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

A field study at a Rocky Mountain spruce-fir-pine forest was undertaken to obtain canopy N uptake (CNU), N reallocation, and foliar N requirement. Wet deposition, dry deposition, and throughfall fluxes of ammonium and nitrate were measured during the 2000 and 2001 growing seasons. Estimation of CNU, for both ammonium and nitrate, was obtained by subtracting throughfall (TF) flux from the sum of wet deposition (WD) and dry deposition (DD): CNU = WD + DD - TF. CNU efficiency (CNU/(WD + DD)) for ammonium (0.9) was consistent across 2000 and 2001. For nitrate, this efficiency was 0.8 and 0.7 for 2000 and 2001, respectively. Foliar N requirement for growth in 2000 and 2001 was about 19 and 22 kg N.ha(-1).year(-1), respectively. Growing season estimates of CNU for 2000 and 2001 were approximately 2 and 3 kg N.ha(-1), respectively. Thus, CNU may contribute 10%-15% of the foliar N requirement for canopy growth. Mountain upslope winds bring substantial amounts of anthropogenic N to this forest during the growing season, thereby contributing to CNU. Given that a sizable fraction of CNU is anthropogenic in origin, the forest's N cycle has likely undergone substantial changes on a decadal time scale.