Ectomycorrhizal Fungi and the Enzymatic Liberation of Nitrogen from Soil Organic Matter: Ecosystem Processes and Underlying Molecular Mechanisms

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The rate at which plants can photosynthesize, thereby capturing carbon from the atmosphere, is limited by a number of factors including the availability of nitrogen (N) in soil. A century of research has presumed that forest trees can exclusively use inorganic nitrogen (i.e., ammonium and nitrate) to fuel their growth. However, recent work has suggested that plants may have access to additional types of nitrogen in the form of more complex organic molecules (organic N) also present in the soil. The key to this story is that nearly all forest trees form symbiotic relationships with fungi that dwell on plant roots below ground. These fungi act to extend the root system of plants and increase plant access to limiting nutrients, including organic N. This research will explore the capacity for certain groups of fungi to provide forest trees with organic forms of nitrogen. More specifically, this work will explore the unknown capacity of such fungi to decompose soil organic matter using enzymes, thereby releasing organic N in soil organic matter for plant uptake. The broad goal of this research project, then is to understand the extent which these root-associated fungi are liberating organic N in soil for use by trees. This project will also provide K-12 teachers with age-appropriate teaching tools to build students' understanding of forest ecology. Recent global level modeling efforts indicate that trees that associate with ectomycorrhizal fungi mutualists will be fertilized by increasing concentrations of atmospheric CO2. Other evidence suggests that the large stocks of C in forest soils are a result of the enzymatic activity of ectomycorrhizal (ECM) fungi. Both models rest upon the hereto untested general claim that ECM fungi mobilize N from the large pools of soil organic matter (SOM). However, direct evidence that ECM fungi can function in this manner is lacking. Understanding the extent to which these organisms obtain organic N by degrading SOM is contingent on whether genes encoding enzymes that mediate both the decay of plant detritus and SOM, were retained during their evolutionary history and are deployed when in symbiosis with plant roots. Multiple lines of indirect evidence, stemming from whole genome sequencing and field data indicate that only certain evolutionary lineages of ECM fungi have the potential to provision their plant hosts with N that is obtained from SOM. Resolving the saprotrophic physiology of ECM fungi across a range of spatial scales and soil N conditions, holds promise to resolve the ecological forces governing where we may expect plants to commonly obtain growth-limiting N from SOM via the activity of their ECM mutualists. Accordingly, this project will use a synthetic whole-plant system to measure the enzymatic expression of genes encoding lignocellulolytic enzymes across a wide phylogenetic range of ECM fungi, and will also trace organic N into intact plants. After validating this mechanism in the laboratory, the occurrence of this phenomenon will be tested using meta-transcriptomic enabled field approaches. Overall, the research will span gradients in soil N availability occurring at both the local and ecosystem level scales to improve understanding of the factors that structure the distribution of ECM fungi that obtain N from SOM.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.