# Phylosymbiosis in the Rhizosphere Microbiome Extends to Nitrogen Cycle Functional Potential
In addition to biotic interactions, microbial communities are influenced by abiotic factors (e.g., temperature and pH) that shape their composition (i.e., taxonomic diversity) and their ability to support specific reactions and environmental services [3; 17; 19; 20; 21]. In the soil, microbes are also affected by plants, and the rhizosphere microbial community differs from the bulk soil. The precise mechanism by which plants modulate their associated microbiome is not clearly understood; however, promoting the growth of specific microbial lineages in the rhizosphere is essential for the plants to develop some immunity against deleterious microbes and to acquire essential nutrients, such as bioavailable N [3; 22; 23]. As some microbial guilds provide necessary services to plants and to the ecosystem (e.g., N fixation), understanding how the rhizosphere microbiome is shaped by the plant is a prerequisite to investigating the molecular mechanisms underlying this symbiosis in natural and managed systems. However, currently, most of the research has focused on understanding the mechanistic interaction between _Arabidopsis thaliana_ and its root microbiome [24; 25; 26]. Many studies investigating the structure of microbial communities across hosts have identified significant correlations between the composition of the microbial community and the host phylogeny [3; 27; 28; 29; 30; 23]. This so-called \"phylosymbiotic\" signal [31] occurs when hosts that are more closely related share more similar microbiomes. A phylosymbiotic pattern suggests that the hosts evolve characteristics (e.g., exuding nutrients, a root morphology that provides habitat, etc.) that make the hosts able to support symbiotic relationships with particular microbial lineages. A phylosymbiotic pattern in the plant root-microbiome association may mean that the host is able to control the composition of the rhizosphere community, regardless of the abiotic environment where it is growing. However, phylosymbiosis is not universally seen across host-microbiome relationships and is generally stronger in internal (e.g., gut and endosphere) than external (e.g., skin and rhizosphere) host-microbiome systems [30]. One explanation for this pattern is that hosts control the conditions in their internal systems, whereas in external systems, the host and abiotic factors interact to shape the microenvironment and the associated microbial community. While plants may impact particular microbial lineages in their rhizosphere, it is also possible that plants and their associated rhizosphere microbiomes' composition would correlate for other reasons. For example, if plant species and microbial lineages share the same habitat, then a co-occurrence pattern would appear without specific beneficial interactions between plants and microbes [30]. Here, we investigated the phylosymbiosis of the plant-rhizosphere microbiome with specific attention to bacterial traits supporting N cycling. We tested for phylosymbiosis using chloroidoid grasses (_Poaceae_, _Chloroidaeae_) from around the world, as grasses are known to efficiently take up N [32]. Specifically, we grew 26 species of chloroid grasses, plus one danthonioid (_Danthoniodeae_) outgroup species, in a common garden experiment at the California Botanic Garden to test if variation in the rhizosphere's microbial community composition (taxonomic diversity) and functional potential for nitrogen cycling correlated with the phylogenetic distance among plants. As the plant's ability to select and control microbes in the rhizosphere is a complex process, likely involving multiple genes [22; 26; 33; 34], we predicted that closely related plants, with a shared evolutionary history and thus functional traits, would display similar abilities and preferences for selecting and promoting microbial lineages in the rhizosphere. In consequence, closely related plants from the same species or genus would display similar microbial communities, whereas more distantly related plants would have more distinct microbial communities.