Optimizing the Carbon and Nitrogen Concentrations and Ratio for the Growth of Bacterial Consortia-based Biofertilizers

The reliance on synthetic agrochemicals has contributed to soil degradation, highlighting the need for sustainable, cost-effective alternatives. Microbial biofertilisers containing beneficial bacteria present an effective solution for enhancing nutrient availability and supporting agricultural productivity. However, their performance is significantly influenced by nutritional conditions, specifically the carbon-to-nitrogen (C/N) ratio, making its optimisation crucial for effective bacterial growth and biofertiliser development. The objective of this study was to optimise carbon (C) and nitrogen (N) concentrations and their ratio to maximise the growth of bacterial consortia-based biofertilizers. Nutrient-specific bacterial consortia comprising nitrogen-fixing bacteria (NFBRS), phosphate-solubilising bacteria (PSBSS) and potassium-solubilising bacteria (KSBSS) derived from biowaste organic resources were obtained from the present laboratory and cultivated in batch culture experiments. Batch culture experiments were conducted to evaluate the growth dynamics of the NFBRS, PSBSS and KSBSS consortia inoculated in nutrient broth across various carbon concentrations (3, 5, 7, 11, 15, 19, 27, and 35 g/L), nitrogen concentrations (1.06, 1.75, 2.45, 3.84, 5.23, 6.62, 8.01, 9.4 g/L), and initial C:N ratios (3:1, 5:1, 10:1, 15:1, 20:1, 25:1, 30:1, 40:1). Growth kinetics were quantified by measuring OD600 over 24 h and specific growth rates were calculated during the exponential growth phase. The growth of NFBRS, PSBSS and KSBSS consortia ranged from 0.009 to 1.828 OD, 0.017 to 1.861 OD, 0.025 to 1.756 OD in different C concentrations, 0.02 to 1.828 OD, 0.02 to 1.820 OD, 0.023 to 1.802 OD in various N concentrations, 0.01 to 1.828 OD, 0.02 to 1.850 OD, 0.011 to 1.802 OD in various carbon to nitrogen ratios, respectively. Optimal C concentrations were identified at 15 g/L for the PSBSS consortium (7.35X), at 11 g/L for NFBRS (7.76X) and the KSBSS (6.33X) consortium. Peak proliferation of the bacterial consortia was found at 2.45 g/L for the PSBSS (7.61X) and KSBSS (7.85X) consortia, and at an N concentration of 5.23 g/L for NFBRS (8.03X). The NFBRS, PSBSS and KSBSS consortia showed 8.03X, 7.64X, 7.85X the highest growth increments at a 25:1 C/N ratio compared with the remaining ratios. The findings indicate that C and N concentrations and their ratio significantly influence the growth kinetics and functional efficiency of the consortia. The study concluded that optimised C and N concentrations and their ratio for the maximum growth of bacterial consortia can provide a practical and environmentally friendly method to enhance soil fertility and productivity through improved nutrient solubilisation and cycling.