Nutritional Imbalance Induced by Phosphorus Overfertilization and Bacterial Mitigation in Common Bean

Common bean (Phaseolus vulgaris L.) growth and productivity depend on the efficient and coordinated use of nitrogen (N) and phosphorus (P). This study evaluated whether P oversupply constrains nutrient-use efficiency and whether selected Antarctic plant-associated bacterial isolates improve N and P utilization in three common bean cultivars. Sapito, Cimarrón, and Zorzal were inoculated with Microbacterium sp. (UA-023-E), Rahnella sp. (UA-025-E), and Bacillus sp. (UA-061-E) at 106, 107, and 108 Colony-Forming Units (CFUs) mL−1. At 14 days after sowing, nutrient-use efficiency, N:P stoichiometry, biomass, root architecture, nodulation, chlorophyll content, and gas exchange were evaluated. Greater P accumulation under P-rich conditions was negatively associated with photosynthetic N-use efficiency (PNUE), biomass, root development, chlorophyll content, and photosynthesis, indicating inefficient nutrient utilization. Bacterial inoculation partially mitigated these responses, increasing biomass by up to 34% in Cimarrón, chlorophyll content by 50–60% in Sapito, and photosynthesis by up to 15%. UA-023-E increased nodulation approximately eight-fold in Sapito, whereas UA-025-E approximately doubled root length in Sapito and Cimarrón. Multivariate analyses associated bacterial responses with PNUE, PPUE, N:P ratio, root architecture, and plant performance, showing the most favorable combination: Sapito+UA-025. Overall, plant performance under P-rich conditions depended on coordinated N and P utilization rather than nutrient accumulation alone, with bacterial effects varying among cultivars, strains, and inoculum concentrations.