Potential plant growth promoting activity of root endophytic and rhizospheric bacteria isolated from mahogany and siratro in Alas Bromo, Indonesia

Abstract Agricultural practices through intensive agrochemical input are still carried out in Alas Bromo agroforestry system and threaten the soil quality and health there. Beneficial endophytes and rhizobacteria have important roles in solubilizing nutrients and producing phytohormones so they act as plant growth promoters in reducing chemical fertilizer use. This study aims to assess root endophytic and rhizospheric bacteria isolated from mahogany and siratro in Alas Bromo as plant growth promoters with the ability to solubilize phosphate and produce IAA. At the first, the surface of plant root samples was sterilized. Bacterial isolation from root and rhizospheric soil were conducted by spread plate method on Pikovskaya medium. The halo zone formation was measured to determine the Phosphate Solubilizing Index (PSI). All isolates were tested for IAA production ability. The highest bacterial population density was obtained from mahogany rhizospheric soil bacteria (24.373×10 5 .CFU.g −1 ), but contrary to Shannon diversity index by siratro rhizospheric bacteria (1.06). The highest PSI from mahogany and siratro were demonstrated by isolates Sm_R_2 (3.84) and Ma_S_4 (3.54), respectively. All isolates were able to produce IAA (0.77-13.90 μg.ml −1 ). Isolate Sm_R_1 from mahogany root has the potential to be developed as a biofertilizer based on high abilities in P-solubilization and IAA production.