Ecosystem-level Consequences of Mutualist Partner Choice in Alder across a Forest Successional Sequence in Interior Alaska

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Most nitrogen-fixing plants form mutualistic relationships between their roots and microorganisms in the soil. This involves the use of plant sugars to support bacteria that capture atmospheric nitrogen gas into the roots of the plant, as well as to support fungi that take up soil phosphorus and also move it into the roots. This study proposes that coordinated changes in host selection for both partners by the tree, red alder, enables persistence of this nitrogen-fixing species as interior Alaskan forests develop over time after disturbances such as fire, and regulates the overall nitrogen inputs to these forests. The identity and physiology of the bacterial and fungal partners will be measured over an age sequence of forest stands. Then shifts in the microbial partners in stands that have been fertilized will be documented and used to help design partner choices for a set of experiments in a greenhouse. Physiological consequences of these shifts for nitrogen as measured in the greenhouse will be scaled back to the ecosystem using measurements of the mass of root nodules in combination with seasonal patterns of measured nitrogen fixation. This project will be the first to study the influence of mutualist partner choice on nitrogen fixation inputs at the ecosystem scale. Understanding controls over nitrogen fixation by native and invasive plants is a problem of fundamental scientific value in Alaska, where invasive nitrogen fixing trees are beginning to appear, and where the range of alders has expanded substantially across the boreal/tundra boundary over the past 150 years. The project has strong ties to the Bonanza Creek Long-term Ecological Research program, where education and public outreach activities will include components of this study. This project will also result in the training of one post-doctoral fellow, two graduate students, and six undergraduates in state-of-the art field, molecular and bioinformatics methods.