DIRECT CANOPY NITROGEN UPTAKE FROM N-15-LABELED WET DEPOSITION BY MATURE RED SPRUCE

We applied N-15-labeled ammonium and nitrate to individual branches of red spruce (Picea rubens Sarg.) to examine the importance of canopy N assimilation in the field. Levels of labeled N were highest in the yo ungest foliage and youngest twigs, and twig ammonium assimilation exce eded nitrate assimilation. Approximately 5% of the ammonium and 1% of the nitrate applied to each branch was assimilated; because of through fall interactions with multiple branches, canopy assimilation rates ar e expected to be 3-6 times larger. Twig N-15 levels exceeded foliar le vels for the younger age-classes in the ammonium-labeled treatments, s uggesting that twigs play an important role in ammonium assimilation. Comparisons of these results with data from trees that assimilated N-1 5 through their roots showed that the pattern of canopy N assimilation differs from root assimilation, primarily by the assimilation of larg e amounts of N by twigs. Our results directly demonstrate for the firs t time that canopy assimilation is a pathway for uptake of N in these high-elevation trees. Canopy assimilation of atmospherically deposited N may represent 2-8% of the total N requirement for spruce in the hig h-elevation forest. While canopy N assimilation may thus reflect only a minor anthropogenic alteration of N acquisition in these forests, th e long-term fate of this N needs to be determined.