Plant interaction traits determine the biomass of arbuscular mycorrhizal fungi and bacteria in soil.

Plant-arbuscular mycorrhizal fungal (AMF) mutualisms are crucial to ecosystem biodiversity and productivity. Yet, our understanding of the functional roles of plants as AMF generalists or specialists, and the consequences of these plant interaction traits for soil ecosystems are virtually unknown. We grew eight pasture plant species under two experimental conditions, sequencing their root AMF communities to assess interaction traits using a range of numeric and phylogenetic diversity metrics, thereby characterizing each plant species' interaction generalism with AMF. We used lipid analysis of rhizosphere soils and Bayesian modeling to explore how host interaction traits affected carbon allocation to AMF and bacteria. We found that plant interaction traits for AMF remained stable despite large variation in soil conditions and AMF pools. Host interaction generalism was linked to contrasting patterns in bacterial and AMF biomass: Phylogenetic diversity in plant interactions was positively associated with AMF biomass, while numeric diversity was negatively associated with bacterial biomass in rhizosphere soils. Explicit consideration of plant interaction niches may enhance understanding of how changes in biodiversity affect ecosystem carbon cycling.