We investigated N source for shoot growth in 50-year-old sessile oak trees using a long term continuous in situ 15N labeling of soil N during bud burst. The recovery of 15N in the growing tissues of sun shoots (leaves, twigs and buds) and in the phloem sap was quantified weekly in order to assess the contribution of soil N uptake to spring growth. During the first 2 weeks of bud burst, remobilized N contributed to more than 90% of total N of the developing leaves and twigs. N uptake from soil started concomitantly to N remobilization and contributed to a small extent to bud burst. The fraction of new N in the total N increased markedly in plant tissues when bud burst was achieved and reached 27% in fully expanded leaves and 18% in developed twigs. In the phloem sap, 15N tracer appeared early after the first labeling and increased until the end of bud burst then decreased at full leaf expansion in June. Among shoot compartments, leaves were the major sink for newly assimilated N, accounting for 80% of new N in shoots whereas twigs and newly formed buds accounted for 17% and 3% respectively. New N partitioning to leaves increased from unfolding to full expansion whereas total N concentration decreased. Our results underline the crucial role of stored N for rapid leaf growth and sustained growth of oak trees. All factors reducing N storage in autumn may handicap spring shoot growth.