Carbon-nitrogen feedbacks in the UVic ESCM
1School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055, Victoria, BC, V8W 3V6, Canada
2Climate Change Research Centre, Level 4, Mathews Building, University of New South Wales, Sydney, NSW, 2052, Australia
3Canadian Center for Climate Modelling and Analysis, Ocean, Earth and Atmospheric Sciences Building A203, University of Victoria, 3800 Finnerty Road, Victoria BC V8P 5C2, Canada
4Centre d'Ecologie Fonctionelle et Evolutive – CNRS, 1919 Route de Mende, 34293 Montpellier cedex 5, France
5Lanser Strasse 30, Top A1, 6080 Igls, Austria
Abstract. A representation of the terrestrial nitrogen cycle is introduced into the UVic Earth System Climate Model (UVic ESCM). The UVic ESCM now contains five terrestrial carbon pools and seven terrestrial nitrogen pools: soil, litter, leaves, stem and roots for both elements and ammonium and nitrate in the soil for nitrogen. Nitrogen cycles through plant tissue, litter, soil and the mineral pools before being taken up again by the plant. Biological N2 fixation and nitrogen deposition represent external inputs to the plant-soil system while losses occur via leaching. Simulated carbon and nitrogen pools and fluxes are in the range of other models and observations. Gross primary production (GPP) for the 1990s in the CN-coupled version is 129.6 Pg C a−1 and net C uptake is 0.83 Pg C a−1, whereas the C-only version results in a GPP of 133.1 Pg C a−1 and a net C uptake of 1.57 Pg C a−1. At the end of a transient experiment for the years 1800–1999, where radiative forcing is held constant but CO2 fertilisation for vegetation is permitted to occur, the CN-coupled version shows an enhanced net C uptake of 1.05 Pg C a−1, whereas in the experiment where CO2 is held constant and temperature is transient the land turns into a C source of 0.60 Pg C a−1 by the 1990s. The arithmetic sum of the temperature and CO2 effects is 0.45 Pg C a−1, 0.38 Pg C a−1 lower than seen in the fully forced model, suggesting a strong nonlinearity in the CN-coupled version. Anthropogenic N deposition has a positive effect on Net Ecosystem Production of 0.35 Pg C a−1. Overall, the UVic CN-coupled version shows similar characteristics to other CN-coupled Earth System Models, as measured by net C balance and sensitivity to changes in climate, CO2 and temperature.