Plant species identity structures soil oomycete communities across natural grasslands
Soil-borne oomycetes include both pathogens and saprotrophs implicated in plant–soil feedback systems that promote coexistence, yet whether individual plant species consistently structure oomycete communities in natural grasslands remains poorly tested. We hypothesised that plant species identity acts as an ecological filter on soil oomycete communities, generating host-specific assemblies enriched in specialised antagonists across large spatial scales and environments. We sampled rhizosphere-associated soils from four grassland species, paired with adjacent bulk soils across 25 grasslands spanning more than 200,000 km² of the Canadian Prairies. Rhizosphere-associated soils harboured significantly higher Oomycete richness than paired bulk soils, and community composition differed significantly among plant species after accounting for spatial variation among sites. The magnitude of community divergence also varied among plant hosts. We found that host-specific Oomycetes were more likely to be classified as plant pathogens, than those found across multiple hosts. The Oomycete community was dominated by Globisporangium and Pythium, but hosts differed in Oomycete genus and trophic guild composition, with D. purpurea supporting the most distinctive assemblage. These results demonstrate that plant species identity consistently filters oomycete communities across natural grasslands, with some plant species generating host-specific assemblages enriched in pathogenic taxa consistent with the theory of specialist enemy accumulation underlying negative plant–soil feedbacks.