Land Use Type Governs Vertical Distribution, Co-Occurrence Network Structure, and Functions of Soil Bacterial Communities in an Alpine Semi-Arid Basin

To reveal how land use type regulates soil bacterial community structure, co-occurrence networks, and carbon–nitrogen functions across 0–100 cm profiles in the alpine semi-arid Huangshui River Basin, soils from five land use types (cropland, grassland, forestland, wetland, and abandoned land) were analyzed by 16S rRNA sequencing. Pseudomonadota, Actinomycetota, Chloroflexota, Acidobacteriota, and Bacillota dominated all communities (77.25–87.04%). Vertical stratification occurred within each land use: copiotrophic taxa dominated surface layers (49.77–70.46%), while stress-tolerant anaerobes increased with depth. Land use was the primary driver of community divergence (56.46% of variation), with grassland clearly separated. Soil organic carbon, total nitrogen, total phosphorus, available phosphorus, and potassium were key drivers. Network analysis showed grassland had the highest positive correlation proportion (53.3%), wetland the highest modularity (0.878), and cropland the highest negative correlation proportion (55.3%) and lowest modularity (0.774). Functional prediction indicated that grassland was enriched in methanotrophy, nitrite respiration, and nitrogen fixation; cropland was characterized by methanol oxidation and ureolysis; and forestland and wetland exhibited strong potential for aromatic compound degradation. These findings clarify land-use regulation of vertical bacterial differentiation and functions, supporting land optimization and soil conservation in alpine semi-arid basins.