Birch, willow, and other shrubs in the Arctic are increasing in response to warming at the expense of grasses and sedges. This increased shrub cover may enhance winter snow capture, increase winter soil temperatures, deepen summer soil thawing, and increase the release of old carbon and nitrogen. The increased flux of sugars from shrubs to shrub-associated symbiotic fungi (termed mycorrhizal fungi) could stimulate decomposition of thawing organic matter and release organic nitrogen for plant uptake previously locked up for millennia. This mycorrhizally-mediated priming of decomposition may influence carbon balances and nutrient availability for further shrub growth. To test this, researchers will use long-running experiments in Arctic Alaska that are examining plant responses to warming in different tundra types. Researchers will use natural levels of carbon and nitrogen isotopes to track the incorporation of new soil-derived organic nitrogen into shrubs and associated mycorrhizal fungi. With the new methodologies developed, organic nitrogen use can be assessed across ecosystems. Models derived from planned measurements can then predict consequences of shrub expansion for ecosystem processes, such as soil energy balances, permafrost melting, and the release and cycling of old soil organic matter. Archived samples of fungi and plants from Alaska and across the Arctic will expand the temporal and geographic relevance of the studies. The project will provide education and research opportunities to undergraduate and graduate students. Researchers will also work with local high school teachers to use fungi as a tool to teach biology and to expand their work in outdoor classrooms.