Preparation and inoculation of Bacillus spp. and Sinorhizobium meliloti strains immobilized on biochar-humic acid improve potted soybean traits and soil parameters

This study examines microbial carriers to enhance soil quality and promote crop development. A composite microbial consortium was developed, consisting of Bacillus velezensis HT-S1, Bacillus subtilis HT-S2, Bacillus megaterium HT-S3, and Sinorhizobium meliloti HT-S4, which were absorbed onto various carriers. The combination of biochar and humic acid was identified as the optimal carrier through characterization analysis and evaluation of its adsorption capacity. The conditions for adsorption were optimized (156 rpm, 1.22 hours, and pH 8.0), resulting in a bacterial adsorption capacity of 3.11 × 10 ¹ ⁰ CFU·g⁻¹ . Kinetic and thermodynamic models indicated that adsorption was a spontaneous exothermic process primarily driven by physical interactions, while desorption was an endothermic process increasing entropy. In pot experiments with soybeans, microbial agents significantly enhanced plant growth, increasing plant height (10.65–17.01 %), stem thickness (12.83–22.62 %), and dry weight (22.6–50.25 %). Soil quality also improved, with increases in ammonium nitrogen (8.49–25.32 %), nitrate nitrogen (10.45–22.28 %), inorganic phosphorus (11.69–20.11 %), quick-acting potassium (7.95–33.01 %), and enzyme activities (sucrase, urease, alkaline phosphatase, and catalase). These results demonstrate that Bacillus spp. and S. meliloti can be immobilized in a biochar-humic acid carrier and colonize soil. The findings provide a theoretical basis for developing microbial fertilizers to improve soil quality in arable land. • A consortium consisting of Bacillus spp. and Sinorhizobium meliloti strains was constructed. • The combination of biochar and humic acid was identified as the optimal carrier. • Carrier microbial agents comprising bacterial consortium immobilized on biochar-humic acid developed. • Adsorption and desorption are spontaneous exothermic and endothermic process, respectively. • Carrier microbial agents significantly improve potted soybean traits and soil parameters.