Seed and Soil Inoculation With a <i>Bacillus</i> Consortium in Durum Wheat. Part I: Soil Phosphorus Content and Plant Morphological and Physiological Performance

ABSTRACT Reducing synthetic fertilizer inputs in durum wheat while maintaining crop performance is a key challenge for Mediterranean cereal systems, where soil constraints frequently limit nutrient availability and canopy function. This study evaluated the physiological and soil‐nutrient responses of durum wheat ( Triticum turgidum subsp. durum cv. Antalis) to a multi‐strain Bacillus consortium ( B. licheniformis , B. methylotrophicus , B. megaterium ) applied through different delivery strategies under fertilized and unfertilized field conditions. Field trials were conducted during the 2023–2024 season at two contrasting Sicilian sites (Campo Carboj and Sparacia) using a randomized complete block design with three replicates. Treatments included seed coating, soil spraying at three rates and combined seed + soil applications. Post‐harvest soil available P (Olsen, expressed as P 2 O 5 ) and plant traits were assessed, alongside physiological indicators of canopy function (SPAD chlorophyll index, stomatal conductance [G s ] and NDVI across key BBCH stages). Across both sites, microbial inoculation increased soil P 2 O 5 availability, with the strongest responses at Sparacia (clay‐rich, more P‐immobilizing conditions), where several microbial treatments produced increases comparable to mineral fertilization and, in some cases, exceeded +50% relative to the unfertilized control. Microbial application also enhanced crop physiology, particularly at Sparacia: non‐fertilized treatments increased plant height (up to +23%), sustained higher SPAD values, improved G s and increased NDVI from tillering through grain filling, indicating improved canopy vigour under nutrient limitation. Responses at Campo Carboj were generally smaller but consistently positive. Combined seed + soil applications (notably S6–S7) provided the most robust improvements, supporting the agronomic value of multi‐modal inoculation strategies. Overall, the consortium partially compensated for fertilizer withdrawal by improving soil P availability and sustaining physiological performance , with the magnitude varying by site and driven by edaphic context.