During the early time after transplanting, the ability of nitrogen (N) uptake in roots of tree stocks is crucial to their future performance. However, it is not clear about the relationship between size or nutrient reserves in nursery-cultured tree seedlings and root N flux (N flow within or out of root) after transplanting. In this study, we pre-fertilized Changbai larch (Larix olgensis Henry.) seedlings with three top-dress applications [N-phosphorus (P)-potassium (K), 20-4-10] at rates of 0 (control), 31.58 (E-Low), 200 (E-Medium), and 525.08 (E-High) kg N ha(-1) to different levels of nutrient storage during the first growing season. After being graded into large and medium sizes, stock N status was indicated to be discriminative in response to pre-fertilization by vector analysis. Overwinter stocks were transplanted into aerated distilled water or nutrient solution to detect root N influx or N efflux, respectively, at the second and fourth weeks under laboratory conditions. Larger stocks had higher net N influx and efflux rates. In large stocks, N influx tended to increase within one month's time in the E-Medium and E-High treatments, whereas N efflux tended to rise as well. In pre-transplanted stocks, root biomass linearly correlated with both N influx and N efflux, but root N concentration only correlated with N influx for large stocks. In conclusion, proper nursery fertilization could induce seedling N reserves, which would result in a higher N influx rate for larger stocks, whose roots, however, would also risk more N efflux in humid but infertile sites during the early time after planting.