Effects of chronic nitrogen additions on nitrogen cycling in a high-elevation spruce–fir stand

Abstract

A 3-year low-level (15–31 kg N•ha−1•year−1) fertilization treatment was conducted in a high-elevation spruce–fir stand, on Mount Ascutney, Vermont. Shortly after fertilization, large concentrations (≤ 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 the 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 of N applied to a site and throughfall N concentration of the site. Increased Basal Area (BA) growth was recorded using two separate techniques, with the greatest increases in living BA occurring on the 25.6 kg N•ha−1year−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 mineralization rates, suggesting a possible C limitation. These results indicate that these slow growing, nutrient conserving ecosystems are responsive to even small increases in N inputs.