Niche-Driven Bacterial Assembly Versus Weak Geographical Divergence of Fungi in the Rhizosheath of Desert Plant Leymus racemosus (Lam.) Tzvel.

The rhizosheath plays a critical but poorly understood role in plant-microbe interactions. However, it still remains unclear how host selection versus geographical isolation contributes to microbial community assembly within the rhizosheath. This study characterized the bacterial and fungal communities in the rhizosheath and surrounding bulk soil of Leymus racemosus using 16S rRNA and ITS high-throughput sequencing. Results showed that the bacterial community was strongly shaped by host selection within the rhizosheath, based on significantly reduced α-diversity and distinct β-diversity (Permutation tests, p < 0.001) compared to bulk soil. Furthermore, the core bacterial community structure was highly similar between the two geographically separated sites (PERMANOVA, p = 0.089). In contrast, the fungal community exhibited weaker habitat specificity but showed significant, though weak, geographical divergence (β-diversity, Permutation tests, p = 0.028). The explanatory power of geographical distance for fungal community variation was low (R2 = 0.095) and less than that of the rhizosheath microhabitat (R2 = 0.142). In conclusion, the rhizosheath imposes a strong filtering effect on bacterial communities. The weaker habitat specificity and stronger geographical signal observed for fungi suggest potential regulation by local dispersal limitation or historical colonization processes. This study provides insights into the assembly mechanisms of the plant rhizosphere microbial community.