The mycorrhizal associated nutrient economy hypothesis: New formulations, context dependencies and research needs

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 new lines of inquiry, leading to new formulations, context dependencies and potential applications. We investigated the body of evidence that supports (or does not support) the MANE hypothesis and evaluated the utility of MANE as a framework for predicting and understanding global change impacts on forests. We found stronger support for MANE in temperate forests vs boreal and sub-tropical/tropical forests, and in eastern relative to western forests of the US. The response variables that most consistently track 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 based on common garden studies. Dominant mycorrhizal associations were also important for predicting forest sensitivity to elevated CO2, N deposition and invasive species. Given reports that MANE dynamics can be detected by remote sensing and incorporated into Earth System Models, the framework may serve 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.