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

## TL;DR

This study shows that using Bradyrhizobium and Azospirillum microbes together improves lima bean growth, nitrogen use, and soil health in dry areas.

## Contribution

The novel finding is that co-inoculation of Bradyrhizobium elkanii and Azospirillum brasilense significantly enhances nitrogen metabolism and nutrient uptake in lima bean genotypes.

## Key findings

- Co-inoculation increased root nodulation up to 277 nodules per plant and shoot nitrogen content by up to 91%.
- Inoculation improved soil indicators like pH and available phosphorus and potassium.
- The 'Raio de Sol' and 'Manteiga' varieties showed the most growth and biomass increases.

## Abstract

Lima bean (Phaseolus lunatus 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 Bradyrhizobium elkanii (strain BR 2003) and Azospirillum brasilense (strain Ab-V5) on nitrogen metabolism, nutrient uptake, plant growth, and residual soil fertility in P. lunatus. Four varieties were subjected to four treatments: control (nitrogen fertilization), single inoculation with B. elkanii or A. brasilense, 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.