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

High pH, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mi>Na</mi></mrow><mrow><mo>+</mo></mrow></msup></mrow></semantics></math></inline-formula>, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo>(</mo><msubsup><mrow><mi>CO</mi></mrow><mrow><mn>3</mn></mrow><mrow><mn>2</mn><mo>−</mo></mrow></msubsup><mo>+</mo><msubsup><mrow><mi>HCO</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula>) 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 (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msubsup><mrow><mi mathvariant="normal">H</mi><mi mathvariant="normal">C</mi><mi mathvariant="normal">O</mi></mrow><mrow><mn>3</mn></mrow><mrow><mo>−</mo></mrow></msubsup><mo> </mo></mrow></semantics></math></inline-formula> + <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mo> </mo><msubsup><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">O</mi></mrow><mrow><mn>3</mn></mrow><mrow><mn>2</mn><mo>−</mo></mrow></msubsup></mrow></semantics></math></inline-formula>) 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.