Grazing reshapes bacterial diversity and ecological networks across vertical soil profiles

As a widespread anthropogenic disturbance and management practice in grassland ecosystems, grazing profoundly influences soil bacterial communities. However, the variation in the effect of the grazing intensity on these communities with soil depth remains uncertain. We investigated the effects of different grazing intensities (no grazing (CK), light grazing (LG), moderate grazing (MG), and heavy grazing (HG)) and soil depths (0–2 cm, 2–10 cm, and 10–20 cm) on the composition, diversity, and co-occurrence networks of soil bacteria on the Loess Plateau. High-throughput sequencing revealed that the Shannon indices of all soil layers under the LG were higher than those of the control, with increases of 0.02, 0.26, and 0.14, respectively. Proteobacteria, Actinobacteria, and Acidobacteria were the dominant phyla regardless of the treatment and depth. Notably, grazing intensity was associated with depth-specific variations in network topology in our exploratory analyses: MG showed relatively higher connectivity in the surface layer, whereas HG showed similar patterns in the subsurface layer. LG appeared to be associated with a more connected network, while HG tended to yield a more modular structure. However, given the limited replication, these descriptive patterns require further validation. Critically, the structural equation modelling results revealed that the potential factors of bacterial diversity and network structure shifted with soil depth, with R 2 values ranging from 0.146 to 0.503 for bacterial diversity and from 0.358 to 0.666 for the co‑occurrence network. Across soil layers, the most stable effect is the suppression of plant inputs, while subsurface C-N regulation and the weakening of grazing effects in the deepest layer together elucidate how grazing differentially reshapes bacterial diversity and ecological networks along the vertical profile. Grazing induces vertical stratification of bacterial communities, with higher diversity and compositional variation in surface than subsurface soils. Grazing consistently reduces plant inputs, but regulatory mechanisms shift with depth. Surface communities are indirectly regulated by plant inputs, mid-layer diversity is promoted by inorganic nitrogen, and deeper layers rely on legacy effects and inherent soil properties.