A new version of ORCHIDEE-GM with coupled carbon-nitrogen-phosphorus cycles: parameter calibration and model evaluation.

The process-based biogeochemical model ORCHIDEE-GM is a version of ORCHIDEE land surface model that includes the grassland management module from PaSim. Accounting for the management practices such as mowing, livestock grazing and fertilizer application on a daily basis, ORCHIDEE-GM proved capable of simulating the dynamics of leaf area index, biomass, and C fluxes of managed grasslands. The previous versions of ORCHIDEE-GM did not include a full nitrogen cycle. The positive effect of nitrogen fertilizers on grassland photosynthesis rates was parameterized with an empirical function calibrated from literature estimate. In this study, ORCHIDEE-GM was merged into the ORCHIDEE-CN-P model, a version with coupled carbon-nitrogen-phosphorus cycles for terrestrial ecosystems. The new ORCHIDEE-GM model is capable to simulate the carbon, nitrogen and phosphorus fluxes among atmosphere, plant, livestock, and soil of managed grasslands. With some of its parameters calibrated, the new model was then evaluated at several grassland sites against eddy covariance fluxes, biometric measurements, and nitrogen-related measurements