- Lovett, Gary M.
Cary Institute of Ecosystem Studies
The effect on forests of excess nitrogen from air pollution is complicated because nitrogen is an essential nutrient for trees, but excessive amounts can be toxic. In this study we re-sampled the trees and soil in research plots that had been receiving added nitrogen for 13 years. The study included six replicate plots dominated by each of 5 species, located in the Catskill Mountains of NY State. Nitrogen was added in a granular form at a rate of 50 kg N/hectare/year; for each fertilized plot there was a paired control plot that received only ambient nitrogen deposition. Even though nitrogen is often considered a limiting element for forests in this region, we found that the trees had only a moderate uptake of the extra nitrogen, as reflected in the nitrogen concentration in their foliage. We also found that soil microbial processes such as soil respiration and nitrogen mineralization were inhibited by the added nitrogen. While the decrease in microbial action should eventually lead to a greater buildup of the forest floor, we did not observe an increase in forest floor mass in this study. All of these responses varied individualistically among the different tree species. We conclude that in the Catskill region the trees are probably less limited by nitrogen than is commonly assumed, but that nitrogen deposition can alter key microbial processes that may eventually lead to increased mass and carbon storage in the forest floor. Policy and management decisions regarding nitrogen pollution should factor in the varying responses of different tree species, and simulation models that predict the effect of nitrogen deposition on the Northern Forest should take individual tree species into account in order to make accurate predictions.