Abstract. The origin of N which contributes to the synthesis of N reserves of in situ forest trees in autumn and to the growth of new organs the following spring is currently poorly documented. To characterize the metabolism of various possible N sources (plant N and soil N), six distinct 20-yearold sessile oaks were 15 N labelled by spraying 15 NH 15 4 NO 3 : (i) on leaves in May, to label the N pool remobilized in the autumn for synthesis of reserves, (ii) on soil in the autumn, to label the N pool taken up from soil and (iii) on soil at the beginning of the following spring, to label the N pool taken up from soil in the spring. The partitioning of 15 N in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of 15 N into the soiltree system; more than 30 % of the administered 15 N was recovered. Analysis of the partitioning clearly revealed that in autumn, roots' N reserves were formed from foliage 15 N (73 %) and to a lesser extent from soil 15 N (27 %). The following spring, 15 N used for the synthesis of new leaves came first from 15 N stored during the previous autumn, mainly from 15 N reserves formed from foliage (95 %). Thereafter, when leaves were fully expanded, 15 N uptake from the soil during the previous autumn and before budburst contributed to the formation of new leaves (60 %).