Effects of the addition of medium and trace elements and their combinations on the growth promoting properties of microbial inoculants

This study was conducted to screen the nutrient elements that can enhance the effective viable count and growth promoting properties of microbial inoculants. By adding different types and concentrations of media and trace elements and their combinations to the microbial inoculants, we explored the optimal amendment amount of different elements and their effects on the effective viable bacterial counts of the inoculants, nitrogen-fixing enzyme activity, phosphorus solubilizing capacity, and secretory indoleacetic acid (IAA). The results showed that when molybdenum (Mo), boron (B), manganese (Mn), and magnesium (Mg) were added at 100, 100, 5, and 10 mg·L⁻¹, the effective viable bacteria counts of the microbial inoculants were increased by 23.53%, 77.65%, 81.88%, and 148.71%, respectively. The orthogonal test showed that the best combination was A_-1B₀C₀D_-1: Mo 100 mg·L⁻¹, Mn 5 mg·L⁻¹, Mg 10 mg·L⁻¹, and B 50 mg·L⁻¹, with boron playing a leading role. The addition of a single element or different combination of elements had different effects on the nitrogenase activity, phosphorus solubilization ability, and IAA secretion ability of the microbial inoculants. The comprehensive evaluation showed that the rankings of the overall ability to influence the growth promoting properties of the microbial inoculants were as follows: among the single-element treatments, the addition of 5 mg·L⁻¹ Mn was the best performer, and among the treatments with a combination of elements added, the best performer was the combination of A₅ (Mo: 100 mg·L⁻¹, B: 150 mg·L⁻¹, Mn: 5 mg·L⁻¹, and Mg: 5 mg·L⁻¹). In summer, the results of this study can be used to improve the formulation of microbial inoculants.