Modeling the Global Carbon Cycle: Nitrogen fertilization of the terrestrial biosphere and the “missing” CO₂ sink

Abstract

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 NOx 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. HUDSON ET AL.' MODELING THE GLOBAL CARBON CYCLE Z•tC + AOc + Z•Bc = I EFossil fuel dt or expressed in terms of rates of change d/dt dAc dOc dBc • + + -EFossil fuel dt dt dt (la) (lb) The rate of change in atmospheric CO2 is the best quantified term in the mass balance at 3.2_+0.2 Pg C yr 4 for the period 1980-1989 [Keeling, 1990]. Fossil fuel emissions are also well quantified at 5.4_+0.5 Pg C yr-• for the same period [Marland, 1990; Marland and Rotty, 1984]. The change in carbon storage in the oceans, assumed to arise primarily from increased absorption of atmospheric CO2, can be modeled by &convolution of the atmospheric CO2 record (reconstructing oceanic uptake using models and historical atmospheric CO2 partial pressure Pco2). By considering the results of a number of such studies, Siegenthaler and Sarmiento [1993] argue that the perturbation in oceanic uptake (~dOc/dt) is 2.0_+0.6 Pg C yr-• for the 1980s. Analysis of measured surface ocean Pco2 values and the distributions, sources, and sinks of atmospheric CO2 using an atmospheric transport model lead Tans et al. [1990] to suggest that the perturbation in oceanic uptake was les...

Nitrogen deposition: a component of global change analyses

“…The remaining C flux was assumed to represent the perturbation of the terrestrial biosphere, including both net emissions from land use change and uptake from fertilization. Using GLOCO's terrestrial submodel to account for land use change, Hudson et al (1994) inferred that significant fertilization must have occurred. The modelled response to historical increases in CO # and temperature accounted for 31 % of the fertilization, implying that N fertilization must account for the rest.…”

“…Gifford et al (1996) concluded that the assumption about how the exogenous N is initially taken up is a critical one that should be further investigated. Hudson, Gherini & Goldstein (1994) employed a very different approach to the calculation of Nstimulated C storage. The historical record of atmospheric CO # concentration was deconvoluted using the historical record of fossil fuel emission and the ocean submodel of their global C cycle model (GLOCO) for the period 1850-1985.…”

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

“…Global warming could increase decomposition and nitrogen mineralization and thereby the primary production, especially in the northern latitudes [Rastetter et al, 1991;Hudson et al, 1994]. This temperature feedback could possibly offset the increased CO 2 releases from the northern soils (temperature feedback on soil respiration).…”

Global terrestrial biogeochemistry: Perturbations, interactions, and time scales

“…More recent estimates of the carbon sink caused by anthropogenic N deposition range from 0.50 to 1.50 Pg yr-' (Kohlmaier et al 1988, Hudson et al 1994, Schindler and Bayley 1993, Townsend et al 1996. Hudson et al (1994) The analysis of atmospheric C02 and 613C has provided another perspective on the global carbon cycle and has allowed a more robust determination of the latitudinal distribution of net terrestrial C02 exchange (e.g., Ciais et al 1995a; see also Section 2.2).…”

“…Hudson et al (1994) The analysis of atmospheric C02 and 613C has provided another perspective on the global carbon cycle and has allowed a more robust determination of the latitudinal distribution of net terrestrial C02 exchange (e.g., Ciais et al 1995a; see also Section 2.2). The latitudinal distribution of the carbon sink derived from N deposition is similar to the latitudinal distribution derived from isotopickoncentration measurements coupled to an inverse modeling framework ( Fig.…”