Beyond broad-spectrum fertilization: genotype-specific responses of cowpea to integrated bio-inorganic amendments in Edo State, Nigeria

Abstract

This study evaluated the in vitro plant growth-promoting traits of isolated rhizobacteria and their in vivo synergistic effects when co-applied with inorganic fertilizers on the yield of three cowpea varieties (IT90K-82-2, IT89KD-391, and IT99K-593-1-1). Molecular characterization via 16S rRNA sequencing identified the biofertilizer isolates as Enterobacter hormaechei, Enterobacter cloacae, Alcaligenes sp., and Clostridium beijerinckii. In vitro biochemical assays revealed that E. cloacae exhibited the highest phosphate solubilization efficiency (239.0 µg/mL) and Indole-3-acetic acid (IAA) production (44.0 µg/mL), coupled with a significant reduction in medium pH. Field evaluations demonstrated that integrated nutrient management (INM) significantly (p < 0.05) enhanced nutrient mobilization, with E. cloacae and inorganic fertilizer combination (B2I2) achieving the maximum depletion of available soil phosphorus post-harvest. Consequently, the combined treatments overwhelmingly outperformed unamended controls across all crop yield parameters. Notably, the study revealed strong genotype-by-treatment interactions. Treatment C2B2I1 gave the greatest increase in plant height (29.56 cm), number of branches (6.90), and leaf area (8.54 cm2), while treatment C1B3I1 gave the highest number of leaves (23.23). Maximum increase in stem girth (3.39 cm) was achieved by treating with C2B2I2. For the Yield Parameters, Treatment C2B2I1 gave the maximum Pods per plant (PPP) (7.00), the highest Number of seeds per pod (NSPP) (7.67), the highest Number of Nodules per plant (NPP) (7.67), and the longest root length (18.80 cm). This study revealed that biofertilizer isolate B2 (E. cloacae) was most effective in enhancing cowpea growth and yield. Combinations with biofertilizer isolate B4 (C. beijerinckii) and inorganic fertilizer 2 should be avoided unless improved formulations are available. Variety C2 was identified as a high-performing genotype under integrated nutrient management. These findings indicate that broad-spectrum fertilization is suboptimal. Instead, the precise, genotype-specific integration of highly efficient microbial inoculants with inorganic fertilizers provides a highly sustainable and effective strategy for maximizing cowpea yield in nutrient-depleted soils.

Keywords

• 16S rRNA
• biofertilizers
• Enterobacter cloacae
• integrated nutrient management
• phosphate solubilizing bacteria
• Vigna unguiculata

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