Biological nitrogen fixation in faba bean (Vicia faba L.) in the Ethiopian highlands as affected by P fertilization and inoculation

N-2 fixation by leguminous crops is a relatively low-cost alternative to N fertilizer for small-holder farmers in developing countries. N-2 fixation in faba bean (Vicia faba L.) as affected by P fertilization (0 and 20 kg P ha(-1)) and inoculation (uninoculated and inoculated) with Rhizobium leguminosarium biovar viciae (strain S-18) was studied using the N-15 isotope dilution method in the southeastern Ethiopian highlands at three sites differing in soil conditions and length of growing period. Nodulation at the late flowering stage was significantly influenced by P and inoculation only at the location exhibiting the lowest soil P and pH levels. The percentage of N derived from the atmosphere ranged from 66 to 74%, 58 to 74% and 62 to 73% with a corresponding total amount of N-2 fixed ranging from 169 to 210 kg N ha(-1), 139 to 184 kg N ha(-1) and 147 to 174 kg N ha(-1) at Bekoji, Kulumsa and Asasa, respectively. The total N-2 fixed was not significantly affected by P fertilizer or inoculation across all locations, and there was no interaction between the factors. However, at all three locations, N-2 fixation was highly positively correlated with the dry matter production and total N yield of faba bean. Soil N balances after faba bean were positive (12-58 kg N ha(-1)) relative to the highly negative N balances (-9-44 kg N ha(-1)) following wheat (Triticum aestivum L.), highlighting the importance of rotation with faba bean in the cereal-based cropping systems of Ethiopia.