Aims: We simultaneously quantified the fate of soil absorbed mineral nitrogen in different tree compartments along with nitrogen immobilization by microorganisms during spring in 20-year-old oak trees. Methods: A soil-applied(15)N solution was traced in situ into the fine roots, medium roots, xylem, phloem, branches, leaves, extractable soil, and microbial biomass before and after budburst, until LAI maximum was reached. Results: During the three weeks following the labeling, around half of the(15)N applied was incorporated into the microbial biomass while the leafless trees absorbed less than 10% of the(15)N. Before and after budburst, the microbial compartment was the main pool of(15)N, and yet the soil-absorbed(15)N still significantly contributed to the leaf nitrogen pool at budburst. This contribution in leaves sharply increased in the days following budburst. Conclusion: The potential competition for mineral nitrogen between trees and the soil microbial biomass is strong during spring. The dependence of the leaf nitrogen pool to internal or external stocks of nitrogen at budburst could be conditioned by environmental conditions.