Plant growth-promoting bacteria isolated from shallot-cultivated karst soils: functional traits and potential applications.

Soil fertility limitations in karst agroecosystems pose a significant challenge to sustainable shallot (Allium ascalonicum L.) production. This study aimed at isolating and characterizing native plant growth-promoting bacteria (PGPB) from shallot rhizospheres in the karst soils of Anggeraja District, Enrekang Regency. A total of 60 bacterial isolates were collected from four villages, Lakawan, Batunoni, Pekalobean, and Tampo, and 19 were selected based on growth and morphology. Biosafety screening using hemolysis and hypersensitivity tests identified six non-pathogenic isolates for further evaluation. These isolates were assessed for key PGP traits, including indole-3-acetic acid and gibberellic acid production, phosphate solubilization, and siderophore formation, and showed clear variability in their functional capacities. Among them, Bacillus sp. (BN11) demonstrated the most consistent multifunctional potential across traits. These findings highlight the promise of karst-adapted rhizobacteria as biofertilizers for nutrient-poor soils and their value in supporting sustainable shallot cultivation in degraded karst landscapes. Future greenhouse and field trials are recommended to validate their efficacy and guide the development of microbial inoculants tailored for marginal environments.