Vadivel Anbuganesan¹, Ramasamy Vishnupradeep¹, Viswanathan Subhadra Varshini¹, Ashok Suma Archana¹, Sundaramoorthy Soundarya¹, L. Benedict Bruno¹, Mani Rajkumar^1,

The objective of the present study was to assess the combined effect of plant growth promoting rhizobacteria (PGPR) and biochar on Ricinus communis growth, physiological response, nutrient uptake and soil enzyme activity. A total of 16 bacterial strains isolated initially, two strains namely ST1NI01 and ST1NI15, demonstrating maximum in vitro plant growth promoting (PGP) potential were selected and identified as Enterobacter hormaechei and Bacillus thuringiensis, respectively. These isolates showed various PGP traits including production of indole-3-acetic acid, siderophores, ammonia and solubilization of phosphate. Similarly, the biochars were prepared from coconut husk by slow pyrolysis at 550°C. The characteristics of prepared biochar were pH 10.95, conductivity 3.52 mS cm^-1, cation exchange capacity 74.6 Cmol Kg^-1, moisture content 2.1 % and carbon 78 %. In pot experiments, inoculation of R. communis with PGPR or amendment of soil with biochar significantly increased the plant growth, protein content, nutrient uptake and soil enzyme activities, whereas decreased stress related metabolites (proline and malondialdehyde contents) and altered the activities of antioxidant enzymes (guaiacol peroxidase, catalase and ascorbate peroxidase) in plant leaves. However, the combined PGPR and biochar treatment exhibited greater effect on plant growth, physiological response, nutrient uptake and soil enzymatic activities as compared with sole treatment of PGPR or biochar. The findings suggest that both PGPR and biochar work synergistically on soil functions and plant growth, which may be utilized as an alternative to chemical fertilizers for improving soil quality and crop productivity.

PGPR, Biochar, plant growth, soil enzyme activity, malondialdehyde