Nitrogen cycling in CMIP6 land surface models: progress and limitations
<p>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 asses...
| Main Authors: | , , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-10-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/17/5129/2020/bg-17-5129-2020.pdf |
| Summary: | <p>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 land surface models with
nitrogen cycles that are used as the terrestrial components of some of the
earth system models in CMIP6. The land surface models were run offline with
a common spin-up and forcing protocol. We use a historical control
simulation and two perturbations to assess the model nitrogen-related
performances: a simulation with atmospheric carbon dioxide increased by 200 ppm and one with nitrogen deposition increased by 50 kgN ha<span class="inline-formula"><sup>−1</sup></span> yr<span class="inline-formula"><sup>−1</sup></span>. There is generally greater variability in productivity response
between models to increased nitrogen than to carbon dioxide. Across the five
models the response to carbon dioxide globally was 5 % to 20 % and the
response to nitrogen was 2 % to 24 %. The models are not evenly distributed
within the ensemble range, with two of the models having low productivity
response to nitrogen and another one with low response to elevated atmospheric
carbon dioxide, compared to the other models. In all five models individual
grid cells tend to exhibit bimodality, with either a strong response to
increased nitrogen or atmospheric carbon dioxide but rarely to both to an
equal extent. However, this local effect does not scale to either the
regional or global level. The global and tropical responses are generally
more accurately modelled than boreal, tundra, or other high-latitude areas
compared to observations. These results are due to divergent choices in the
representation of key nitrogen cycle processes. They show the need for more
observational studies to enhance understanding of nitrogen cycle processes,
especially nitrogen-use efficiency and biological nitrogen fixation.</p> |
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| ISSN: | 1726-4170 1726-4189 |