Mechanisms of Chlorella sorokiniana and Bacillus combination enhanced tomato (Solanum lycopersicum) growth and soil quality

Introduction Microorganisms are capable of enhancing plant growth by improving the soil microenvironment, supplying nutrients, and facilitating root development. Different microorganisms employ distinct mechanisms to achieve promotion of plant growth. Methods This study employed a partitioned plastic greenhouse experiment with three treatments— Chlorella sorokiniana ( C. sorokiniana ) inoculation, a Bacillus combination inoculation, and a water control—applied via root irrigation at each tomato growth stage, aiming to investigate the effects of C. sorokiniana and Bacillus combination on tomato growth, rhizosphere soil properties, and associated microbial communities. Results The results demonstrated that the combination of C. sorokiniana and Bacillus significantly promoted plant growth and improved soil quality. C. sorokiniana primarily enhanced soil physicochemical properties, as evidenced by a 53.68% decrease in electrical conductivity (EC). It also increased the activities of key soil nitrogen cycle-related enzymes, including urease (42.34%) and protease (24.14%), and elevated soil organic carbon (SOC) content (19.39%). Collectively, these modifications in soil conditions contributed to enhanced plant growth, resulting in increased plant height (11.72%) and stem diameter (9.78%). In contrast, the Bacillus combination enhanced soil nutrient availability (alkaline-hydrolyzable nitrogen (AN) 5.55%, available phosphorus (AP) 21.57%, available potassium (AK) 6.53%) by modulating the structure and function of the soil microbial community. Furthermore, it positively regulated the concentrations of metal ions (Ca 2+ 0.51%, Mg 2+ 1.99%), increased stress resistance in tomato plants (peroxidase (POD) 66.18%, superoxide dismutase (SOD) 98.67%, and catalase (CAT) 31.79%), and consequently enhanced plant growth metrics, including height (17.36%) and stem diameter (18.36%). These changes in soil physicochemical parameters may be related to increased rhizosphere microbial activity. Further correlation analysis indicated a positive relationship between the relative abundance of the Patescibacteria phylum and soil EC values. Discussion This study elucidates key mechanisms underlying growth promotion in tomatoes and establishes a theoretical foundation for the application of Bacillus combination and C. sorokiniana as microbial fertilizers.