Optimizing nitrogen-fixing bacteria viability and plant growth-promoting traits through liquid media composition

Developing effective liquid biofertilizers depends on compatible microbial strains, suitable carrier formulations, and proper scaling-up conditions.The study aimed to assess the effects of N and P enrichment on a molasses-based liquid carrier formula and various agitation conditions on the cell viability of nitrogen-fixing bacteria, Azotobacter, Priestia, and Bacillus.All selected strains, isolated from the strawberry rhizosphere, showed mutual growth without inhibiting nutrient agar, indicating their suitability for forming a consortium.The liquid carrier, formulated with 1% molasses, 0.1% NH 4 Cl, and 0.2% phosphate buffer, produced the highest bacterial populations, reaching 17.3 log CFU/mL in N-free Ashby's agar and 16.4 log CFU/mL in Tryptic Soy Agar after nine days of incubation.This liquid carrier formulation also maintained a more stable pH (8.1) and lower electrical conductivity (1.5 mS/ cm) compared to other treatments, suggesting these conditions are optimal for sustained bacterial activity.All agitation speeds (115-135 rpm) showed similar growth curves, with OD reaching a maximum of 3.9-4.0on day six.However, 135 rpm resulted in the most significant growth rate (slope 0.638, R² 0.926) and higher levels of indole-3-acetic acid, gibberellin, zeatin, and phosphatase activity.These results demonstrate that carrier formulation enhances population growth and stability.This optimized process provides a promising approach for developing a multi-strain liquid carrier biofertilizer for strawberry cultivation.