Representation of the nitrogen cycle and its coupling with the carbon cycle in ISBA (SURFEX v9) the land surface model: evaluation using two Free-Air CO₂ Enrichment experiment sites

Abstract. Nitrogen (N) is a critical nutrient, that controls photosynthesis and decomposition processes. It is important to include the N cycle in the land component of climate models to improve the exchange fluxes of CO₂ between land and atmosphere. We present here the implementation of the N cycle in the CNRM land surface model, namely ISBA. We evaluate the model on two Free-Air  Enrichment (FACE), experiments sites: Duke and Oak Ridge. In particular, the response to elevated CO₂ is studied. We compare the reference version without the N cycle (C) and the new version in which it is included (CN). A comparison to a multi model analysis shows encouraging results, since the computed NPP and N assimilation flux fall in the inter model range. The CN version performs better than the C version for NPP. Next, we focus on the carbon cycle by confronting simulation results to observations. The CN version improves the carbon stocks, largely overestimated by the C version. In particular, at elevated CO₂, in the CN version, photosynthesis is downregulated by the N limitation. This yields a reduction of C accumulation in soil and biomass in comparison to the C version. In the literature, diverging strategies are observed to overcome N limitation. The model reproduces well the main features but fails to represent some sites characteristics. Finally, a detailed analysis of the simulated N dynamics is presented.