Implementation of nitrogen cycle in the CLASSIC land model

Abstract

Abstract. 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 CO2 at the land–atmosphere boundary. CLASSIC is similar to most models and its gross primary productivity 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 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 (Vcmax) prognostically. An increase in atmospheric CO2 decreases leaf N amount 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; denitrification; 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. An increase in atmospheric CO2 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, NH3 volatilization, and gaseous losses of N2, N2O, 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 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.

Ocean biogeochemistry in the Canadian Earth System Model version 5.0.3: CanESM5 and CanESM5-CanOE

“…This strategy allows us to quantify how much of the improvement in model skill is due to the physical circulation, which is in fact substantial (e.g., Figure 8). The CanESM terrestrial carbon model is also undergoing important new developments (e.g., Asaadi and Arora, 2021) and we expect CanESM to continue to offer a credible contribution to global carbon cycle studies, as well as advancing regional downscaling and impacts science.…”

CLASSIC v1.0: the open-source community successor to the Canadian Land Surface Scheme (CLASS) and the Canadian Terrestrial Ecosystem Model (CTEM) – Part 2: Global Benchmarking

“…The main deficiencies that should be addressed in future model development are the (i) positive albedo bias and resulting SW radiation bias in parts of the NH extratropics and Tibetan Plateau, (ii) out-of-phase seasonal GPP cycle in the humid tropics of South America and Africa, (iii) lacking spatial correlation of annual mean NEE measured by FLUXNET sites, (iv) underestimation of fractional area burned and corresponding emissions in the boreal forests, (v) negative soil organic carbon bias in high latitudes, and (vi) time lag in seasonal LAI maxima in the NH extratropics. Recent model development has started addressing some of those issues already, including the improvement of LAI seasonality through the incorporation of non-structural carbohydrates, which will form part of the next model version release (Asaadi et al, 2018). Further research is required to separate the impact of observational uncertainties on biases in LAI and above-ground biomass.…”

“…The main processes simulated by CTEM include photosynthesis, canopy conductance, tissue turnover, allocation of carbon, and phenology (Arora and Boer, 2005b); dynamic root distribution (Arora and Boer, 2003); maintenance, growth, and heterotrophic respiration (Melton et al, 2015); wildfires (Arora and Boer, 2005a;Arora and Melton, 2018); competition for space between PFTs (Arora and Boer, 2006;Melton and Arora, 2016), and land use change (Arora and Boer, 2010). For the purpose of this study, CLASSIC's recently added nitrogen cycle has been turned off (Asaadi and Arora, 2021). A more detailed description of the model and its history is documented in Melton et al (2020).…”

Simulating shrubs and their energy and carbon dioxide fluxes in Canada's Low Arctic with the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC)

“…As the photosynthetic capacity of Arctic shrubs is seasonally variable and has been shown to depend on day length and maximum insolation (Chapin and Shaver, 1985;Shaver and Kummerow, 1992;Oberbauer et al, 2013), especially in the fall, a seasonal variation of the maximum carboxylation rate by the Rubisco enzyme (V cmax , mol CO 2 m −2 s −1 ) was implemented for shrubs as it was previously included for deciduous tree species in CLASSIC. Following Bauerle et al (2012) and Alton (2017), seasonality was included by modifying V cmax such that…”