An Analysis of Nitrogen Controls on Terrestrial Carbon and Energy Dynamics Using the Carbon-Nitrogen Coupled CLASS-CTEMN+ Model

Abstract:

The advent of biophysical land surface schemes, in which photosynthesis and the structure of plant functional types is modelled explicitly, allows detailed carbon budgets to be simulated in Earth System Models (ESMs), including the response of ecosystems to increasing atmospheric CO₂. Projections of future carbon balances are often viewed in terms of enhanced photosynthesis in response to increased atmospheric CO₂, the so-called ‘CO₂ fertilization effect’, versus increased respiration caused by warming. However, most ESMs do not represent nutrient cycles, most notably nitrogen (N), the availability of which can act as a strong constraint on photosynthesis, and carbon turnover in the soil.

In the Canadian ESM (CanESM), surface processes are represented by the Canadian Land Surface Scheme (CLASS), which models surface energy and water exchanges, coupled with the Canadian Terrestrial Ecosystem Model (CTEM), which models carbon-related processes. We present global and site-level results from incorporating a nitrogen cycle (C-N coupled) into CLASS coupled with CTEM. Flux, forcing and initializing data sets developed by the North American Carbon Program (NACP) and NACP- Multi-Scale Synthesis and Terrestrial Model Intercomparison Project (MsTMIP) were used.

The C-N coupled model yielded global annual estimates (over 1980-2010) of 122.7 Pg C yr⁻¹ for gross ecosystem production (GEP), and 62.7 Pg C yr⁻¹ for net primary productivity (NPP). Ecosystem respiration (Re) was 119.1 Pg C yr⁻¹ which is about 25% larger than observed, and results in a low estimate of 3.64 Pg C yr⁻¹ for net ecosystem productivity (NEP = GEP – Re). On regional and site-level scales, larger differences were seen between the C-only and C-N coupled model, especially at high latitudes during summer months where N is limiting. Analysis of the long-term annual variations over 1901-2010 also showed different responses to evolving climate, CO₂ and N deposition. For 1970-2010, the C-N coupled model indicated a strong N constraint on the rate of increase of GEP and NPP compared to the C-only model.