This study measured the amount of uptake of labeled nitrogen (15N) of lodgepole pine (Pinus contorta Dougl. var. latifolia Engelm) seedlings, at three different phenological stages, in a growth chamber experiment. Thirty days after 15N application, the amount of 15N recovered in seedlings as a percentage of the total 15N fertilizer applied was 4% in early spring, 43% in summer and 33% in autumn. The total 15N recovered in the plant–pot system ranged from 80 to 96%, and is higher than reported in other studies. Total 15N recovered from the pot compartment alone ranged from 48 to 95%, suggesting that substantial pools of N remain in the soil. Results suggest that low 15N uptake in the spring was associated with limited development of new root as a result of low spring soil temperatures. The lack of unsuberized roots in spring could be a key factor decreasing the effectiveness of early spring fertilization in the boreal forest.