Aaron S. David, Khum B. Thapa‐Magar, Michelle E. Afkhami

A key challenge to understanding microbiomes and their role in ecological processes is contextualizing their effects on host organisms, particularly when faced with environmental stress. One influential theory, the Stress Gradient Hypothesis, might predict that the frequency of positive interactions increases with stressful conditions such that microbial taxa would mitigate harmful effects on host performance. Yet, equally plausible is that microbial taxa could exacerbate these effects. Here, we introduce the "Mitigation-Exacerbation Continuum" as a novel framework to conceptualize microbial mediation of stress. We (1) use this continuum to quantify microbial mediation of stress for six plant species, and (2) test the association between these continuum values and natural species abundance. We factorially manipulated a common stress (allelopathy) and the presence of soil microbes to quantify microbial effects in benign and stressed environments for two critical early life-history metrics, seed germination and seedling biomass. Although we found evidence of both mitigation and exacerbation among the six species, exacerbation was more common. Across species, the degree of microbial-mediated effects on germination explained >80% of the variation of natural field abundances. Our results suggest a critical role of soil microbes in mediating plant stress responses, and a potential microbial mechanism underlying species abundance. This article is protected by copyright. All rights reserved.