The assembly and evolution of diazotrophic communities among the ancient disjunct forest soils of the northern hemisphere

Here we characterized the intercontinental assembly patterns of soil diazotrophic communities (SDC) among the ancient disjunct forests of eastern North America (ENA) and eastern Asia (EA). This well-known floristic disjunction shares approximately 65 seed plant genera including many tree genera. The "Out of Asia" hypothesis contends that many of these plant genera migrated from EA to ENA approximately 30 Mya during the Tertiary era. We asked whether the diazotrophs in these forest soils also followed a disjunct pattern. In total, 2,855 soil samples were collected across two continents and among ten disjunct plant genera from eleven forest sites. We used amplicon sequence variants translated to the corresponding peptide sequences (NifH tASVs) to assess the SDC similarity between ENA and EA. The patterns of assembly of the SDC presented here are correlated with geographic location and not with the plant genera with which the diazotrophs are associated. There did not appear to be patterns of assembly associated within a particular disjunct plant genus where the distribution and diversity of the SDC were similar. There was more similarity observed within a forest site and across sites within a continent versus broadly between continents. The abundance of the SDC was significantly higher in EA than in ENA. In addition, the relative abundance of Firmicutes in EA was significantly higher than that in ENA while the Proteobacteria were more abundant in the ENA samples. The NifH tASVs in common between the two continents exhibited greater heterogeneity in EA compared to the tASVs distinct to EA or ENA. Phylogenetic analysis of diazotrophs supports the "Out of Asia" hypothesis at the Phylum, but not tASV, level.