Implementation of nitrogen cycle in the CLASSIC land model

Ali Asaadi,Vivek K. Arora

doi:10.5194/bg-18-669-2021

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.

Highlights

The uptake of carbon (C) by land and ocean in response to the increase in anthropogenic fossil fuel emissions of CO2 has served to slow down the growth rate of atmospheric CO2 since the start of the industrial revolution
As long as photosynthesis is not water limited, the uptake of carbon in response to increasing anthropogenic CO2 emissions is driven by two primary factors: (1) the CO2 fertilization of the terrestrial biosphere and (2) the increase in temperature, both of which are associated with increasing [CO2]
We present the implementation of the N cycle in the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC) model, which serves as the land component in the family of Canadian Earth System models (Arora et al, 2009, 2011; Swart et al, 2019)

Summary

Introduction

The uptake of carbon (C) by land and ocean in response to the increase in anthropogenic fossil fuel emissions of CO2 has served to slow down the growth rate of atmospheric CO2 since the start of the industrial revolution. About 55 % of total carbon emitted into the atmosphere is taken up by land and oceans (Le Quéré et al, 2018; Friedlingstein et al, 2019). As long as photosynthesis is not water limited, the uptake of carbon in response to increasing anthropogenic CO2 emissions is driven by two primary factors: (1) the CO2 fertilization of the terrestrial biosphere and (2) the increase in temperature, both of which are associated with increasing [CO2]. The CO2 fertilization effect increases photosynthesis rates for about 80 % of the world’s vegetation that uses the C3 photosynthetic pathway and is currently limited by [CO2] (Still et al, 2003; Zhu et al, 2016). Warming increases carbon uptake by vegetation in mid- to high-latitude regions where growth is currently limited by low temperatures (Zeng et al, 2011)

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