Nitrogen fixation by beans in crop mixtures is influenced by functional traits of associated species

Abstract Purpose Legumes can fix atmospheric nitrogen and facilitate nitrogen availability to their companion plants in crop mixtures. However, biological nitrogen fixation (BNF) of legumes in intercrops varies largely with the identity of the legume species. The aim of our study was to understand whether BNF and concentration of essential plant nutrients by common bean is influenced by the functional traits of the companion plant species in crop mixtures. Methods In this greenhouse pot study, common beans were cultivated with another legume (chickpea) and a cereal (Sorghum). We compared BNF, crop biomass and nutrient assimilation of all plant species grown in monocultures with plants grown in crop mixtures. Results We found beans to exhibit low levels of BNF, and to potentially compete with other species for available nitrogen in crop mixtures. The BNF of chickpeas however, was enhanced when grown in mixtures. Furthermore, biomass, phosphorous and potassium values of chickpea and Sorghum plants were higher in monocultures, compared to in mixtures with beans; suggesting competitive effects of beans on these plants. Concentration of calcium, magnesium and zinc in beans was higher when grown with chickpeas than with Sorghum. Conclusion It is generally assumed that legumes benefit their companion plant species. Our study highlights the contrary and shows that the specific benefits of cereal-legume mixtures are dependent on the functional biology of the species concerned. We further highlight that the potential of legume-legume mixtures is currently undervalued and may play a strong role in increasing nitrogen use efficiency of intercrop-based crop systems.