Implementation of nitrogen cycle in the CLASSIC land model

A terrestrial nitrogen (N) cycle model is coupled to the 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 CO 2 at the land–atmosphere boundary. CLASSIC is similar to most models and its gross primary productivity increases in response to increasing atmospheric CO 2 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 CO 2 . In the new model when nitrogen (N) and carbon (C) cycles are coupled, cycling of N through the coupled soil–vegetation system facilitates the simulation of leaf N amount and maximum carboxylation capacity ( V cmax ) prognostically. An increase in atmospheric CO 2 decreases leaf N amount 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; denitrification; ammonia volatilization; leaching; and the gaseous fluxes of NO, N 2 O, and N 2 . 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 CO 2 , change in climate, increase in N deposition, and increasing crop area and fertilizer input, over the historical period. An increase in atmospheric CO 2 increases the C:N ratio of vegetation; climate warming over the historical period increases N mineralization and leads to a decrease in the vegetation C:N ratio; N deposition also decreases the vegetation C:N ratio. Finally, fertilizer input increases leaching, NH 3 volatilization, and gaseous losses of N 2 , N 2 O, and NO. These model responses are 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 −1 and compares reasonably well with the quasi observation-based estimate from the 2019 Global Carbon Project (2.1 Pg C yr −1 ). The contribution of increasing CO 2 , 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.

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