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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2019-513
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2019-513
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 10 Jan 2020

Submitted as: research article | 10 Jan 2020

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Nitrogen Cycling in CMIP6 Land Surface Models: Progress and Limitations

Taraka Davies-Barnard1,2, Johannes Meyerholt2, Sönke Zaehle2, Pierre Friedlingstein1,3, Victor Brovkin4, Yuanchao Fan5,6, Rosie A. Fisher7,8, Chris D. Jones9, Hanna Lee5, Daniele Peano10, Benjamin Smith11,12, David Wårlind11,12, and Andy Wiltshire9 Taraka Davies-Barnard et al.
  • 1University of Exeter, Exeter, UK
  • 2Max Planck Institute for Biogeochemistry, Jena, Germany
  • 3Laboratoire de Meteorologie Dynamique, Institut Pierre-Simon Laplace, CNRS-ENS-UPMC-X, Departement de Geosciences, Ecole Normale Superieure, 24 rue Lhomond, 75005 Paris, France
  • 4Max Planck Institute for Meteorology, Hamburg, Germany
  • 5NORCE Norwegian Research Centre, Bjerknes Centre for Climate Research, Bergen, Norway
  • 6Harvard University, Cambridge, USA
  • 7National Center for Atmospheric Research, Boulder, Colorado, USA
  • 8Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique, Toulouse, France
  • 9Met Office Hadley Centre, Exeter, UK
  • 10Fondazione Centro euro-Mediterraneo sui Cambiamenti Climatici, Bologna, Italy
  • 11Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden
  • 12Hawkesbury Institute for the Environment, Western Sydney University, Richmond, Australia

Abstract. The nitrogen cycle and its effect on carbon uptake in the terrestrial biosphere is a recent progression in earth system models. As with any new component of a model, it is important to understand the behaviour, strengths, and limitations of the various process representations. Here we assess and compare five models with nitrogen cycles that will be used as the terrestrial components of some of the earth system models in CMIP6. We use a historical control simulation and two perturbations to assess the models' nitrogen-related performance: a simulation with atmospheric carbon dioxide 200 ppm higher, and one with nitrogen deposition increased by 50 kg N ha−1 yr−1. We find that, despite differing nitrogen cycle representations, all models simulate recent global trends in terrestrial productivity and net carbon uptake commensurate with observations. The between-model variation is likely more influenced by other, non-nitrogen parts of the models. Globally, the productivity response to increased carbon dioxide is commensurate with observations for four of the five models, but highly spatially variable within and between models. The productivity response to increased nitrogen is significantly lower than observed in two of the five models. The global and tropical values are generally better represented than boreal, tundra, or other high latitude areas. These results are due to divergent though valid choices in the representation of key processes. They show the need for better understanding and more provision of observational constraints of nitrogen processes, especially nitrogen-use efficiency and biological nitrogen fixation.

Taraka Davies-Barnard et al.
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Taraka Davies-Barnard et al.
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