Similar biogeographic patterns of abundant and rare bacterial communities are driven by distinct assembly mechanisms in grassland soils

Background: The regional species pool and local community assembly processes shape the biogeographic patterns of soil bacterial community diversity. However, how community assembly mechanisms regulate biogeographic patterns in rare and abundant bacterial communities remains unclear. Methods: Soil samples of 16 grassland habitats across the Inner Mongolian Plateau and Qinghai-Tibet Plateau (QTP) transects were collected to investigate the variation of beta-diversity in rare taxa (RT) and abundant taxa (AT). High-throughput sequencing analysis of 16S rRNA gene amplicons was implemented on an Illumina MiSeq platform. Results: Significant distance-decay relationships of beta-diversity in RT and AT were observed at transect and habitat scales, and the turnover rate increased from desert to meadow steppe in both taxa. For variations of beta-diversity along environmental gradient, the regional species pool had a limited effect on both taxa except RT in QTP. Deterministic processes, including homogeneous selection (85.1%-97.3%) and heterogenous selection (48.1%-64.2%), dominated the assembly of RT at both the transect and habitat scales. In contrast, the assembly of AT exhibited habitat specificity and was dominated by homogeneous selection (47.2%-80.6%), heterogenous selection (42.1%-54.2%), and dispersal limitation (41.8%) in different transects and habitats. Moreover, the local assembly processes of the AT community were more stochastic than those of the RT community. Mean annual precipitation (MAP) was the dominant driver of community assembly at the transect scale, with extreme MAP (<200 or >400mm) resulting in more deterministic processes and a moderate level of MAP (200-400mm) leading to more stochastic processes. However, the effects of geographical distance and soil properties on different grassland habitats cannot be ignored. Conclusions: Although both bacterial taxa exhibited significant distance-decay patterns, different assembly mechanisms shaped the beta-diversity of AT and RT communities in grassland soils. Our results suggested that MAP can mediate community assembly of soil bacteria on a large scale.