Inorganic nitrogen application improves the yield and yield traits of common bean (Phaseolus vulgaris L.) irrespective of the indigenous rhizobial population

Abstract

The effect of number and efficiency of indigenous rhizobia on the need for external inorganic nitrogen (N) application to boost common bean production is unknown. Hence, field experiments were conducted at four locations with soil populations of <100, 100–1 000 and >1 000 rhizobia to evaluate the effect of the indigenous rhizobial population on N requirement of common bean in eastern Ethiopia. The treatments comprised six levels of N application arranged in a randomised complete block design with three replications. The results showed decreases in nodule number (NN) and nodule dry weight (NDW) with increasing rate of inorganic N application in all soil types. Application of 20 kg N ha−1 resulted in a significant increase in the NN and NDW in soil with a low and moderate rhizobial population but not in soils with a high rhizobial population. The highest yield and yield traits in soil containing a high and low rhizobial population was recorded with <40 and >60 kg N ha−1 application, respectively. The association between grain yield and N application was negative in soils with a high and moderate rhizobial population, and positive in soil containing a low rhizobial population. In general, the results revealed that the number of indigenous rhizobia could affect the quantity of N applied.