Integrative evaluation of symbiotic performance and biomass allocation in rhizobial inoculated cowpea under contrasting soil microbial conditions

Reliable evaluation of rhizobial inoculant performance requires integrative assessment of nodulation and biomass responses under contrasting soil microbial environments. A controlled pot experiment was conducted to evaluate cowpea (Vigna unguiculata L. Walp.) responses to inoculation with Sinorhizobium fredii, Bradyrhizobium japonicum, mineral nitrogen (urea), and a non-inoculated control under heat-treated and unsterilized sandy loam soils. Treatments were arranged in a completely randomised design with three replicates and monitored up to 10 weeks after planting (WAP). Nodulation, vegetative growth, shoot biomass, and root biomass were assessed at 2, 4, 6, 8, and 10 WAP. Integrative indices, including nodulation biomass efficiency ratio (NBER), biomass allocation ratio (BAR), and symbiotic advantage index (SAI), were calculated at 10 WAP, and Pearson correlation analysis was performed. Nodulation differed significantly among treatments under both soil conditions (P ≤ 0.001). In heat-treated soil, S. fredii produced 45.33 nodules plant−1 compared with 27.00 nodules plant−1 under B. japonicum, while non-inoculated plants showed limited nodulation (5.00 nodules plant−1), indicating residual indigenous rhizobial influence. Under unsterilized soil, nodulation increased to 74.00 and 46.00 nodules plant−1 under S. fredii and B. japonicum, respectively. S. fredii also produced the highest shoot (5.70 g plant−1) and root biomass (2.70–2.80 g plant−1). Integrative indices significantly differentiated treatment performance, confirming S. fredii as the most effective inoculant and demonstrating the value of combined nodulation-biomass assessment for evaluating rhizobial effectiveness.