Interactions of Carbon Cycling, Nitrogen Cycling, and Nitrogen (N) Stable Isotopes in Plants as Affected by Mycorrhizal Fungi, N Availabilty and N Form

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ABSTRACT: Nitrogen (N) availability often limits plant growth in forestry and agriculture, while excess N in atmospheric deposition increasingly harms forests throughout the US. Mycorrhizal fungi are key plant symbionts and directly influence plant growth and N cycling through the uptake, retention, and transfer of soil-derived N. At present, mycorrhizal symbioses are poorly understood. Consequently, they are largely absent from conceptual pictures of forest functioning and ecosystem models used to predict forest growth and stress responses. Foliar 15N:14N ratios appear useful markers of carbon and N allocation in the symbiosis, but we have limited understanding of biochemical mechanisms regulating 15N:14N distributions. We will measure complete carbon and N isotope budgets in mycorrhizal Pinus sylvestris seedlings supplied with different N forms, at different N availabilities, and with different fungal symbionts. Photosynthesis, belowground respiration, and aboveground respiration will also be quantified. We will also investigate the underlying biochemical mechanisms causing 15N:14N patterns using data from mass spectrometry and nuclear magnetic resonance measurements. From these experiments in culture, we will be poised to (1) measure carbon allocation to mycorrhizal fungi in ecosystems under different environmental conditions, (2) use foliar 15N:14N measurements to indicate allocation to mycorrhizal fungi, and (3) explicitly incorporate mycorrhizal fungi into ecosystem-level models. BROADER IMPACTS: This work will improve understanding of what regulates forest productivity and the functioning of mycorrhizal symbioses under anthropogenic N deposition. There will be at least 3 graduate students trained on this grant.