Mechanisms of tomato growth promotion in three soils after applying Bacillus combinations

The application of Bacillus combinations is a promising strategy to increase plant growth by increasing phosphorus availability in different soil environments. In this study, we investigated the effects of Bacillus inoculation on tomato seedling growth in Haplic Chernozems, Haplic Calcisols, and Haplic Luvisols soils. The results revealed that Bacillus colonization varied significantly among the soil types, with better colonization observed in the Haplic Calcisols and Haplic Luvisols soils, which was positively correlated with the total phosphorus content of the soil. Inoculation significantly increased tomato plant height and weight, particularly in Haplic Calcisols and Haplic Luvisols soils, due to the activation of insoluble phosphorus by increased alkaline phosphatase and phytase activities. Furthermore, Bacillus combinations modulated the rhizosphere microbial community structure and function, increasing the abundance of key genera such as Streptomyces in the Haplic Calcisols soil, Pseudoxanthomonas and Flavihumibacter in the Haplic Luvisols soil, and Flavisolibacter in the Haplic Chernozems soil. These changes facilitated phosphorus solubilization and nutrient uptake, which was supported by increased levels of rhizosphere organic acids. Functional predictions revealed that Bacillus inoculation enhanced processes related to nutrient cycling, biofilm formation, and phytohormone production, contributing to improved plant growth. Our findings highlight the importance of soil-specific responses to microbial inoculation and provide insights into the mechanisms by which Bacillus combinations promote phosphorus availability and plant growth. This study provides a foundation for the targeted application of Bacillus biofertilizers to optimize soil fertility and sustainable agricultural practices.