Changes in soil microbial activity, essential oil quantity, and quality of Thai basil as response to biofertilizers and humic acid

Thai basil (Ocimum basilicum var. thyrsiflorum) is an important medicinal plant that used in pharmaceutical, cosmetic, and food industries. In order to evaluate the effect of biofertilizers and humic acid on secondary metabolites of Thai basil, a factorial experiment; fertilizer treatments (Control, NF (nitrogen-fixing bacterium), PS (phosphate solubilizing bacteria), combination of NF and PS, chemical fertilizer, combination of NF and PS + 50% chemical fertilizer) and humic acid (0 and 20 kg ha−1) was conducted. The soil microbial respiration and carbon biomass, soil organic matter, leaf nitrogen and phosphorus elements, number and size of glandular trichomes as well as the quantity and quality of the essential oil were evaluated. Results showed that the application of biofertilizers greatly caused to increase 60% in soil microbial respiration (1.11 CO2 g−1 dry soil day−1) and 3-fold in carbon biomass (117.47 mg C 100 g−1 dry soil) than control. The use of humic acid enhanced the soil organic matter by 2-fold (1.15%) more than control. The highest value of phosphorus element (0.67%), size of glandular trichomes in upper (411.11 μm2) and lower (419.31 μm2) surfaces, and essential oil content (2.81%) were obtained in the inoculation of plants by PS and the lowest values were observed in control. The combination of NF and PS + 50% chemical fertilizer led to increment 45% in the nitrogen element (3.42%), 1.5-fold in number of glandular trichomes in upper (5.03 number mm−2) and lower (6.22 number mm−2) surfaces and 80% essential oil yield (7.89 g m−2) compared to the control. The maximum amount of methyl chavicol (66.26%), eucalyptol (12.98%), linalool (5.42%), and β-pinene (2.15%) was recorded in combination of NF and PS + 50% chemical fertilizer and the minimum amounts were detected in control. Despite the positive effect of the simultaneous application of humic acid and biofertilizers treatment on the soil organic matter, nitrogen element and some major oil components, humic acid had no significant impact on the phosphorus concentration, size of glandular trichomes and essential oil content and yield. Further investigation is needed to use humic acid in plant nutrition management. In conclusion, combination of NF and PS + 50% chemical fertilizer can be recommended as a suitable solution to reduce the demand for chemical fertilizers and their environmental hazards in warm regions.