Implementation of nitrogen cycle in the CLASSIC land model

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 (V_cmax) prognostically. An increase in atmospheric CO₂ decreases leaf N content, and therefore V_cmax, 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.