We quantified the distribution of nitrogen (N), dry-matter (biomass) and of soil-applied (15) N in tree and soil compartments in five naturally-growing 20-year-old oak trees. After applying (15) N solution to soil at the base of the trees in spring, all the trees were felled in the fall, their root system excavated, biomass, nitrogen and (15) N content measured in all compartments. Xylem rings-compartment contains most biomass (47%) while branches and coarse-root contains most nitrogen (29% and 14% respectively). The labelled (15) N absorbed throughout the vegetation season, was found in all compartments except the heartwood. The majority of recovered (15) N was in the leaves (24%). Some often overlooked compartments (coarse root, stump, xylem and other branches) together recovered 45% of the (15) N. (15) N was found in all the sapwood rings, from the ring formed in the current year up to 10 year-old rings, marking the limit of the heartwood. More (15) N was found in the younger rings compared to older rings. The (15) N allocated to ancient rings can originate from different, non-mutually exclusive, sources: whether directly from the soil via the (15) N uptake throughout the vegetation season and transport in the xylem sap, from the autumnal resorption of (15) N first allocated to the leaves, or from the (15) N mobility once allocated to the forming ring to older rings through ray parenchyma. With about 6% of the initial (15) N retrieved in the microbial biomass at the end of the growing season, we confirmed the role of microbial biomass as forest nitrogen sink.