Biogeographical patterns of rhizosphere microbial communities in Robinia pseudoacacia forests along a north–south transect in the Loess Plateau

Rhizosphere microbiota play an essential role in plant productivity; however, their biogeographical patterns in restored forest ecosystems across large spatial scales remain poorly understood. We used the Ion S5TM XL sequencing platform to explore the distributions of microbial communities in the rhizosphere and bulk soil of Robinia pseudoacacia at 11 mature forest sites along a north–south precipitation gradient in the Loess Plateau, China. Microbial alpha-diversity mostly peaked in the south zone at lower latitudes, and significant distance-decay patterns were observed in both bacterial and fungal communities. Fungal communities exhibited more distinct spatial patterns in the rhizosphere than bacterial communities. The relative influence of geographic factors on bacterial/fungal community turnover in the rhizosphere (12.4/16.7%) and bulk soil (16.1/17.1%) was greater than that of local environmental factors. Microbial community assembly was mainly governed by deterministic processes (98% of nearest taxon index > 2), despite their relative importance decreasing from bulk soil to the rhizosphere in the case of bacteria. Co-occurrence patterns of sensitive taxa were more complex in the bacterial network than in the fungal network owing to a marked regional effect. Interactions of bacterial taxa differentially enriched between soil compartments were much closer than fungal interactions, indicating a stronger rhizosphere effect on bacteria. The results provide novel insights into the assembly of rhizosphere microbial communities in large-scale forest ecosystems under vegetation restoration.