Elevated pCO 2 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 ₂ partial pressure ( pCO ₂ ) 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 ₂ . The N-fluxes within the plants were estimated by comparing the fate of newly acquired ¹⁵ N-NO ₃ ^- 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 ¹⁵ 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 ₂ increased total biomass and the root : shoot ratio at deficient N-supply, but had no effect at sufficient N-supply. Elevated pCO ₂ 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 ₂ also enhanced the allocation of newly acquired ¹⁵ N to the youngest leaves. Root nitrate reduction as a possible factor controlling the root : shoot ratio and N-allocation is discussed.