Nitrogen and phosphorus benefits from faba bean (Vicia faba L.) residues to subsequent wheat crop in the humid highlands of Ethiopia

Abstract

Faba bean–wheat rotation is one of the traditional cropping systems in most parts of the temperate, Mediterranean and tropical highland areas. However, the net contribution of legumes to soil nutrient balance is determined by the extent to which crop residue is removed from the field. Therefore, we assessed two possible faba bean residue management scenarios and their role in the faba bean–wheat rotation system in a two-phase field experiment. We further tested to what extent high N₂-fixing and P efficient faba bean varieties could benefit subsequently grown wheat. In the first phase, three improved faba bean varieties (Degaga, Moti, Obse) were grown at four levels of P fertilization (0, 10, 20 and 30 kg P ha⁻¹) along with local faba bean and reference wheat but without any fertilization. N₂-fixation, soil N balance and P uptake were determined for the faba beans. The N balance was determined via two possible residue management scenarios: scenario-I assumed that all the aboveground biomass is exported from the fields; scenario-II assumed that all the above ground biomass except grains and empty pods is incorporated to the soil. In the second phase, the N and P benefits of faba beans to rotational wheat were assessed. Scenario-I gave a negative net N balance (kg N ha⁻¹) in the range of −86.5 ± 5.8 (Degaga) to −9.4 ± 8.7 (Moti) with significant differences between varieties. Scenario-II showed that all balances were significantly (P < 0.01) improved and the varieties were positively contributing N to the system in the range of 50.6 ± 13.4 (Degaga) to 168.3 ± 13.7 (Moti) kg N ha⁻¹, which is equivalent to 110–365 kg N ha⁻¹ in the form of urea (46 % N). In the second crop phase, biomass and grain yield of wheat grown after the faba beans improved significantly (P < 0.05) by 112 and 82 %, respectively compared to the yield of wheat after wheat. Phosphorus application to the preceding faba bean varieties significantly improved total biomass and grain yield of the succeeding wheat (R² = 0.97). The incorporated legume root, nodule and straw clearly played a role in improving wheat yield through N addition via BNF and straw P. The study demonstrates the prospects and importance of improved faba bean germplasm and management as a key component for sustainable wheat based cropping systems in the humid tropical highlands.