The mycorrhizal associated nutrient economy hypothesis: New formulations, context dependencies and potential applications

Richard P. Phillips, Elizabeth Huenupi, Ashley K. Lang and Sierra B. Perez, Department of Biology, Indiana University, Bloomington, IN, Katilyn V. Beidler, Biology, Indiana University, Bloomington, IN, Christopher Blackwood, Department of Biological Sciences, Kent State University, Kent, OH, Edward R. Brzostek, Department of Biology, West Virginia University, Morgantown, WV, Matthew E. Craig, Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, David M. Eissenstat, Intercollege Graduate Degree Program in Ecology, The Pennsylvania State University, University Park, PA, Joshua B. Fisher, Joint Institute for Regional Earth System Science and Engineering, University of California, Los Angeles, CA, Insu Jo, Forestry and Natural Resources, Purdue University, West Lafayette, IN, Adrienne B. Keller, Dept. of Ecology, Evolution, and Behavior,, University of Minnesota, St Paul, MN, Stephanie Kivlin, Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Knoxville, TN, Guigang Lin, Institute of Applied Ecology,, CAS Key Laboratory of Forest Ecology and Management, Chinese Academy of Sciences, Shenyang, China, Ryan M. Mushinski, School of Public and Environmental Affairs, Indiana University, Bloomington, IN, Jonathan Raff, Department of Chemistry, Indiana University, Bloomington, IN, Kurt A. Smemo, Environmental Studies and Sciences, Skidmore College, Saratoga Springs, NY, Nadia Soudzilovskaia, Environmental Biology Department, Leiden University, Leiden, Netherlands, César Terrer, Lawrence Livermore National Lab, Livermore, CA

Background/Question/Methods
Nearly all plant species associate with a single type of mycorrhizal fungi, and there is mounting evidence that the type of fungi that a plant associates with gives rise to a constellation of nutrient use traits that influence ecosystem functioning. This idea forms the basis of the mycorrhizal associated nutrient economy (MANE) hypothesis, which predicts that trees and their associated microbes possess an integrated suite of stoichiometrically-constrained traits that both reflect and determine biogeochemical variation across landscapes or within regions. The MANE hypothesis was first described in 2013 and since its publication, the hypothesis has generated multiple new lines of inquiry, leading to new formulations, a greater appreciation of context dependencies and potential applications. We investigated the body of evidence that supports (or does not support) the MANE hypothesis, examined how MANE dynamics have been incorporated into models and evaluated the utility of MANE as a framework for predicting and understanding global change impacts.
Results/Conclusions
Overall, we found stronger evidence for the MANE hypothesis in temperate forests relative to boreal and sub-tropical/tropical forests, and stronger support for MANE in eastern relative to western forests of the US. The response variables that most consistently tracked the relative abundance of AM vs. ECM trees were soil variables, including N cycling, C:N, pH and microbial community composition. We found support for the idea that mycorrhizal associations are at least partially responsible for observed differences in soil properties (e.g., based on common garden studies), yet also found evidence that environmental filtering and tree-mycorrhizal reinforcement of syndromes may be common in “natural” forests. Dominant mycorrhizal association were more important for predicting forest sensitivity to elevated CO2, N deposition and invasive species relative to forest sensitivity to warming and drought. Given reports that MANE dynamics can be detected by remote sensing and incorporated into Earth System Models, we found support for the framework as a useful tool for predicting ecosystem impacts to global change. Finally, we identified key knowledge gaps pertaining to MANE, including the need for: 1) improved quantification of the costs/benefits of mycorrhizal-mediated nutrient uptake, 2) better understanding of how belowground processes affect soil organic matter formation, stabilization and turnover, and 3) enhanced knowledge about how mycorrhizal community composition and mycorrhizal colonization levels can affect MANE dynamics.