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Accounting for carbon and nitrogen interactions in the global terrestrial ecosystem model ORCHIDEE (trunk version, rev 4999): multi-scale evaluation of gross primary production
Ist Teil von
Geoscientific Model Development, 2019-11, Vol.12 (11), p.4751-4779
Ort / Verlag
Katlenburg-Lindau: Copernicus GmbH
Erscheinungsjahr
2019
Link zum Volltext
Quelle
EZB Free E-Journals
Beschreibungen/Notizen
Nitrogen is an essential element controlling ecosystem
carbon (C) productivity and its response to climate change and atmospheric
[CO2] increase. This study presents the evaluation – focussing on
gross primary production (GPP) – of a new version of the ORCHIDEE model
that gathers the representation of the nitrogen cycle and of its
interactions with the carbon cycle from the OCN model and the most recent
developments from the ORCHIDEE trunk version. We quantify the model skills at 78 FLUXNET sites by simulating the observed
mean seasonal cycle, daily mean flux variations, and annual mean average GPP
flux for grasslands and forests. Accounting for carbon–nitrogen interactions
does not substantially change the main skills of ORCHIDEE, except for the
site-to-site annual mean GPP variations, for which the version with
carbon–nitrogen interactions is in better agreement with observations.
However, the simulated GPP response to idealised [CO2] enrichment
simulations is highly sensitive to whether or not carbon–nitrogen
interactions are accounted for. Doubling of the atmospheric [CO2]
induces an increase in the GPP, but the site-averaged GPP response to
a CO2 increase projected by the model version with carbon–nitrogen
interactions is half of the increase projected by the version without
carbon–nitrogen interactions. This model's differentiated response has
important consequences for the transpiration rate, which is on average 50 mm yr−1 lower with the version with carbon–nitrogen interactions. Simulated annual GPP for northern, tropical and southern latitudes shows
good agreement with the observation-based MTE-GPP (model tree ensemble gross primary production) product for present-day
conditions. An attribution experiment making use of this new version of
ORCHIDEE for the time period 1860–2016 suggests that global GPP has
increased by 50 %, the main driver being the enrichment of land in
reactive nitrogen (through deposition and fertilisation), followed by the
[CO2] increase. Based on our factorial experiment and sensitivity analysis, we conclude that
if carbon–nitrogen interactions are accounted for, the functional responses
of ORCHIDEE r4999 better agree with the current understanding of photosynthesis
than when the carbon–nitrogen interactions are not accounted for and that
carbon–nitrogen interactions are essential in understanding global
terrestrial ecosystem productivity.