Globally Consistent Drivers of Plant Microbiome Diversity Across Hosts and Continents

All multicellular organisms host a diverse microbiome composed of microbial pathogens, mutualists, and commensals, and changes in microbiome diversity or composition can alter host fitness and function. Nonetheless, we lack a general understanding of the drivers of microbiome diversity, in part because it is regulated by concurrent processes spanning scales from global to local. Global-scale environmental gradients can determine microbiome diversity at single sites, however an individual host microbiome also may reflect its local micro environment. We fill this knowledge gap by experimentally manipulating two potential mediators of plant microbiome diversity (soil nutrient supply and herbivore density) at 23 grassland sites spanning global-scale gradients in soil nutrients, climate, and plant biomass. In unmanipulated plots, leaf-scale microbiome diversity depended on the total microbiome diversity at each site, which was highest at sites with high soil nutrients and plant biomass. An experimental test of this relationship produced concordant results. Adding soil nutrients and excluding herbivores increased microbiome diversity by increasing plant biomass, which created a shaded microclimate. This demonstration of consistent drivers of microbiome diversity across a wide range of host species and environmental conditions suggests the possibility of a general, predictive understanding of microbiome diversity.