Soil mutualisms potentially determine the reintroduction outcome of an endangered legume

Successful recovery of populations of endangered plant species requires conservation of existing populations as well as the creation of new populations through reintroduction. However, the ecological requirements of many rare plant species are poorly understood, and many reintroduced populations are unable to survive long term. Effective reintroduction of rare plants, such as the federally endangered Astragalus bibullatus, may depend on developing a greater understanding of the symbiotic relationships that these rare species form with the soil microbiome, as well as determining whether these are species- and site-specific. We inoculated seedlings of A. bibullatus, its more widespread congener A. tennesseensis, and the common grass Schizachyrium scoparium, with soil biota collected from five glade sites where A. bibullatus is historically present (HP) and four glade sites where the species is historically absent (HA). We examined the impacts of soil microbes from HP and HA glade sites as well as from each species on the growth, arbuscular mycorrhizal fungi (AMF) colonization, and formation of root nodules of A. bibullatus and its congener A. tennesseensis. Astragalus bibullatus grew significantly larger when grown in soil from HP glade sites compared to its growth in soil from HA glade sites. Astragalus bibullatus also formed significantly more root nodules when grown in HP soil, but no difference was detected in AMF colonization based on glade history. Our findings suggest that the successful establishment of rare plant species may depend on species-specific associations with soil mutualists such as nitrogen-fixing rhizobial bacteria and should consider whether essential microbes are present.