Enhancing lemon balm’s resistance to drought with poultry manure compost fortified by biological, chemical, and organic amendments

• Integrated poultry compost + biochar + microbes boosts lemon balm drought resilience. • Soil pH reduction & nutrient enrichment via fortified compost under water stress. • Synergistic amendments elevate soil enzymes, microbial activity, and biomass yield. • Biochar-enriched compost enhances micronutrient solubility in calcareous soils. • Sustainable strategy for arid agriculture: Organic-chemical-microbial synergy. Lemon balm ( Melissa officinalis L . ) is a valuable medicinal and aromatic herb that is susceptible to the adverse effects of drought stress, a growing concern due to climate change. This study aimed to investigate how the biomass of lemon balm plants under water shortage conditions in calcareous soil was influenced by enriched compost of poultry manure, biochar derived from poultry manure, nitrogen, potassium, and phosphorus fertilizers, sulfur, and beneficial microorganisms ( Trichoderma harzianum and Thiobacillus thioparus ). The experiment was conducted in a greenhouse using a completely randomized factorial design, incorporating three levels of water deficit stress (95–100 %, 75–80 %, and 55–60 % field capacity) alongside various soil amendments, applied either singly or in combination. The results indicated that, compared to the control, the combined application of nitrogen, potassium, and phosphorus fertilizers, Th. thioparus with elemental sulfur, T. harzianum , and compost enriched with biochar significantly enhanced the biomass of lemon balm under water-deficient conditions. This integrated approach led to decreased soil pH, increased electrical conductivity, enhanced availability of nutrients (including N, P, K, Mn, Fe, Zn, and Cu), as well as improvements in microbial respiration, microbial biomass carbon, and the activity of soil enzymes (urease, acid and alkaline phosphatase, and dehydrogenase). The findings suggest that the combined effects of these amendments can effectively mitigate the negative impacts of water deficiency stress on lemon balm production. This comprehensive strategy promotes sustainable agricultural practices that enhance the resilience of aromatic and medicinal plants in water-limited environments. Further research is needed to optimize application rates and explore the long-term effects of this integrated approach on soil health and lemon balm productivity.