Bradyrhizobial inoculation and P application effects on haricot and mung beans in the Ethiopian Rift Valley

Abstract

To investigate factors limiting biological nitrogen fixation (BNF) by grain legumes in the Ethiopian Rift Valley in soils with different input legacies. Two field experiments were set up, at a high-input (Hawassa) and a low-input (Lokabaya) site, comparing each two varieties of haricot bean (Phaseolus vulgaris var. Hawassa dume and Awash1) and mung bean (Vigna radiata var. N26 and Sunaina) throughout two cropping seasons, with and without commercially available inoculants and/or phosphorous addition (2nd year only). Symbiotic performance was evaluated by estimating the proportion of N derived from the atmosphere (%Ndfa) using 15N natural abundance. Mean Ndfa of haricot bean ranged from 31 to 57% at Hawassa and 56 to 57% at Lokabaya with inconsistent differences between varieties and years. Mung beans had lower Ndfa values than haricot beans, but performed better at the low-input (47–53%) than the high-input (9–24%) sites. Overall, haricot bean fixed more N at Hawassa (71–99 kg ha−1) than at Lokabaya (17–36 kg ha−1), while BNF by mung bean was small at both sites (8–25 kg N ha−1). Both Ndfa and N yields responded positively to P addition in the second year, which roughly resulted in 50% increase in BNF by both beans at the low-input site Lokabaya but not at the high-input site Hawassa. Combining P addition with rhizobial inoculation increased BNF and N yields, but there was no significant difference to P addition alone. Low soil fertility typical for the Rift Valley smallholder farms strongly constrains grain legume yields despite good symbiotic performance. In marginal soils, inoculation together with P addition may increase Ndfa to high values (> 60%) in haricot bean, but N yields remain small relative to yields obtained in high-input soil. This indicates that the large yield gap commonly experienced by smallholder farmers cannot readily be overcome by the use of inoculants or P fertilization and that long-term soil fertility management is needed to increase haricot bean productivity in the region. By contrast, mung bean, which had similar yields with haricot bean in the marginal soil, is responsive to P addition and may thus be a valid alternative for small holders with marginal soils.