Growth and nutrient uptake enhancement of Bambara groundnut under drought stress as impacted by Bradyrhizobium japonicum and Bacillus subtilis co-inoculation

Although Bambara groundnut (food security crop) is relatively drought-tolerant, water stress can still adversely affect its growth, physiological performance, and nutrient uptake. Enhancing nutrient acquisition under drought conditions is therefore essential for improving its productivity. This study evaluated the effects of B. japonicum and B. subtilis BD234 co-inoculation on growth, biomass production and nutrient uptake by landraces of Bambara groundnut under simulated drought conditions. A greenhouse experiment was conducted using a completely randomized design in a 5 × 3 × 2 factorial arrangement with four replicates. Treatments included i) inoculation with B. subtilis BD234 alone, ii) inoculation with B. japonicum alone , iii) co-inoculation with both microbial strains, iv) ammonium nitrate (NH₄NO₃) as a positive control, and v) a non-inoculated control. The drought stress treatment was imposed at three levels: 100% field capacity (well-watered), 50% field capacity (moderate stress), and 25% field capacity (severe stress), using two Bambara groundnut landraces (red and cream). Co-inoculation with B. japonicum and B. subtilis BD234 had a significant effect ( p < 0.05) on growth, biomass production and nutrients accumulation in shoots of Bambara groundnut under drought stress and landrace response was significant, with the red landrace showing superior adaptation compared to the cream landrace. Co-inoculated plants were taller with more leaves, longer roots, and higher biomass production. Additionally, drought stress influenced nutrient uptake and the associations among them and landrace response was also significant. Under well-watered conditions, co-inoculation enhanced phosphorus (P) uptake by 67% and 61% in the red and cream landraces, respectively, compared to their respective uninoculated controls, while under severe drought stress, increases of 14% and 17% were recorded for the red and cream landraces, respectively. Likewise, under moderate drought conditions, co-inoculation enhanced Ca absorption by 65% and 64% in shoots of the red and cream landraces (respectively) compared to uninoculated controls. Co-inoculation markedly improved uptake of both macro-nutrients (P, K, Ca, Mg) and micro-/trace elements (Na, Cu, Mn, Fe, and B) in shoots of Bambara groundnut landraces subjected to simulated drought stress. These findings imply that combined microbial inoculation can be a viable, sustainable strategy to improve drought resilience and nutrient use efficiency in Bambara groundnut cultivation, particularly when selecting drought-tolerant landraces.