Purpose: Phosphorus (P) deficiency is a key limitation in calcareous soils with high lime content, where fixation restricts nutrient availability and crop productivity. This study assessed the interactive effects of P fertilization and Trichoderma koningii inoculation—including its mutant strain—on barley (Hordeum vulgare L.) performance under rainfed conditions, with emphasis on improving phosphorus use efficiency (PUE). Methods: A two-year field trial (2023–2024) was conducted in northwest Iran using four P rates (12, 24, 36, and 48 kg ha⁻¹) and three inoculation treatments (non-inoculated, wild-type T. koningii, and mutant strain). Morphological traits, yield components, and P efficiency indices—PUE, P uptake efficiency (PUpE), and P recovery efficiency (PRE)—were evaluated. Results: Phosphorus application improved growth and yield, with the strongest responses at 36–48 kg ha⁻¹. Inoculation with T. koningii, particularly the mutant strain, further increased grain yield (up to 85%) and biological yield (up to 77%) relative to the uninoculated control. Both P and inoculation enhanced P efficiency indices, with the mutant strain showing the highest PUE (398–279), PUpE (0.77–0.80), and PRE (25–26%). Significant P × inoculation interactions indicated synergistic effects on yield formation, while factor analysis revealed stronger associations between yield traits and P efficiency under inoculation. Conclusion: Moderate P fertilization (36 kg ha⁻¹) combined with T. koningii, particularly its mutant strain, effectively improves P efficiency and barley productivity in calcareous soils. This integration of biofertilization and chemical input represents a sustainable strategy for nutrient management in rainfed systems.
