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-year-old sessile oaks were sup15/supN labelled by spraying sup15/supNHsub4/subsup15/supNOsub3/sub: (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 sup15/supN in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of sup15/supN into the soil–tree system; more than 30?% of the administered sup15/supN was recovered. Analysis of the partitioning clearly revealed that in autumn, roots' N reserves were formed from foliage sup15/supN (73?%) and to a lesser extent from soil sup15/supN (27?%). The following spring, sup15/supN used for the synthesis of new leaves came first from sup15/supN stored during the previous autumn, mainly from sup15/supN reserves formed from foliage (95?%). Thereafter, when leaves were fully expanded, sup15/supN uptake from the soil during the previous autumn and before budburst contributed to the formation of new leaves (60?%).
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