Seed and soil inoculation with a Bacillus consortium in durum wheat. part II: yield components, grain technological quality and mineral composition

Reducing chemical fertilizers while ensuring food security is a major challenge for Mediterranean cereal systems. This study (Part II) evaluated the effects of a multi-strain Bacillus consortium (B. licheniformis, B. methylotrophicus, B. megaterium) on yield components, grain technological quality, and mineral composition of durum wheat (cv. Antalis) under fertilized and unfertilized conditions across multiple delivery strategies (seed, soil, and combined applications) at two contrasting Sicilian sites (Sparacia and Campo Carboj). Results showed that productivity and quality traits were strongly shaped by site-specific factors, with combined seed + soil treatments consistently performing best. At Campo Carboj (less limiting soil), the most intensive combined application closed the yield gap between fertilized and unfertilized plots without compromising technological quality. At Sparacia (heavy clay soil), the consortium strongly stimulated biomass (+141%) and grain protein deposition. Crucially, interaction analysis revealed that the highest protein content (17.85%) occurred under the intensive combined inoculation plus mineral fertilization, indicating synergistic effects with nitrogen inputs. Furthermore, the intensive combined treatment promoted a notable increase in grain phosphorus and induced autonomous calcium biofortification at Campo Carboj, where the unfertilized inoculated plots matched the fertilized ones, offering physiological advantages against terminal stress. A marked accumulation of grain nitrates was also recorded under intensive treatments, providing clear evidence of microbially altered rhizosphere nitrogen dynamics. Overall, the Bacillus consortium acted as a complementary bio-stimulatory tool rather than a direct replacement for chemical fertilizers but provided context-dependent compensation and improving nutrient assimilation. Combined multimodal inoculation represents a powerful supplementary strategy to support sustainable durum wheat intensification and biofortification under reduced chemical inputs in Mediterranean field environments.