Direct canopy nitrogen uptake from 15N-labeled wet deposition by mature red spruce

Abstract

We applied 15N-labeled ammonium and nitrate to individual branches of red spruce (Picearubens 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 15N 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 15N 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.