Elevated pCO affects N-metabolism of young poplar plants (Populus tremula × P. alba) differently at deficient and sufficient N-supply.

•  The effects of N-availability and elevated atmospheric CO partial pressure ( pCO ) on growth, allometry and N-metabolism of poplar plants are reported here. •  Poplar plants were grown hydroponically at deficient and sufficient N-supply under ambient and elevated pCO . The N-fluxes within the plants were estimated by comparing the fate of newly acquired N-NO in plants either severely N-limited or with sufficient N-supply. •  At deficient N-supply, plants accumulated less biomass and exhibited an increased root : shoot ratio compared with sufficient N-supply; a larger fraction of newly acquired N was allocated to the youngest leaves immediately after exchange of the nutrient solution. Increasing the external N-supply from deficient to sufficient shifted the site of nitrate reduction from roots to leaves. •  Elevated pCO increased total biomass and the root : shoot ratio at deficient N-supply, but had no effect at sufficient N-supply. Elevated pCO decreased rates of N-uptake in both treatments. Increased root : shoot ratio at deficient N-supply coincided with enhanced nitrate reduction in the root and elevated pCO also enhanced the allocation of newly acquired N to the youngest leaves. Root nitrate reduction as a possible factor controlling the root : shoot ratio and N-allocation is discussed.