Canopies of forest ecosystems frequently intercept and retain >50% of atmospheric nitrogen (N) deposition. Atmospheric N reaching the forest floor often is retained in litter and soil pools, but if atmospheric N retained by the canopy is assimilated through foliar uptake, then N inputs may have greater effects on tree N requirements and growth than suggested by N additions to forest floors alone. To quantify foliar and root uptake of N deposition by trees and to determine patterns of within-tree N allocation, we performed a tightly controlled 15N tracer experiment using constructed, seminatural forest mesocosms. Mesocosms contained seedling trees, which assimilated modest amounts of the aqueous 15NH4Cl applied to their canopies and surrounding soils (10% and 14%, respectively). The seedlings allocated most of the 15N that they took up to leaf biomass (40%–50%), regardless of uptake pathway. Soil pools retained >80% of the 15N applied to mesocosm forest floors, with soil organic matter concentration explaining 79% of the variation in soil 15N recovery across sampling depths. The low value for 15N movement into biomass and its retention in soil pools suggests that atmospheric N deposition measured at a nearby forest site has no short-term effects on tree N content or growth.