4.3 Distinct assembly processes underlie similar biogeography of abundant and rare bacteria
Disentangling the biogeography and community assembly of abundant and rare microbial taxa is essential for understanding microbe-driven ecosystem processes and functions in the Yarlung Tsangpo River. We observed that both abundant and rare bacteria exhibited significant distance-decay relationships (Fig. 2), which was consistent with previous studies (Ji et al., 2020; Jiao et al., 2017; Mo et al., 2018). Although both subcommunities yielded similar biogeographic patterns, we found that the relative contributions of ecological processes to the assembly of abundant and rare subcommunities were different (Fig. 4b). Null model showed that the assembly of abundant subcommunity was dominated by dispersal limitation (84%), and is in line with previous investigations on inland lakes (Liu et al., 2015), epipelagic waters (Wu et al., 2017), and agricultural soils (Jiao and Lu, 2020). This result may be due to the strong environmental adaptation of abundant taxa as mentioned above, which make them less sensitive to environmental filtering and thus being more influenced by dispersal limitation. Additionally, the VPA results in our study indicated the pure effect of spatial variables surpassed those of environmental factors and land use types for abundant taxa, suggesting the greater significance of dispersal limitation in the abundant subcommunity assembly (Fig. S4). By contrast, assembly of the rare subcommunity was dominated by heterogeneous selection (77%), which was contradictory to previous findings that the rare subcommunity was primarily controlled by homogeneous selection (Jiao and Lu, 2020; Zhang et al., 2021a). Heterogeneous selection refers to selection under heterogeneous abiotic and biotic environmental conditions leading to more dissimilar structures among communities (Zhou and Ning, 2017). These discrepancies may be ascribed to the environmental heterogeneity in fluvial sediments. Due to the interplay of climate change, increasing anthropogenic, and complex geomorphologies, the water quality and biogeochemical nutrients in the Yarlung Tsangpo River were suspected to be highly variable over time and space (Wang et al., 2017; Wang et al., 2020b; Zhang et al., 2021c). Therefore, we would expect a strong heterogeneous selection on rare taxa in the investigated fluvial sediments.