9629842 Pregitzer Cycling of NO3-N in Northern Hardwood Forests: Regulation and Consequences of N Saturation A series of experiments will be conducted in Michigan sugar maple forests to determine the potential consequences of chronic atmospheric nitrate (NO3-) deposition. The experiments will utilize a tracer, the stable isotope 15N, to do studies to determine the immediate and long-term fates of added NO3-. The experiments will monitor processes important to forest health and productivity on study plots that receive four times the amount of normal atmospheric NO3--N inputs. The researchers will follow internal ecosystem cycling of the added NO3-, enabling the assessment of the susceptibility of Michigan sugar maple forests to N saturation from atmospheric nitrogen inputs. It is hypothesized that long-term NO3- additions will gradually increase N availability, altering above- and below-ground forest productivity. When the forest's capacity to assimilate added NO3- is finally exceeded, N saturation should occur, resulting in large leaching losses of NO3- and other nutrients such as calcium and magnesium, potentially leading to nutrient imbalances and decreased productivity. This Michigan sugar maple forest study will test these hypotheses and provide a more fundamental understanding of the internal mechanisms through which ecosystems regulate N saturation.
