In a first experiment two year old trees of 12 species (Acacia melanoxylon, Acacia dealbata, Eucalyptus debeuzevillei, Eucalyptus niphophila, Eucalyptus pauciflora, Quercus ilex, Quercus robur, Fagus sylvatica, Alnus glutinosa, Pinus sylvestris, Picea abies and Pseudotsuga menziesii) were exposed for several months in climate chambers to actual (380ppm) and elevated (1000ppm) CO₂ concentrations. In a second experiment sprouts of Chenopodium album were exposed for five weeks on a FACE system to CO₂ concentrations of 400ppm and 510ppm. Plants were watered daily with nutrient solution containing the following nitrogen concentrations: 0, 1, 5 and 25mmol l^-1.

At the start and end of the exposition periods dry weight, carbon and nitrogen concentration of shoot, leaves, roots and fine roots were measured to calculate nitrogen uptake and allocation of nitrogen and carbon. During the exposition gas exchange measurements were carried out to calculate photosynthesis, respiration and the regulation of stomatal conductance. The concentrations of carbohydrates and nitrogen in leaves and whole plants were used to characterize the plant nitrogen status.

Plants grown under elevated CO₂ contained higher concentrations of soluble carbohydrates and starch, while their nitrogen concentration was lower. This was interpreted as a shift of the nitrogen status towards the carbon budget.

At the same time they achieved higher dry mass and contained more nitrogen. The increase of dry mass was almost proportional to the increase of nitrogen content, with the strongest increase in plants with intermediate nitrogen nutrition. A general limitation of the CO₂ nutrition effect by low nitrogen nutrition was not observed, with the exception of nitrogen-free nutrient solution.

All investigated processes depended on the nitrogen status of the plants. The dependence of photosynthesis and respiration was almost equal among different species; nitrogen uptake, allocation and regulation of stomatal conductance were highly specific for different species.

With the exception of respiration, the interrelation of processes and nitrogen status was independent of the way the nitrogen status was varied: either by different nitrogen nutrition or CO₂ concentration. So the influence of elevated CO₂ on the processes could be explained by the influence of elevated CO₂ on the nitrogen status.

The influence of elevated CO₂ on the nitrogen status lead to enhanced nitrogen investment to processes involved in nitrogen uptake and assimilation and less investment to processes involved in carbon assimilation. This influence of high CO₂ on the nitrogen budget was important for the CO₂ fertilization effect, because the CO₂ fertilization effect was almost proportional to the increase of nitrogen uptake.

Process-based models to predict the CO₂ fertilization effect should consider the influence of elevated CO₂ on the nitrogen status, because all investigated processes depended on the nitrogen status of the plants. With the exception of respiration, it is concluded that the current models are adequate to model plant growth in elevated CO₂, if they consider the influence of elevated CO₂ on the nitrogen status.