N-15 PARTITIONING AND PRODUCTION OF N-15-LABELED LITTER IN BEECH TREES FOLLOWING [N-15]UREA SPRAY

The leaves of 10-year-old beech trees grown in a plantation were spray ed once in late summer in 1993, 1994 and 1995 with [N-15]urea, to dete rmine the N-15 utilization by beech (Fagus sylvatica L.) and to obtain homogeneous N-15 labelled litter. One day after spraying, leaves had incorporated 42 % (1993) to 55 % (1995) of the applied [N-15]urea. The leaf amino acid content and N concentration increased shortly after a pplication. During leaf senescence, approximately 88 % of the incorpor ated N-15 was translocated and mainly stored in the above-ground bioma ss. After spraying, N concentration and N-15 enrichment of leaves were measured until abscission. In spring, trees were sampled and N-15 all ocation in above-and below-ground organs were determined to assess N-1 5 partitioning. Buds and bark showed the highest N-15 enrichment, but the largest amounts of foliarly applied N-15 were stored in bark and w ood. N-15 enrichment of fallen leaves (i.e. litter) increased after ea ch N-15 urea spray, from 2.11 % N-15 atom excess in 1993, to 2.97 % N- 15 in 1994 and 3.14 % N-15 in 1995. Annual litter contained 4.7 %, 7.3 % and 7.8 % of the sprayed N-15. Soluble and insoluble N fractions sh owed an identical N-15 atom excess indicating a homogeneous distributi on of N-15 in the labelled leaves as well as litter. ((C) Inra/Elsevie r, Paris.).