Direct and indirect effects of tree species on forest nitrogen retention in the Catskill Mountains, New York

The aim of this research was to understand the influence of tree species on nitrogen (N) retention and cycling within forests of the Catskill Mountains, NY. These forests receive some of the highest rates of N deposition in the northeastern United States and many watersheds are beginning to show signs of N saturation. However, watersheds vary in the amount of N export even though they receive approximately uniform amounts of N deposition. I hypothesized that tree species of the Catskill Mountains vary in their influence on forest N retention and loss. Results from a laboratory study (Chapter 1) show that while tree species influence soil microbial biomass and organic soil C:N, the variation in these two properties cannot explain differences in potential net mineralization, net nitrification, or microbial uptake of 15N. Therefore, properties other than microbial biomass and soil C:N need to be examined to understand the factors controlling microbial N transformations and potential retention in these forest soils. A greenhouse experiment (Chapter 2) shows that sugar maple and hemlock seedlings take up more NH₄ + than NO₃−, while beech seedlings take up more NO₃− than NH₄ +. Results from a 300-day field 15N experiment (Chapter 3) show that most of the N deposited onto forests is retained within the forest floor, but the magnitude of N retention varies among tree species. Sugar maple stands retain the least N, while red oak stands retain the most. The fertilized treatment shows that red oak stands are most likely to have the greatest decrease in N retention if availability increases in the future. These results suggest that sugar maple stands currently export significantly more N than the other forested stands and this can be explained by a combination of soil and plant processes. Sugar maples are not able to take up a significant amount of NO ₃− the form of N that is produced in the largest amount in their stands and is most susceptible to leaching losses. Results from this study suggest that tree species composition can be a strong regulator of forest N retention, and differences among species may change depending upon N inputs.