Altitudinal patterns of bacterial communities across soil layers in the alpine meadows of the Qinghai-Tibet Plateau

• Soil bacterial communities vary in altitudinal patterns across layers. • Bacteria in different soil layers exhibit distinct community assembly mechanisms. • TOC predicted the altitudinal pattern of surface soil bacteria. • Salinity predicted the U-shaped altitudinal pattern of subsurface soil bacteria. Microorganisms in the soils of alpine meadows on the Qinghai-Tibet Plateau are pivotal in sustaining ecosystem functionality and stability. The community structure and functional composition of these microbes exhibit pronounced altitudinal variations; however, specific distribution patterns across different soil layers remain inadequately elucidated within alpine meadow ecosystems. This study employed high-throughput sequencing to examine bacterial communities across various soil layers along an altitudinal gradient in the alpine meadow ecosystem of the Qilian Mountains. The α-diversity of surface soil bacteria demonstrated a hump-shaped altitudinal trend (P < 0.05), whereas subsurface bacterial α-diversity followed a U-shaped distribution along the altitude gradient (P < 0.001). Community assembly in surface soils was predominantly governed by homogeneous selection (71.5 %), whereas in subsurface soils, it was co-dominated by heterogeneous selection (40 %) and homogeneous selection (39.5 %). Network analysis revealed that subsurface bacterial networks exhibited greater complexity and a higher prevalence of negative correlations compared to surface soil networks. Environmental variables, notably salinity, emerged as the most significant predictors of altitudinal disparities in bacterial community patterns across soil layers. Specifically, salinity may exert a dominant influence on subsurface soil community composition by enhancing negative interactions and imposing direct selective pressures. Conversely, alterations in surface soil bacterial community composition were more strongly correlated with total organic carbon (TOC).These findings elucidate distinct altitudinal patterns in bacterial community diversity and assembly between soil layers. This enhanced understanding is critical for advancing knowledge regarding the functional roles of soil microorganisms in alpine meadow ecosystems and their adaptive responses to climatic fluctuations.