Investigating the Effect of Anaerobic Digestion Residue on Basil Growth, Secondary Metabolite Synthesis, and Growing Substrate Properties

To assess digestate’s efficacy as a fertilizer for basil development, a two-year pot experiment was established, comprising four fertilization treatments: namely, mineral fertilizer (F), digestate (D), combined mineral fertilizer and digestate (1:1, FD), and unfertilized control (C). Key metrics assessed included plant height, chlorophyll concentration index (CCI), total biomass (TB), leaf production (LP), essential oil yield, and composition. Post-harvest analysis evaluated nutrient and heavy metal content and pathogen contamination in the growing substrate and leaves. FD treatment produced the highest TB (68.2 g plant−1) and LP (52.7 g plant−1). Digestate application substantially enhanced substrate nutrient availability, increasing extractable phosphorus by 68.5%, potassium by 134.4%, and organic matter by 54.7%. The essential oil yield was significantly higher in the control plants. whereas different fertilization regimes altered secondary metabolite synthesis. Specifically, fertilization with digestate favored sesquiterpenes synthesis, inorganic fertilization enhanced methyleugenol and β-farnesene synthesis, and the control showed higher limonene, eugenol, and linalool. Heavy metal accumulation in the growing substrate was negligible, remaining well within regulatory limits. Salmonella spp., were not detected. Pathogen concentration in the growing substrate was low, while Enterococcus faecalis levels were marginally below EU safety limits (100 cfu g−1) on the leaves. Continuous monitoring of soil chemical properties and plant products after digestate application is essential to ensure soil health and food safety.