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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-147
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-147
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 May 2020

14 May 2020

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This preprint is currently under review for the journal BG.

Implementation of nitrogen cycle in the CLASSIC land model

Ali Asaadi and Vivek K. Arora Ali Asaadi and Vivek K. Arora
  • Canadian Centre for Climate Modelling and Analysis, Environment Canada, University of Victoria, Victoria, B.C., V8W 2Y2, Canada

Abstract. A terrestrial nitrogen (N) cycle model is coupled to carbon (C) cycle in the framework of the Canadian Land Surface Scheme Including biogeochemical Cycles (CLASSIC). CLASSIC currently models physical and biogeochemical processes and simulates fluxes of water, energy, and CO2 at the land-atmosphere boundary. Similar to most models, gross primary productivity in CLASSIC increases in response to increasing atmospheric CO2 concentration. In the current model version, a downregulation parameterization emulates the effect of nutrient constraints and scales down potential photosynthesis rates, using a globally constant scalar, as a function of increasing CO2. In the new model when N and C cycles are coupled, cycling of N through the coupled soil-vegetation system facilitates the simulation of leaf N content and maximum carboxylation capacity (Vcmax) prognostically. An increase in atmospheric CO2 decreases leaf N content, and therefore Vcmax, allowing the simulation of photosynthesis downregulation as a function of N supply. All primary N cycle processes, that represent the coupled soil-vegetation system, are modelled explicitly. These include biological N fixation, treatment of externally specified N deposition and fertilization application, uptake of N by plants, transfer of N to litter via litterfall, mineralization, immobilization, nitrification, ammonia volatilization, leaching, and the gaseous fluxes of NO, N2O, and N2. The interactions between terrestrial C and N cycles are evaluated by perturbing the coupled soil-vegetation system in CLASSIC with one forcing at a time over the 1850–2017 historical period. These forcings include the increase in atmospheric CO2, change in climate, increase in N deposition, and increasing crop area and fertilizer input, over the historical period. The model response to these forcings is consistent with conceptual understanding of the coupled C and N cycles. The simulated terrestrial carbon sink over the 1959–2017 period, from the simulation with all forcings, is 2.0 Pg C/yr and compares reasonably well with the quasi observation-based estimate from the 2019 Global Carbon Project (2.1 Pg C/yr). The contribution of increasing CO2, climate change, and N deposition to carbon uptake by land over the historical period (1850–2017) is calculated to be 84 %, 2 %, and 14 %, respectively.

Ali Asaadi and Vivek K. Arora

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Ali Asaadi and Vivek K. Arora

Ali Asaadi and Vivek K. Arora

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Latest update: 10 Aug 2020
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Short summary
More than a quarter of the current anthropogenic CO2 emissions are taken up by land reducing atmospheric CO2 growth rate. This is because of the CO2 fertilization effect which benefits 80 % of the global vegetation. However, if nitrogen and phosphorus nutrients can't keep up with increasing atmospheric CO2 our terrestrial biosphere can't provide us this ecosystem service. This manuscript implements nitrogen constraints on photosynthesis in a model to understand the mechanisms involved.
More than a quarter of the current anthropogenic CO2 emissions are taken up by land reducing...
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