EFFECTS OF CHRONIC NITROGEN ADDITIONS ON NITROGEN CYCLING IN A HIGH-ELEVATION SPRUCE-FIR STAND

A 3-year low-level (15-31 kg N . ha-1 . year-1) fertilization treatmen t was conducted in a high-elevation spruce-fir stand, on Mount Ascutne y, Vermont. Shortly after fertilization, large concentrations (less-th an-or-equal-to 900 ppm) of both NH4-N and NO3-N were recovered in ion- exchange resin bags buried at the base of the forest floor. Despite an initial loss of added N, we found significant correlations between th e amount of fertilizer applied and measured ecosystem parameters. Bulk deposition for the plots equalled 5.1 kg N.ha-1-year-1, of which 50% fell as NO3-N in snow. No correlations were found between the amount o f N applied to a site and throughfall N concentration of the site. Inc reased Basal Area (BA) growth was recorded using two separate techniqu es, with the greatest increases in living BA occurring on the 25.6 kg N.ha-1.year-1 treatment. As N fertilization increased, foliar %N, net forest floor net N mineralization, dead BA, total (living + dead) BA, first year net N mineralization, and coniferous and deciduous litter % N also increased. On plots receiving high rates of fertilization, net N mineralization rates remained constant or decreased during the third year while low N addition plots experienced increased net N mineraliz ation rates, suggesting a possible C limitation. These results indicat e that these slow growing, nutrient conserving ecosystems are responsi ve to even small increases in N inputs.