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 derived 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, anthropogenic N emissions, or climate, has fertilized terrestrial ecosystems. To address this question, we herein describe and apply GLOCO, a global carbon cycle model. GLOCO's ocean submodel combines a box diffusion model with representations of chemical equilibria and biological processes to simulate the distributions and cycling of inorganic and organic carbon, phosphate, and alkalinity. The terrestrial submodel divides the biosphere into seven natural biomes with dynamic carbon and nitrogen 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 incorporated in the model. Our analysis confirms previous suggestions that because temperate and boreal forests are N limited, CO₂ fertilization of these forests is less than predicted by short‐term CO₂ response factors. Modeling of temperate/boreal forest fertilization by anthropogenic N deposition suggests that CO₂ is initially sequestered at a C:N ratio of ∼100, rather than the steady state value for the ecosystem of ∼30. If N deposition is to account for the 40–70% of the fertilization of the terrestrial biosphere not explainable by CO₂ fertilization and temperature increases, then we estimate that 26‐30 Tg N yr⁻¹ of anthropogenic deposition in the temperate and boreal zones would be required. Recent anthropogenic NO_x and NH₃ deposition fluxes at northern temperate latitudes have been estimated to be 20–28 Tg N yr⁻¹. Thus fertilization by anthropogenic N emissions likely constitutes a significant portion of the missing CO₂ sink.