Biogeography and the driving factors affecting forest soil bacteria in an arid area.

Bacteria are one of the most abundant and diverse groups and mediate many critical terrestrial ecosystem processes. Despite the crucial ecological role of bacteria, our understanding of their large-scale biogeography patterns across longitude (east-west transect), and the processes that determine these patterns lags significantly behind that of macro-organisms. Here, we used 16S rRNA gene sequencing to evaluate the geographic distributions of bacterial diversity and their driving factors across different longitude sites along an 800-km east-west transect in the Loess Plateau. Twenty-four phyla were detected across all soil samples and the most sequence-abundant bacterial phyla were Acidobacteria, Actinobacteria, Alphaproteobacteria, Betaproteobacteria, Chloroflexi and Gemmatimonadetes (average relative abundance >5%). Soil bacterial α-diversity, expressed by the richness of soil bacterial communities and Shannon diversity, differed among climates (MAP) but showed strong correlations with MAP (r=-0.537 and r=-0.42, respectively; p<0.05 in both bacterial diversity indices). Variation partition analysis demonstrated that the bacterial community structure was closely correlated with environmental variables and geographic distance, which together explained 62% of the community variation. Soil properties contributed more to bacterial community variation than the combined geographic distance (historical contingencies) and climate factors. Among all environmental factors, soil pH exhibited a dominant role in structuring bacterial communities in this arid area. Our findings provide new evidence of bacterial biogeography patterns in an arid area (MAP ranged from 473mm to 547mm). Additionally, the results indicated a close linkage among soil bacterial community, climate and edaphic variables, which is critical for predicting promoting sustainable ecosystem services in the Loess Plateau.