TREE SPECIES EFFECTS ON NITROGEN CYCLING AND RETENTION: A SYNTHESIS OF STUDIES USING 15N TRACERS

摘要

The forests of the Catskill Mountains in New York receive some of the highest rates of nitrogen (N) deposition in the northeastern United States, and many watersheds are beginning to show signs of N saturation. The watershed export of N is variable, despite relatively uniform N deposition. I tested the hypothesis that tree species vary in their influence on forest N retention and loss. A laboratory study showed that tree species influenced soil microbial biomass and organic soil C:N, but that variation in these properties cannot explain differences in potential net mineralization, net nitrification, or microbial uptake of 15N. A greenhouse experiment showed that sugar maple seedlings take up more NH4+ than NO3", while beech seedlings take up more NO3" than NH41. Results from a 300-day 13N experiment in the field showed that most of the N deposited onto forests is retained within the forest floor, and the magnitude of N retention varied among tree species. Sugar maple stands retained the least N, and red oak stands retain the most. A fertilization experiment showed that red oak stands are most likely to have the greatest decrease in N retention if availability increases in the future. Sugar maple stands currently export substantially more N than the other forests, in part due to a limited capacity to take up NO3". Tree species composition can be a strong regulator of forest N retention, but differences among species may depend on N inputs. Future research needs to address the interacting effects of increased N inputs and changes in tree species composition, as nonlinear effects may likely occur in biogeochemical cycles.