Improvement of cowpea productivity by rhizobial and mycorrhizal inoculation in Burkina Faso

Cowpea is one of the most important food legume crops in Burkina Faso. It is able to associate with arbuscular mycorrhizal fungi (AMF) and rhizobia. This dual symbiosis improves nitrogen and phosphorus nutrient uptake in cowpea. As the application of exotic inoculants frequently lacks positive responses in field experiments, this study set out to select well-adapted native symbiotic rhizobial and AMF strains. Soil samples were collected from six study sites in three different climatic zones of Burkina Faso to investigate their native symbiotic strains. Soil-extraction of native spores led to the identification of four AMF genera ( Scutellospora , Gigaspora , Glomus and Entrophospora ) by morpho-anatomical characterization. The two most effective cowpea fungal strains were selected after spore isolation from field-collected soils, multiplication on maize roots and inoculation on cowpea seedlings in a greenhouse experiment. Cowpea-nodulating rhizobial strains were trapped in the greenhouse by planting cowpea seeds in collected soil samples and the strains were characterized using molecular methods. This characterization led to the rhizobial isolates being classified in four clusters on the phylogenetic tree (using the Maximum-Likelihood Phylogenies method). All strains belonged to the Bradyrhizobium genus and most of them were included in the B. japonicum branch. Some groups were clearly distinct species already identified and may be new species. The two most effective strains for cowpea yield improvement in the field were selected after cowpea inoculation in a greenhouse experiment. The inoculation design in the field experiment consisted of four single inoculation treatments, either rhizobial or mycorrhizal, along with four dual inoculations, one treatment with chemical fertilizers, and one uninoculated control. The results showed that cowpea productivity was significatively improved by dual inoculation with native rhizobial and mycorrhizal strains, reaching the same level as the application of commonly used chemical fertilizers [Nitrogen, Phosphorus and Potassium fertilizers (NPK)]. In addition, dual inoculation resulted in the highest iron content in cowpea leaves.