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 youngest foliage and youngest twigs, and twig ammonium assimilation exceeded nitrate assimilation. Approximately 5% of the ammonium and 1% of the nitrate applied to each branch was assimilated; because of throughfall interactions with multiple branches, canopy assimilation rates are expected to be 3-6 times larger. Twig N-15 levels exceeded foliar levels for the younger age-classes in the ammonium-labeled treatments, suggesting that twigs play an important role in ammonium assimilation. Comparisons of these results with data from trees that assimilated N-15 through their roots showed that the pattern of canopy N assimilation differs from root assimilation, primarily by the assimilation of large amounts of N by twigs. Our results directly demonstrate for the first 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 high-elevation forest. While canopy N assimilation may thus reflect only a minor anthropogenic alteration of N acquisition in these forests, the long-term fate of this N needs to be determined.