Soybean Seed Coinoculation with <i>Priestia megaterium</i> ILBB592 and <i>Bradyrhizobium elkanii</i> Increases the Efficiency of Phosphorus Fertilization and Modulates the Rhizospheric Bacterial Community in the Field

Technologies that improve phosphorus (P) use efficiency in crops can reduce P fertilization needs and P losses, enhancing sustainability. This study evaluated the coinoculation of soybean seeds (cv. INIA Genesis 5602) with Bradyrhizobium elkanii and two P-mobilizing plant growth-promoting rhizobacteria (PGPR), Priestia megaterium ILBB592 and Bacillus pumilus ILBB44, formulated as biofertilizers. Field trials were conducted over two years with and without P fertilization. Coinoculation with ILBB592 improved P, N, and K uptake and the shoot dry weight regardless of P input, while ILBB44 increased the nodule dry weight. Both PGPR increased rhizospheric bacterial diversity and predicted P cycling gene abundance, especially ILBB592. Phosphorus agronomic efficiency rose when P fertilization was combined with either PGPR. Overall, coinoculation with P. megaterium ILBB592 consistently enhanced plant performance under both P regimes, highlighting its potential to sustainably reduce fertilizer use.