Wheat response to N2 fixed by faba bean (Vicia faba L.) as affected by sulfur fertilization and rhizobial inoculation in semi-arid Northern Ethiopia

Nitrogen fixation in faba bean (Vicia taba cv. Mesay) as affected by sulfur (S) fertilization (30kg S ha(-1)) and inoculation under the semi-arid conditions of Ethiopia was studied using the N-15-isotope dilution method. The effect of faba bean-fixed nitrogen (N) on yield of the subsequent wheat crop (Trificurn aestivum L.) was also assessed. Sulfur fertilization and inoculation significantly (p < 0.05) affected nodulation at late flowering stage for both 2004 and 2005 cropping seasons. The nodule number and nodule fresh weighs were increased by 53% and 95%, relative to the control. Similarly, both treatments (S fertilization and inoculants) significantly improved biomass and grain yield of faba bean on average by 2.2 and 1.2 Mg ha(-1). This corresponds to 37% and 50% increases, respectively, relative to the control. Total N and S uptake of grains was significantly higher by 59.6 and 3.3 kg ha(-1), which are 76% and 66% increases, respectively. Sulfur and inoculation enhanced the percentage of N derived from the atmosphere in the whole plant of faba. bean from 51% to 73%. This corresponds to N-2 fixation varying from 49 to 147kg N ha(-1). The percentage of N derived from fertilizer (%Ndff) and soil (%Ndfs) of faba bean varied from 4.3% to 2.8 %, and from 45.1 % to 24.0%, corresponding to the average values of 5.1 and 47.9kg N ha(-1). Similarly, the %Ndff and %Ndfs of the reference crop, barley, varied from 8.5 % to 10.8% and from 91.5% to 89.2%, with average N yields of 9.2 and 84.3 kg N ha(-1). Soil N balance after faba bean ranged from 13 to 52kg N ha(-1). Beneficial effects of faba bean on yield of a wheat crop grown after faba bean were highly significant, increasing the average grain and N yields of this crop by 1.11 Mg ha(-1) and 30kg ha(-1), relative to the yield of wheat grown after the reference crop, barley. Thus, it can be concluded that faba bean can be grown as an alternative crop to fallow, benefiting farmers economically and increasing the soil fertility.