SG: Continental and Century Scale Assessment of Forest Nitrogen Cycling in the United States Using Dendroisotopes

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Nitrogen is one of the most important substances on Earth. Modern agriculture depends on it because it is the key component of fertilizer. However, much of the nitrogen used by humans can escape to water and the air, where it can cause problems for human and ecosystem health. Although nitrogen supply to ecosystems is increasing due to human activities, many ecosystems seem to have a high capacity to retain or remove nitrogen from harmful pathways. This project will identify which places have this capacity, and what is causing these patterns, by measuring long-term records of nitrogen cycling. Trees in most forests of the U.S. form annual growth rings that preserve the history experienced by that tree. A portion of nitrogen taken up by an individual tree is preserved in its wood, providing a record of past nutrient conditions in that place. This research will greatly expand the few individual data points that have been measured from nitrogen in wood, and it will extend the timeframe of study much longer than previous efforts. Further, this project will deliver benefits to society and the larger scientific community through outreach about tree ring research to middle school students, open access of the data and research findings, and increased understanding of how to better manage nitrogen so that it does not cause harm to people or ecosystems.This project will generate the first continental-scale sampling of coupled tree-ring growth and nitrogen isotope records by sampling trees at 47 sites across the United States. This will produce three key data sets about the nitrogen isotopic composition of wood and the terrestrial nitrogen cycle. The three research goals are: (1) to generate the first continental-scale reconstructions of long-term patterns of nitrogen availability, particularly testing whether forests are experiencing increasing or decreasing nitrogen availability, (2) to identify the key mechanisms and drivers of trajectories in nitrogen availability over the past century, especially slow processes that are difficult to study with other approaches, and (3) to accomplish three methodological advances for broader use of wood nitrogen isotopes.