Effect of Bio-Enriched Vermicompost with Plant Growth-Promoting Bacteria on Basil Yield and Soil Biological Properties

Background and Objectives: The rapid growth of the global population and the increasing demand for high-quality agricultural and medicinal products have intensified the importance of optimizing agricultural inputs. In modern farming systems, chemical fertilizers are often indispensable for supplying essential nutrients quickly to crops. However, excessive and unbalanced use of these fertilizers has led to serious environmental consequences, including contamination of groundwater and surface water, loss of soil biodiversity, and degradation of soil structure and fertility. Such issues have emphasized the necessity of adopting sustainable agricultural practices that can maintain or enhance crop productivity while reducing environmental risks. Among organic inputs, vermicompost has emerged as a valuable bio-organic fertilizer. Produced through the biological decomposition of organic residues by earthworms such as Eisenia foetida, vermicompost is rich in bioavailable nutrients, humic substances, and beneficial microorganisms. It also improves the soil’s physical, chemical, and biological characteristics. Despite these advantages, achieving optimal agronomic results typically requires applying large quantities of vermicompost, which can present economic and logistical challenges. To address this limitation, bio-enrichment of vermicompost with plant growth-promoting bacteria (PGPB) has been proposed as an innovative approach to enhance its effectiveness. These beneficial bacteria are capable of improving plant nutrition and soil health through mechanisms such as nitrogen fixation, solubilization of insoluble phosphates, production of plant growth hormones like auxins and gibberellins, and stimulation of microbial activity in the rhizosphere. Basil (Ocimum basilicum L.), an economically important medicinal and aromatic plant widely used in the pharmaceutical, food, and cosmetic industries, is highly sensitive to soil conditions. Therefore, using bio-enriched vermicompost has the potential to improve both the growth and quality of basil. Based on this premise, the present study aimed to investigate the effects of vermicompost enriched with different bacterial strains on growth parameters of basil and on soil biological properties under controlled greenhouse conditions.
Materials and Methods: This study was conducted in 2019 at the research greenhouse of Razi University, Kermanshah, Iran, using a completely randomized design with six treatments and four replications. The experimental treatments included: vermicompost without bacterial inoculation (positive control); well-decomposed animal manure (control); vermicompost enriched with Bacillus megaterium strain 56; vermicompost enriched with Bacillus velezensis strain FOL; vermicompost enriched with Bacillus pumilus strain INR7; and vermicompost enriched with Pseudomonas putida strain 54. The bacterial strains were sourced from two origins: some were isolated from the bodies of earthworms and from vermicompost, while others were obtained from the microbial culture collection of the Department of Plant Pathology at Razi University. For bacterial isolation, 60 bacterial isolates were obtained and purified from combined samples of vermicompost and earthworms through serial dilution and culturing on agar medium. For screening, their effects on basil growth traits were evaluated in a greenhouse experiment with 60 treatments and 3 replications. Finally, four strains with superior performance were selected for vermicompost enrichment. Vermicompost was produced by composting a mixture of fully decomposed cow manure and organic plant residues (including apple pomace, carrot waste, and eggshells) at a 3:1 w/w, which was decomposed over a period of three months by Eisenia foetida earthworms. After composting, each 10 kg batch of vermicompost was inoculated with 500 mL of bacterial suspension containing approximately 1×109 cells. Sweet basil (Ocimum basilicum L.) seeds of a commercial sweet variety were surface-sterilized, and then sown in pots filled with a sterilized mixture of field soil, washed sand, and vermicompost in a 1:1:2 ratio. Irrigation was applied every other day based on the field capacity of the substrate. Sixty days after sowing, before the flowering stage, the plants were harvested, and growth-related parameters, including fresh and dry weight of shoot and root, as well as plant height, were measured. In addition, several soil biological properties were evaluated, including basal microbial respiration, microbial biomass carbon (MBC), metabolic quotient (qCO2), and substrate-induced respiration (SIR). The collected data were analyzed using SAS software version 9.4, and means were compared using Duncan’s multiple range test at significance levels of 1% and 5%.
Results: The treatments had a statistically significant effect (p < 0.01) on all measured basil growth parameters. The highest shoot fresh weight (30.72 g/pot) was recorded in the treatment with vermicompost enriched with Bacillus megaterium strain 56, while the lowest (12.47 g/pot) was observed in the vermicompost enriched with Pseudomonas putida strain 54. This difference highlights the influence of bacterial strain selection on the efficiency of vermicompost enrichment. Plant height was also significantly affected by the treatments. The greatest height (26.6 cm) was achieved with the vermicompost enriched with Bacillus megaterium strain 56, whereas the lowest height (13.17 cm) occurred in the animal manure treatment. Regarding root parameters, the highest root fresh weight (19.62 g/pot) was recorded in the uninoculated vermicompost treatment, while the highest root dry weight (2.33 g/pot) was obtained with vermicompost enriched with Bacillus velezensis strain FOL. The lowest root fresh and dry weights were consistently observed in the treatment with Pseudomonas putida strain 54. In terms of soil biological properties, vermicompost enriched with Bacillus megaterium strain 56 exhibited the highest basal microbial respiration (275 mg C /kg soil. day), indicating enhanced microbial activity. In contrast, the lowest value (190.6 mg C /kg soil. day) was observed in the animal manure treatment. The highest microbial biomass carbon (317.98 mg/kg soil) was recorded in the uninoculated vermicompost treatment, whereas the lowest (238.48 mg/kg soil) occurred in the treatment enriched with Pseudomonas putida strain 54. However, no significant differences among treatments were observed for the metabolic quotient (qCO2) and substrate-induced respiration (SIR), which may reflect limited sensitivity of these indices or minor differences in microbial metabolic efficiency under the experimental conditions.
Conclusion: The findings of this study demonstrate that enriching vermicompost with plant growth-promoting bacteria, particularly Bacillus megaterium strain 56, can significantly improve basil growth performance and enhance soil biological properties. These improvements are likely due to increased nutrient availability, production of plant growth hormones, and stimulation of beneficial microbial communities in the soil. The observed differences among bacterial strains, including the weaker performance of Pseudomonas putida strain 54, underscore the importance of selecting effective and compatible strains for vermicompost enrichment. This research suggests that bio-enriched vermicompost can serve as a sustainable alternative to reduce dependency on chemical fertilizers, improve soil fertility, and produce high-quality medicinal plants such as basil. However, further studies at the field scale and under diverse environmental conditions are necessary to confirm the practical applicability and economic viability of this approach. The adoption of bio-enriched vermicompost represents a promising step toward sustainable agriculture and the production of medicinal and aromatic plants with enhanced quality and yield.