Contribution of previous year's leaf N and soil N uptake to current year's leaf growth in sessile oak

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 ¹⁵N labelled by spraying ¹⁵NH₄¹⁵NO₃: (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 ¹⁵N in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of ¹⁵N into the soil–tree system; more than 30 % of the administered ¹⁵N was recovered. Analysis of the partitioning clearly revealed that in autumn, roots' N reserves were formed from foliage ¹⁵N (73 %) and to a lesser extent from soil ¹⁵N (27 %). The following spring, ¹⁵N used for the synthesis of new leaves came first from ¹⁵N stored during the previous autumn, mainly from ¹⁵N reserves formed from foliage (95 %). Thereafter, when leaves were fully expanded, ¹⁵N uptake from the soil during the previous autumn and before budburst contributed to the formation of new leaves (60 %)