From sugar industry by-product to resource: Utilizing retentate concentrate of sugarcane juice for novel plant growth promoter development.

Ceramic membrane clarification technology is a novel physical clarification process for sugar production. The Retentate Concentrate of Sugarcane juice (RCS) contains substances difficult to clarify through conventional methods, limiting resource utilization and technology industrialization. The rich nutrients in RCS make it worthy of resource utilization, particularly suitable as a microbial culture medium. This study investigated the use of RCS as a substrate for developing plant growth-promoting microbial formulations, thereby addressing its resource utilization challenges. The dynamic changes in RCS microbial community composition during 60 days of storage were monitored using high-throughput amplicon sequencing. Eight selected Bacillaceae strains with PGP potential were included, including phosphate solubilization, nitrogen fixation, indole-3-acetic acid (IAA) production, extracellular enzyme secretion, osmotic stress tolerance and no antagonism. Mixed strains were used to prepare the RCS agent with increased colony count (8.57 log10 CFU mL-1), sporulation rate (2.33%), and maintained enzyme activity using 10 × diluted RCS medium. Brassica chinensis L. (Pakchoi) potting experiments confirmed RCS agent significantly improved its growth quality (plant height +17.35%, fresh weight +38.00%, chlorophyll a +35.66%) and enhanced rhizosphere soil nutrients (NH4+ +17.21%, available phosphorus +4.62%, NO3- +12.45%) compared to controls. These results demonstrate a novel strategy for the value-added utilization of RCS by converting it into an effective plant growth-promoting microbial formulation.