Multifunctional microbial consortium for bio- enriched compost

Abstract

The research work on “Multifunctional microbial consortium for bio-enriched compost” was performed at the Department of Agricultural Microbiology, College of Agriculture, Vellayani, Thiruvananthapuram during 2024-2025, with the objective of developing a lignocellulolytic microbial consortium with plant growth promotion potential for conversion of agricultural residues to bio-enriched compost. Microorganisms capable of decomposing lignin, hemicellulose and cellulose in plant material can be effectively used for the management of agricultural residues. Many lignocellulolytic microorganisms possess the ability to promote plant growth by enhancing the nutrient supply and supplying plant growth hormones. Ten lignocellulolytic bacterial isolates obtained from two previous studies were procured from the Department of Agricultural Microbiology and screened for lignocellulolytic potential and plant growth promotion ability. The isolates with superior lignocellulolytic activity and PGP activities were checked for compatibility and used for composting of banana residues. The potential of the resultant bio-enriched compost for supporting plant growth was evaluated by pot culture studies using red amaranthus var. Vaika. Among the ten isolates tested, isolate M1045 recorded the highest cellulolytic index (5.86) and CLSD07 exhibited the maximum ligninolytic index (5.77). Carboxymethyl cellulase (CMCase) activity of the isolates ranged from 3.43 to 9.88 U mL⁻¹ min⁻¹, with M1045 showing the highest activity, followed by CLSD07. Filter paper cellulose (FPase assay) activity was maximum in CLSD07 (2.56 U mL⁻¹ min⁻¹) which was followed by M1045 (2.40 U mL⁻¹ min⁻¹) after 96 h of incubation. Laccase activity peaked in CLSM03 (1.53 U mL⁻¹) and CLSM02 (1.39 U mL⁻¹), while lignin peroxidase and manganese peroxidase were maximally produced by M1045 (18.60 and 3.56 U mL⁻¹, respectively). All isolates synthesized indole-3-acetic acid (IAA), with CLSD07 producing the highest amount (55.36 μg mL⁻¹ in the presence of the precursor, tryptophan). Gibberellic acid production was highest in M1045 (98.54 μg mL⁻¹), followed by G1051 (84.11 μg mL⁻¹). Extracellular ammonia production was maximum in CLSD07 (7.46 μmol mL⁻¹). Six isolates were capable of solubilizing inorganic phosphate in NBRIP medium, with M1045 showing the highest phosphate solubilization index (2.87), followed by G1051 (2.67). Both M1045 and CLSD07 produced siderophores, and four isolates demonstrated antagonistic activity against Rhizoctonia solani, with G1051 showing the greatest inhibition (40.23% over control), followed by W1048 (36.48%). Compatibility analysis revealed that the best performing isolates M1045, CLSD07, W1048, and G1051 exhibited no mutual inhibition, confirming their suitability for consortium development. Morphological and biochemical characterization of the isolates revealed that all the five isolates were Gram-positive, endospore-forming rods. The 16S rRNA gene sequencing showed maximum sequence similarity of the isolates with Calidifontibacillus erzurumensis (M1045), Bacillus velezensis (CLSD07) and Bacillus spp. (W1048 and G1051). The microbial consortium prepared from these isolates was evaluated for its composting efficiency. The four bacterial isolates were cultured in nutrient broth and consortium was prepared by mixing equal volumes of all four isolates. The consortium (~1x108 cfu mL-1) was sprayed over banana residue chopped into small pieces at the rate of 0.1%, 0.2% and 0.5% (v/w). The treatment with 0.5% consortium (T₃) achieved rapid decomposition, attaining the highest temperature (48.2°C) during the fourth week, lowest C:N ratio (12.70:1) by the ninth week, minimum moisture content (21.54%), and lowest bulk density (0.185 g cm⁻³). The resulting compost was nutrient-enriched, with elevated levels of nitrogen (1.41%), phosphorus (0.43%), potassium (1.28%), zinc (14.65 mg kg⁻¹), and copper (53.75 mg kg⁻¹), while heavy metals remained below detectable limits, ensuring safety for agricultural use. However, the C:N ratio fell below 20:1 at the 5th week itself, which is the level acceptable for compost for agricultural purpose.

From the initial screening, the treatment with 0.5% microbial consortium (T1) was identified as the most effective based on rapid decomposition rate and minimum time required to attain a lower C:N ratio. The selected treatment was therefore used for subsequent studies comparing its performance with an uninoculated control. The compost used for pot culture studies was prepared by mixing 0.5% v/w of bacterial consortiumwith 600 kg of banana residue. This treatment showed improved compost quality, registering a temperature of 68.64°C during the third week, pH of 7.81, EC of 2.71 dS m⁻¹, and a reduced C:N ratio of 13.05:1, with enhanced nutrient status (N 1.31%, P 0.466%, K 1.243%, Zn 15.01 mg kg⁻¹). Application of bio-enriched compost significantly improved seed germination (96.5%) and germination index (106.3) in red amaranthus. The treatment T1 – bio-enriched compost markedly enhanced growth attributes, including plant height (52.43 cm), number of leaves (57), branches (8.33), and root length compared to control. Overall, the lignocellulolytic bacterial consortium with plant growth promotion potential demonstrated superior composting efficiency, producing a nutrient-rich, agriculturally safe compost that promoted plant growth and could serve as a sustainable alternative to chemical fertilizers.