Engelmann spruce forest responses to nitrogen deposition in northern Colorado

Abstract

Atmospheric nitrogen (N) deposition to high-elevation sites in northern Colorado is greater east of the Continental Divide compared to the west (3-5 vs. 1-2 kg N ha-1 y-). Three approaches were used to address the effects of elevated N deposition on high-elevation old-growth Englemann spruce forests: a fertilization experiment, comparative study, and modeling exercise. Ammonium-nitrate fertilizer was applied (25 kg N ha-1 y-1) at two sites, one west and the other east of the Continental Divide over four years. We hypothesized that due to differences in atmospheric N deposition, the western site response would be minimal to fertilization due to N limitation while the eastern site would respond rapidly. The eastern site responded rapidly to fertilization. Nitrogen cycling rates and estimated N leaching losses, in particular, increased dramatically. Western sites, as expected, showed little change in N cycling rates or leaching loss estimates. Six forested sites east and six forested sites west of the Continental Divide were compared. Higher N deposition sites had characteristics comparable to forests in the northeastern US and Europe that have been impacted by elevated N deposition. Current levels of N deposition, even though low by comparison to other regions, have caused measurable changes in forest biogeochemistry. These include increased soil and foliar N, decreased soil and foliar C:N ratios, and increased foliar N:Mg ratios and soil net mineralization rates. CENTURY, a biogeochemical cycling model, was used to address the sensitivity of Englemann spruce forests to alterations in climate, soil texture and N inputs. The objective was to simulate potential ecosystem responses to increased annual precipitation, altered soil texture and elevated N deposition, because while field data from the comparative study indicate atmospheric N deposition has affected Englemann spruce forest biogeochemistry, variation in precipitation and soil texture between sites were plausible alternative explanations for the observed differences among sites. Similarities between model results and field data indicate elevated N inputs, not dissimilarities in precipitation or soil texture, drove the observed contrasts between high and low N deposition sites.