The Improved Remediation Effect of the Combined Use of Earthworms with Bacillus subtilis-Loaded Biochar in Ameliorating Soda Saline–Alkali Soil

High pH, Na+, and (CO32-+HCO3-) are the primary characteristics of soda saline-alkali soil. Current strategies for ameliorating soda saline-alkali soil often involve the combined use of cow manure and maize straw, the addition of biochar (BC), and the inoculation of <i>Bacillus subtilis</i> (BS). In this study, <i>B. subtilis</i>-loaded biochar (BSC) was prepared using an adsorption technique. An incubation experiment was conducted. The treatments were as follows: soda saline-alkali soil amended with maize straw and cow manure (T₁), which was used as a control; T₁ supplemented with earthworms (T₂); and T₂ supplemented with BS (T₃), BC (T₄), or BSC (T₅). After a 60-day incubation, T₅ showed the most significant reduction in pH, ESP, and (HCO3- + CO32-) concentrations, with reductions of 0.24 units, 3.26%, and 120 mg kg^-1, respectively, compared to the T₁ treatment. The content of soil humic acid, available potassium, and available nitrogen and the activities of β-glucosidase and urease were highest in T₅, increasing by 33.5%, 70.1%, 26.1%, 19.0%, and 17.9%, respectively. Microbial sequencing analysis revealed that the <i>Bacillus</i> abundance in T₃ was highest during the first 45 days (2.51-3.65%), while the <i>Bacillus</i> abundance in T₅ peaked at 3.22% after the 60-day incubation. The soil that was cultivated for 60 days in the experiments was then used for planting alfalfa. T₅ showed the highest alfalfa aboveground biomass and peroxidase, increasing by 30.1% and 73.1%, respectively, compared with T₁. This study demonstrated that loading onto biochar is beneficial for the survival of <i>B. subtilis</i> in soda saline-alkali soil. When traditional organic materials are used, the combination of earthworms and <i>B</i>. <i>subtilis</i>-loaded biochar significantly alleviates the constraints of soda saline-alkali soil.