Biochar-Loaded Purpureocillium lilacinum Formulation Enhances Soil Water Retention, Drought Tolerance, and Essential Oil Productivity of Spearmint (Mentha spicata L.)
Abstract
: Water deficit severely constrains the productivity and essential oil yield of medicinal and aromatic crops grown in sandy soils. This study assessed whether a biochar-loaded Purpureocillium lilacinum formulation could improve soil properties, drought tolerance, growth, mineral nutrition, soil biological activity, and essential oil production of spearmint ( Mentha spicata L.) under full and deficit irrigation. A two-season field experiment was conducted in 2024 and 2025 using a split-plot design arranged in a randomized complete block with three replicates. Irrigation regimes, including 100% and 75% of crop evapotranspiration (ETc), were assigned to main plots, whereas bioformulation treatments, including untreated control, biochar alone, free P. lilacinum , and biochar-loaded P. lilacinum , were assigned to subplots. The formulation maintained high viability during storage, with fungal counts declining from 8.30 × 10 7 to 5.10 × 10 7 colony-forming units (CFU) g − 1 after 60 days, while pH and electrical conductivity remained relatively stable. Deficit irrigation reduced soil organic matter, water-holding capacity, available macronutrients, plant growth, chlorophyll content, relative water content, leaf nutrient concentrations, essential oil yield, and dehydrogenase activity, while increasing proline accumulation. Conversely, bioformulation treatments improved most measured traits, with biochar-loaded P. lilacinum producing the strongest response. This treatment enhanced soil water retention and nutrient availability, reduced bulk density, improved biomass accumulation and physiological status, and maintained high essential oil yield under both irrigation regimes. Overall, biochar-loaded P. lilacinum represents a promising bioformulation for improving spearmint productivity, drought resilience, soil biological activity, and essential oil performance in sandy soils under deficit irrigation