Composition of plant growth promoting rhizobacteria in forest and systematic plantations of tasar silkworm food plants and their relationship with soil and leaf nutrients

Aim: To investigate the status of bacterial load and number of isolates of plant growth promoting rhizobacteria in forest and systematic plantation of Terminalia tomentosa and T. arjuna, and their relationship with soil and leaf nutrient levels. Methodology: Soil samples were collected from the rhizosphere of food plants in four tasar silkworm rearing regions of India. Bacterial load of total bacteria, Nitrogen fixing bacteria (NFB), Phosphate Solubilizing Bacteria (PSB) and bacteria grown on King's B media (KBB) was enumerated on specific media by employing serial dilution technique. Based on the colony morphology and cell type, distinct colonies on specific media were counted as isolate. Nutrient levels in soil and leaf samples collected from different plantations were analyzed. Multivariate correlation analysis was performed between PGPR microbial load and isolates with soil physio-chemical parameters and leaf nutrients. Results: The rhizosphere of forest plants possessed the highest total bacterial load, the systematic plantation possessed the highest NFB load, whereas PSB and KBB load was equal in both the plantations. Forest plantations of Keonjhar and Sundergarh regions had the highest number of NFB isolates per rhizosphere soil sample and systematic plantations possessed the highest number of KBB (Keonjhar and Sundergarh regions) and PSB isolates (West Singhbhum region). Positive correlation was observed between PSB load and the number of isolates with soil organic carbon, KBB load with soil electrical conductivity and available potassium, PSB load and KBB isolates with soil available sulphur. NFB and KBB load and their isolates showed positive correlation with leaf N and K content. Interpretation: Results suggest the possibility of improving leaf nutrient quality of tasar silkworm food plants by enriching PGPR microbial load through biofertilizer application. Key words: Rhizobacteria composition, Soil nutrients, Silkworm, Terminalia tomentosa, Terminalia arjuna