More than a quarter of the current anthropogenic CO₂ emissions are taken up by land, reducing the atmospheric CO₂ growth rate. This is because of the CO₂ fertilization effect which benefits 80 % of global vegetation. However, if nitrogen and phosphorus nutrients cannot keep up with increasing atmospheric CO₂, the magnitude of this terrestrial ecosystem service may reduce in future. This paper implements nitrogen constraints on photosynthesis in a model to understand the mechanisms involved.