Analysis of nitrogen controls on carbon and water exchanges in a conifer forest using the CLASS-CTEM^N+ model

A carbon (C) and nitrogen (N) cycle-coupled model, CLASS-CTEM^N+ was developed by incorporating soil and plant N cycling algorithms in the Canadian Land Surface Scheme (CLASS) and the Canadian Terrestrial Ecosystem Model (CTEM), used in the Canadian Global Climate Model. Key soil and plant N cycling processes incorporated in the model include biological fixation, mineralization, nitrification, denitrification, leaching and N controls on plant photosynthesis capacity. The model was used to analyse N controls on C and water exchanges in a 70-year-old temperate conifer forest in southern Ontario, Canada from 2003 to 2007. The simulated values of soil-plant N contents and fluxes - including N₂O flux - were generally in good agreement with observations. When N controls on C and water cycling were included in the model, simulated daily gross ecosystem productivity (GEP), ecosystem respiration (R_e), net ecosystem productivity (NEP) and evapotranspiration (ET) fluxes showed improved agreement with eddy covariance flux measurements. The five-year mean annual NEP predicted by the N-coupled model was 121gCm^-2yr^- for 2003-2007, compared to 273gCm^-2yr^-1, which was simulated by the model when N controls were switched off (non-N model). N-coupled model estimates compared well with the measured five-year mean (± standard deviation) annual NEP of 136±59gCm^-2yr^-1. Simulated annual mean ET over five-years was 384mmyr^-1 for the N-coupled model, and 433mmyr^-1 for non-N model, compared with the measured five-year mean annual value of 405±44mmyr^-1. Model results confirmed that a proper representation of N controls on photosynthetic uptake and canopy conductance could result in more plausible simulations of observed C and water fluxes. The model results also suggested that N limitations in spring and early summer were generally more important in controlling NEP. Discrepancies between simulated and measured annual variations of C exchanges occurred in years that included extreme weather periods (e.g. low soil water content and warm spring/summer temperatures).