Evaluation of the Nitrogen Cycle in the Land Surface Model of CNRM

Earth system models (ESMs) need to include nutrient limitation to predict realistic land carbon (C) uptake. In particular, Nitrogen (N) is a critical nutrient, that controls photosynthesis and decomposition processes. We have implemented an explicit N cycle in the land component of the CNRM-ESM model. Here we present an evaluation of the model using data from the Free-Air CO2 Enrichment (FACE) experiments conducted over 10 years in North America. We compare the reference version of the model without the N cycle (C) to the new version in which it is included (CN). We investigate the response of the Net Primary Prodcution (NPP) to elevated CO2 and confront our results to a multi-model analysis carried out on these two sites. Our results fall in the inter model range. We show that the CN version of the model performs better than the C version because NPP is reduced by N limitation. In the literature, diverging strategies are observed to overcome N limitation. We analyse the simulated N dynamics and show that the model reproduces well the main features but fails to represent some sites characteristics.