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

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 P2O5) and plant traits were assessed, alongside physiological indicators of canopy function (SPAD chlorophyll index, stomatal conductance [Gs] and NDVI across key BBCH stages). Across both sites, microbial inoculation increased soil P2O5 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 Gs 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.