Mechanisms of cotton growth and water use efficiency in saline-alkali soil using deep vertical rotary tillage and Bacillus subtilis

Deep vertical rotary tillage (DVRT) enhances the soil structure, creates a low-salt environment, and enhances the potential of Bacillus subtilis to improve severely saline-alkali soil. However, there is little research on the synergistic improvement of saline-alkali land via these two technologies. Therefore, in this study, two years of field experiments were conducted during 2023 and 2024. The experiment included two tillage methods (conventional tillage at 20 cm and DVRT at 60 cm) and five B. subtilis application rates (0, 9, 12, 15, and 18 kg·ha–1). The results revealed that DVRT significantly reduced soil bulk density by 4.09 %, increased soil water content by 19.30 %, and decreased soil salinity by 13.51 %. Concurrently, B. subtilis inoculation enhanced bacterial diversity by 18.29 % and improved cotton stress physiology through increased proline content and superoxide dismutase activity with reduced malondialdehyde content. The synergistic interaction between DVRT and B. subtilis collectively increased cotton plant height (31.41 %), stem diameter (92.58 %), and leaf area index (257.85 %), while boosting yield (3.32 %–54.67 %) and water use efficiency (1.68 %–41.07 %). Structural equation modeling demonstrated that: DVRT primarily mediated improvements in soil physical/chemical structure through bulk density reduction; B. subtilis directly enhanced soil bacterial diversity; Their interaction synergistically optimized soil hydro-salinity conditions, creating favorable micro-environments for cotton development. The optimal synergy occurred at 12 kg·ha⁻¹ B. subtilis under DVRT. These findings provide practical strategies for synergistic soil remediation in saline-alkali systems.