Conifer and broadleaf trees show a strong co-evolution with rhizosphere diazotrophic microbiome

Background and Aims Free living, non-symbiotic nitrogen-fixing bacteria (diazotrophs) that inhabit plant rhizosphere substantially contribute to nitrogen input in forest ecosystems. Different plant species provide heterogeneous habitats for rhizosphere diazotrophs by releasing root exudates containing potential resources for microbial utilization, but the ecological processes of diazotrophic community assembly and association with plant species are not fully understood. Methods We investigated the diazotrophic abundance (as assessed by real-time quantitative PCR), diversity, and composition of rhizosphere diazotrophic communities (based on nifH -amplicon sequencing) for four non-leguminous broadleaf tree species and three conifer tree species in a forest ecosystem. Results The analysis revealed that the rhizosphere of four broadleaf tree species harbored higher diazotrophic abundance and diversity by 239% and 50.0% respectively, as compared to the conifer tree species. Further, phylogenetic distance between the seven tree species was positively correlated (Mantel r = 0.482, P < 0.001) with the dissimilarity of diazotrophic community assembly, thus determining rhizosphere potential nitrogen fixation indicated by nifH gene abundance. Of the top 20 genera (ranked by percentage increase in Mean Squared Error) that contribute to potential nitrogen fixation, 11 rare genera were at low relative abundances (< 1%). Network analysis further showed the central position of rare genera in the diazotrophic co-occurrence network of four broadleaf tree species, as the closeness centrality of rare genera was significantly higher than that of abundant genera. Conclusion Overall, the study suggested that the identity of tree species impresses the assembly of diazotrophic communities in its rhizosphere. Possible implication of our findings for forest management is discussed for maintaining nitrogen sustainability in forest ecosystems.