Impact of Bradyrhizobium elkanii and Azospirillum brasilense Co-Inoculation on Nitrogen Metabolism, Nutrient Uptake, and Soil Fertility Indicators in Phaseolus lunatus Genotypes

Lima bean (<i>Phaseolus lunatus</i> L.), an important legume in semiarid environments, often exhibits low yield, requiring strategies to enhance symbiotic nitrogen fixation and nutrient-use efficiency. This study evaluated the effects of single and combined inoculation with <i>Bradyrhizobium elkanii</i> (strain BR 2003) and <i>Azospirillum brasilense</i> (strain Ab-V5) on nitrogen metabolism, nutrient uptake, plant growth, and residual soil fertility in <i>P. lunatus</i>. Four varieties were subjected to four treatments: control (nitrogen fertilization), single inoculation with <i>B. elkanii</i> or <i>A. brasilense</i>, and co-inoculation. All inoculation strategies significantly increased root nodulation, nitrogen assimilation, and the accumulation of key macronutrients. Root nodulation increased from 1 to 12 nodules per plant in the control treatments to up to 277 nodules per plant under inoculation, while shoot nitrogen content increased by up to 91% in 'Raio de Sol' and 87% in 'Cearense'. Increases in P and K were also observed, including a 48% increase in shoot P in 'Manteiga' and up to a 100% increase in shoot K in 'Raio de Sol', whereas root K increased by up to 90% under co-inoculation. The 'Raio de Sol' and 'Manteiga' varieties exhibited the most pronounced increases in growth and biomass. Additionally, inoculation improved post-cultivation soil indicators, including pH and available P and K in specific genotype-microbe combinations, and reduced electrical conductivity. These results demonstrate the strong contribution of microbial inoculation to nitrogen assimilation and nutrient acquisition, supporting its use as a promising alternative to conventional nitrogen fertilization in lima bean cultivation.