Effectiveness, competitiveness and persistence of inoculant Rhizobium for perennial African clovers in a highland Vertisol

Abstract

The effectiveness of 20 strains of Rhizobium leguminosarum bv. trifolii was evaluated with Trifolium semipilosum and T. burchellianum grown in a Vertisol soil in the glasshouse at the International Livestock Research Institute (ILRI) in Addis Ababa. Several effective strains were identified for both species. In T. semipilosum, inoculation significantly increased nodule DM and root N yield over the uninoculated control; Ethiopian Rhizobium isolates outperformed isolates from other sources for shoot DM and N yield. In T. burchellianum, contrast analysis revealed that there was no significant response to inoculation, although one effective strain was identified. Inoculant strains failed to overcome the competitive dominance of indigenous strains as reflected in mean nodule occupancies by inoculant strains of 15 and 7% in T. semipilosum and T. burchellianum, respectively. The 20 strains showed variable persistence following a 5-week drought period; only two of eight (T. semipilosum) and six of eight (T. burchellianum) strains were recovered from nodules on seedlings planted in the soil following the drought period. Overcoming the constraints of low nodule occupancy and variable persistence will require further understanding of the competitive interaction and the factors affecting access to nodule infection sites if superior Rhizobium–clover combinations are to be identified and developed.