Mineral Fertilizer Demand for Optimum Biological Nitrogen Fixation and Yield Potentials of Legumes in Northern Ethiopia

Farmers in Northern Ethiopia integrate legumes in their cropping systems to improve soil fertility. However, biological nitrogen fixation (BNF) potentials of different legumes and their mineral nitrogen (N) and phosphorus (P) demands for optimum BNF and yields are less studied. This study aimed to generate the necessary knowledge to enable development of informed nutrient management recommendations, guide governmental public policy and assist farmer decision making. The experiment was conducted at farmers' fields with four N levels, three P levels, and three replications. Nodule number and dry biomass per plant were assessed. Nitrogen difference method was used to estimate the amount of fixed N by assuming legume BNF was responsible for differences in plant N and soil mineral N measured between legume treatments and wheat. The result revealed that the highest grain yields of faba bean (2531 kg ha(-1)), field pea (2493 kg ha(-1)) and dekeko (1694 kg ha(-1)) were recorded with the combined application of 20 kg N ha(-1)and 20 kg P ha(-1). Faba bean, field pea and dekeko also fixed 97, 38 and 49 kg N ha(-1), respectively, with the combined application of 20 kg N ha(-1)and 20 kg P ha(-1); however, lentil fixed 20 kg ha(-1)with the combined application of 10 kg N ha(-1)and 10 kg P ha(-1). The average BNF of legumes in the average of all N and P interaction rates were 67, 23, 32 and 16 kg N ha(-1)for faba bean, field pea, dekeko and lentil, respectively. Moreover, faba bean, field pea, dekeko and lentil accumulated a surplus soil N of 37, 21, 26 and 13 kg ha(-1), respectively, over the wheat plot. The application of 20 kg N ha(-1)and 20 kg P ha(-1)levels alone and combined significantly (p< 0.05) increased the nodulation, BNF and yield of legumes; however, 46 kg N ha(-1)significantly decreased BNF. This indicated that the combination of 20 kg N ha(-1)and 20 kg P ha(-1)levels is what mineral fertilizer demands to optimize the BNF and yield of legumes. The results of this study can lead to the development of policy and farmer guidelines, as intensification of the use of legumes supplied with starter N and P fertilizers in Northern Ethiopian cropping systems has the multiple benefits of enhancing inputs of fixed N, improving the soil N status for following crops, and becoming a sustainable option for sustainable soil fertility management practice.