Symbiotic N2 fixation in cowpea varieties is markedly enhanced by inoculation with elite Bradyrhizobium strains

Because of its excellent ability to fix atmospheric nitrogen, cowpea [Vigna unguiculata (L.) Walp] makes a significant contribution to soil sustainability and productivity in the resource limited tropical regions. However, due to in part to ineffectiveness and limited availability of bio-inoculant, its symbiotic N contribution and yield remained low in the field. Therefore, this study examined the effect of elite cowpea infecting Bradyrhizobium strains (CP-24 and CP-37) on shoot biomass and symbiotic nitrogen contributions of four cowpea varieties (Keti, TVU, Black eye bean, and White wonderer trailing). For this a two-year field experiment was carried out at three sites using a factorial randomized complete block design with four replications. The natural abundance of the ¹⁵N technique was used to compute the symbiotic N contribution. Bradyrhizobium inoculation led to significantly higher nodule formation, % Ndfa, amounts of N fixed, and shoot biomass, demonstrating the effectiveness and ability of the strains to enhance soil fertility. Inoculating cowpea with CP-24 strain increased shoot N content, % Ndfa and N fixed by 40%, 15%, and 41%, respectively, in comparison to the un-inoculated control. Furthermore, the inoculant by variety interaction had a significant effect on nodule number, nodule dry weight, and amount of N fixed, with TVU and White Wonderer trailing in combination with CP-24 exhibiting the most outstanding performance. There was also a strong positive correlation between biomass accumulation and N fixed, as well as N fixed and seed yield. Therefore, Bradyrhizobium inoculation on cowpea varieties TVU and White Wonderer trailing with CP-24 strain is recommended at all three tested sites and similar agro-ecologies for improved symbiotic N contribution and associated yield advantage of cowpea. This study highlights that, the use of elite and crop specific Bradyrhizobium strains can boost symbiotic nitrogen contribution, soil fertility, and the yield performance of legumes. Thus, it helps resource-poor farmers who are suffering from rising mineral fertilizer cost to achieve food security while reducing climate change risks.