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. Nitrogen is an essential element controlling ecosystemcarbon (C) productivity and its response to climate change and atmospheric[CO2] increase. This study presents the evaluation – focussing ongross primary production (GPP) – of a new version of the ORCHIDEE modelthat gathers the representation of the nitrogen cycle and of itsinteractions with the carbon cycle from the OCN model and the most recentdevelopments from the ORCHIDEE trunk version. We quantify the model skills at 78 FLUXNET sites by simulating the observedmean seasonal cycle, daily mean flux variations, and annual mean average GPPflux for grasslands and forests. Accounting for carbon–nitrogen interactionsdoes not substantially change the main skills of ORCHIDEE, except for thesite-to-site annual mean GPP variations, for which the version withcarbon–nitrogen interactions is in better agreement with observations.However, the simulated GPP response to idealised [CO2] enrichmentsimulations is highly sensitive to whether or not carbon–nitrogeninteractions are accounted for. Doubling of the atmospheric [CO2]induces an increase in the GPP, but the site-averaged GPP response toa CO2 increase projected by the model version with carbon–nitrogeninteractions is half of the increase projected by the version withoutcarbon–nitrogen interactions. This model's differentiated response hasimportant 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 showsgood agreement with the observation-based MTE-GPP (model tree ensemble gross primary production) product for present-dayconditions. An attribution experiment making use of this new version ofORCHIDEE for the time period 1860–2016 suggests that global GPP hasincreased by 50 %, the main driver being the enrichment of land inreactive nitrogen (through deposition and fertilisation), followed by the[CO2] increase. Based on our factorial experiment and sensitivity analysis, we conclude thatif carbon–nitrogen interactions are accounted for, the functional responsesof ORCHIDEE r4999 better agree with the current understanding of photosynthesisthan when the carbon–nitrogen interactions are not accounted for and thatcarbon–nitrogen interactions are essential in understanding globalterrestrial ecosystem productivity.

geoscientific model development

Accounting for carbon and nitrogen interactions in the global terrestrial ecosystem model ORCHIDEE (trunk version, rev 4999): multi-scale evaluation of gross primary production