What principles govern plant nitrogen uptake responses to changes in CO2, climate and nitrogen deposition?

Land surface models and dynamic global vegetation models have been developed to reflect the vegetation response to changes in atmospheric CO2 and climate, and recent developments have added the nitrogen cycle to many of these models. The plant response to the new coupled carbon-nitrogen cycle in these models, however, is still very under-developed with little connection to ecophysiological understanding. Here, we describe how plant nitrogen acquisition can be physiologically-based and globally-robust in these models, and how this affects vegetation responses to changes in CO2, climate and nitrogen deposition. The model is validated with data from a wide range of sites, including four FACE sites, three sites from a nitrogen fertilization experiment in the Peruvian Andes, and two more sites in Europe. We use these data to compare model responses to the experimental manipulations to test if the model can adequately capture the ecosystem responses to changes in CO2 and nitrogen addition. Finally, we show the global effect of implementing this model into five dynamic global vegetation models (HYLAND, LPJ, ORCHIDEE, SDGVM, TRIFFID).