known as the ''missing'' CO2 sink, suggests that some changing environmental factor, such as CO2, anthro pogenic N emissions, or climate, has fertilized terrestrial ecosystems . To address this question, we herein describe and apply GLOCO, a glob al carbon cycle model. GLOCO's ocean submodel combines a box diffusion model with representations of chemical equilibria and biological proc esses to simulate the distributions and cycling of inorganic and organ ic carbon, phosphate, and alkalinity. The terrestrial submodel divides the biosphere into seven natural biomes with dynamic carbon and nitro gen cycling in both vegetation and soils. Anthropogenic influences on the functioning of the carbon and nitrogen cycles, such as fossil fuel combustion, forestry, and agricultural development, are also incorpor ated in the model. Our analysis confirms previous suggestions that bec ause temperate and boreal forests are N limited, CO2 fertilization of these forests is less than predicted by short-term CO2 response factor s. Modeling of temperate/boreal forest fertilization by anthropogenic N deposition suggests that CO2 is initially sequestered at a C:N ratio of similar to 100, rather than the steady state value for the ecosyst em of similar to 30. If N deposition is to account for the 40-70% of t he fertilization of the terrestrial biosphere not explainable by CO2 f ertilization and temperature increases, then we estimate that 26-30 Tg N yr(-1) of anthropogenic deposition in the temperate and boreal zone s would be required. Recent anthropogenic NOx and NH3 deposition fluxe s at northern temperate latitudes have been estimated to be 20-28 Tg N yr(-1). Thus fertilization by anthropogenic N emissions likely consti tutes a significant portion of the missing CO2 sink.