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Soil bacteria, which are active in shrub encroachment, play key roles in regulating ecosystem structure and function. However, the differentiation characteristics and assembly process of bacterial communities in scrubbed grasslands remain unknown. Taking the Qinghai-Tibet Plateau, a hotspot of shrub encroachment, as the study area, we collected 192 soils near nine natural typical shrubs' roots on a trans-longitude transect (about 1800 km) and investigated the bacterial communities using 16S rRNA amplicon sequencing. We found that the bacterial communities exhibited plant-specific and geographic-specific differentiation. On the one hand, bacterial communities differed significantly across plant species, with widely distributed shrubs harboring high diversity communities but few plant-specific taxa, and narrowly distributed shrubs possessing low diversity communities but more plant-specific taxa. Besides, there was a significant negative correlation between bacterial community similarity and plant phylogenetic distance. On the other hand, bacterial communities differed across geographic sites, with a significant decay in bacterial community similarity with geographic distance. The bacterial alpha diversity varied in an inverted V-shape from west to east, peaking at 91°E, which could be largely driven by mean annual temperature, soil pH and soil total carbon content. Community differentiation increased with the heterogeneity degree of assembly processes, and the dominant assembly process in these two specific differentiations differed. Dominated by stochastic and deterministic forces, respectively, geography diverged bacterial communities primarily through increased dispersal limitation, whereas plants diverged bacterial communities primarily through increased variable selection. Our study provides new insight into the characteristics and mechanisms of root-surrounding soil bacteria differentiation in scrubbed grasslands, contributing to the scientific management of degraded grasslands and the prediction of bacterial community structure and ecosystem function in response to global change., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Soil CO 2 -fixing microbes play a significant role in CO 2 -fixation in the terrestrial ecosystems, particularly in the Tibetan Plateau. To understand carbon sequestration by soil CO 2 -fixing microbes and the carbon cycling in alpine meadow soils, microbial diversity and their driving environmental factors were explored along an elevation gradient from 3900 to 5100 m, on both east and west slopes of Mila Mountain region on the Tibetan Plateau. The CO 2 -fixing microbial communities were characterized by high-throughput sequencing targeting the cbbL gene, encoding the large subunit for the CO 2 -fixing protein ribulose 1, 5-bisphosphate carboxylase/oxygenase. The overall OTU (Operational Taxonomic Unit) abundance is concentrated at an altitude between 4300  and  4900 m. The diversity of CO 2 -fixing microbes is the highest in the middle altitude area, and on the east slope is higher than those on the west slope. In terms of microbial community composition, Proteobacteria is dominant, and the most abundant genera are Cupriavidus, Rhodobacter, Sulfurifustis and Thiobacillus. Altitude has the greatest influence on the structural characteristics of CO 2 -fixing microbes, and other environmental factors are significantly correlated with altitude. Therefore, altitude influences the structural characteristics of CO 2 -fixing microbes by driving environmental factors. Our results are helpful to understand the variation in soil microbial community and its role in soil carbon cycling along elevation gradients., (© 2022. The Author(s).)