Predicting Forest N Dynamics Using Ecosystem-Scale 15-N Tracers

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9408794 Nadelhoffer The goal of this project is to test and improve computer models of forest nutrient cycles by incorporating information from on-going studies in Maine and Massachusetts forests using a naturally occurring, non-radioactive isotope of nitrogen (15N) as a tracer. These 15N tracer studies are unique in that they employ tracers at large scales--either on large plots with intact, mature trees, or in entire forested watersheds (10 hectares). Our use of 15N tracers at ecosystem scales has allowed us to measure the importance's of different vegetation (e.g. leaves, wood, tree species) and soil (mineral soil, forest floor, litter) pools as "sink" for anthropogenic N inputs to forest. As a result of these experiments, the distributions of 15N in our forest plots and experimental catchment now differ markedly from 15N distributions in natural forest. As nitrogen continues to cycle within these forests, the 15N tracers will continue to be redistributed even though no additional 15N will be added. This poses a unique opportunity to test and refine simulation models of forest ecosystems that are being used to predict responses of forests to a changing climate and chronic atmospheric N deposition that is now occurring downwind of industrialized regions of the northern hemisphere. Improving these models using information derived from our large-scale tracer studies will lead to a better understanding of the interactions between nutrient cycles, forest growth and ecosystem drainage water quality and to better predictions of forest ecosystem responses to changes in climate and air quality.