PARENT SESSION
Poster Session #27: Nutrient Cycling I.
Tuesday, August 6. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Canopy uptake of atmospheric nitrogen and new growth nitrogen requirement at a Colorado subalpine forest.

Tomaszewski, Timothy^*,1, Torizzo, Jonathan¹, Boyce, Richard², Sievering, Herman¹, ¹ University of Colorado at Denver, Denver, CO² University of Denver, Denver, CO

ABSTRACT- A field study at a Rocky Mountain spruce-fir-pine forest was undertaken to obtain growing season measurements of canopy nitrogen uptake (CNU) and annual N requirement for tree growth. Wet deposition (WD), dry deposition (DD), and throughfall (TF) fluxes of ammonium and nitrate were obtained during both the 2000 and 2001 growing seasons. CNU was estimated for ammonium and nitrate by subtracting TF flux from the sum of WD and DD: CNU=WD+DD-TF. The N required for twig and needle growth in 2000 and 2001 was 22 and 35 kg N ha^-1, respectively. Growing season estimates of CNU for each of these years was 2 and 3 kg N ha^-1,. Thus, CNU may contribute substantially to annual N growth requirement at this site. Low rates of net N mineralization, attributed to cold soils and low litterfall carbon to nitrogen (C/N) ratios, indicate the N supply from soil for canopy new growth is minimal. Comparison of new growth N to litterfall foliar N suggests reallocation of N may contribute as much as 60% of the annual new growth N demand. Thus, reallocation of N appears to be quite significant within this N limited system. Given that a sizable fraction of CNU at this site is anthropogenic in origin, the forest's natural N allocation pattern has likely undergone disruption. Additionally, carbon sequestration may be enhanced due to the increment in canopy available N that CNU represents.

KEY WORDS: Nitrogen, Reallocation, Uptake, Requirement