Native symbiotic rhizobia strains improve yield and nitrogen fixation in underutilized Bambara groundnut (Vigna subterranean L. Verdc.)

Bambara groundnut (BG) (Vigna subterranean L. verdc) is a highly nutritious and economically important leguminous crop of African origin that forms symbiotic relationships with nitrogen-fixing rhizobia, but it has a low yield. Currently, there is a paucity of information on effective rhizobium inoculant (to improve the yield) for BG, which are cheaper and safer alternatives to chemical fertilizers. Indigenous rhizobia strains were trapped from 54 farm soils collected at three Nigerian states (Niger, Kaduna, and Kano). Six selected rhizobium strains (Bradyrhizobium spp.) were identified using 16S rDNA sequencing, applied as an inoculant in a field experiment using a completely randomized block design, and compared with USDA110 strain (Bradyrhizobium japonicum), urea fertilizer, and uninoculated plants on two selected BG varieties (TVSU1248 and TVSU 631). Nutrient utilization and nitrogen fixation in plants were determined at 6 weeks, while leaf chlorophyll was determined fortnightly. Seed yield was determined at maturity. The strains increased the %nitrogen (1.33 ± 0.10–≥1.37 ± 0.05), %phosphorous (0.48 ± 0.02–≥0.51 ± 0.02), %nitrogen fixed (21.07 ± 2.38–≥61.30 ± 6.13), nodulation (72.50 ± 21.075–154.00 ± 23.79), and leaf chlorophyll, which was ≥49.74 ± 1.47 mg/L, ≥51.82 ± 1.86 mg/L, and ≥49.65 ± 1.56 mg/L at second, fourth, and sixth weeks after planting, respectively, and the yield of BG (from an average of 398.6 to 1454.725 kg/ha). Seed yield was highest (1869.85 ± 273.68 kg/ha) in BG variety TVSU1248 using BN5 strain. Indigenous Bambara-symbiotic strains significantly increased the seed yields and plant nutrients, showing their potential use as an inoculant to improve the yield of BG.