Deep-rooted plant species recruit distinct bacterial communities in subsoil than in topsoil

Deep-rooted plants can obtain water and nutrients from the subsoil, making them resilient to climatic changes. Plant growth and health may depend on interactions with root-associated bacteria, but the composition and assembly dynamics of deep root-associated bacterial communities are unknown, as are their ability to supply plants with nitrogen (N). Here, we investigated the root-associated communities of the three deep-rooted perennial crops, lucerne (Medicago sativa), intermediate wheatgrass (Thinopyrum intermedium), and rosinweed (Silphium integrifolium), grown in 4 m tall RootTowers, under semi-natural conditions. Across the plant species, higher bacterial abundance and lower diversity were found in the root-associated communities compared to the bulk soil communities. The deep root-associated communities were enriched in the genera Pseudarthrobacter, Pseudomonas, Rhizobium and Streptomyces, genera found to harbor a wide variety of bacterial species expressing plant beneficial traits. The composition of the deep root-associated bacterial communities were plant species specific, and clearly distinct from the shallow communities. Additionally, the deep root-associated communities comprised primarily amplicon sequence variants (ASVs) that were omnipresent in the bulk soil, and to a limited extent ASVs that could have been transported from the topsoil or potentially from the seed. Abundances of genes involved in N-cycling: amoA, nifH, nirK, nirS and nosZ showed plant species specific patterns, and indicated that intermediate wheatgrass and lucerne recruit N-fixing bacteria even at 3 m depth for N supply. This work provides the first steps toward understanding plant-microbe interactions of deep-rooted crops, which are important for evaluating these crops for use in future sustainable cropping systems.