Indigenous rhizobia population influences the effectiveness of Rhizobium inoculation and need of inorganic N for common bean (Phaseolus vulgaris L.) production in eastern Ethiopia

Supplement with inorganic N application is essential to improve the common bean production in sub-Saharan Africa. However, the influence of indigenous rhizobial population on the inorganic N requirement with Rhizobium inoculation to secure sustainable way of common bean production system is not well known. The effect of different rates of N application either alone or in combination with Rhizobium inoculation on the nodulation, yield and yield traits of common bean cultivated in soils with different rhizobial population were conducted. Twelve treatments were produced by factorially combined six levels of N fertilizer (0, 20, 40, 60, 80 and 100 kg N ha −1 ) and two Rhizobium inoculation treatments (inoculated and uninoculated). The treatments were laid out in randomized completely block design and all treatments were replicated three times. Regardless of soil types, nodule number and nodule dry weight decreased with increasing rates of N application. 20 kg N ha −1 both alone and in combination with Rhizobium inoculation resulted in the largest nodulation in all soil types. The largest nodulation were induced in soil with large rhizobial population. Rhizobium inoculation significantly ( P < 0.05) improved yield and yield traits of common bean. Moreover, our result revealed that the largest values of investigated traits were observed in inoculated treatment, as compared to the corresponding N rates of uninoculated treatments. The 20, 100 and 40 kg N ha −1 treatments resulted in significantly greater plant total tissue N at soil types with small, medium and large rhizobial population, respectively, as compared to unfertilized control. The highest total biomass yield (TBY) and grain yield (GY) at soil types with small and medium rhizobial population were obtained by the 100 kg N ha −1 treatment in combination with Rhizobium inoculation, while 20 and 40 kg N ha −1 applications produced the greatest TBY and GY, respectively, in soil with large rhizobial population. These results indicate that N requirement is varied based on rhizobial population and effectiveness of native rhizobia in N 2 fixation.