Deep tillage enhances the spatial homogenization of bacterial communities by reducing deep soil compaction

The pressures from machinery use and continuous agricultural production practices exacerbate soil compaction. Deep tillage (DT) can reduce subsoil compaction, but the spatial composition patterns and community assembly mechanisms of subsoil vs. topsoil bacteria mediated by DT are unknown. Here, we collected soil profile samples from 18 agricultural fields within a multiple cropping area of southern China to investigate the status of soil compaction. The effects of DT on soil physicochemical properties and bacterial communities at different soil depths were investigated by establishing three long-term experimental sites, and the spatial composition mechanisms of the bacterial community were preliminarily explored. Our results showed that soil compaction occurred at soil depths greater than 20 cm, as evidenced by higher soil bulk density, and sharp decreases in water and nutrient contents and bacterial community diversity. Although the ameliorative effect of DT on deep soil compaction diminished in the fourth year, the water and organic matter content and bacterial α diversity remained high. DT resulted in a more homogeneous bacterial community across the soil profile in terms of community similarity and compositional stability, along with increased alpha diversity, all of which were associated with reduced heterogeneity in the soil variables, increased soil organic matter content, and the importance of homogeneous selection in the community assembly mechanism. Additionally, the homogenization of bacterial communities under DT may promote enhancement of bacterial network complexity and stability. Collectively, our findings reveal the importance of deep tillage for deep soil improvement and spatial homogenization of bacterial communities, which has far-reaching implications for comprehensively understanding the spatial dynamics and assembly mechanisms of tillage-mediated soil microbial communities in agroecosystems.