Inoculation with native Bradyrhizobium strains formulated with biochar as carrier improves the performance of pigeonpea (Cajanus cajan L.)

Abstract

Abstract Pigeonpea (Cajanus cajan) originated in India and is now a major source of protein in countries in tropical and subtropical regions worldwide. However, little is known about the effect on crop yields of inoculation with appropriate rhizobia strains. The objective of this study was to explore the possibility of improving pigeonpea yields in the Caribbean region, by inoculating with selected N-fixing symbionts. To achieve the study objective, we selected three elite strains of the genus Bradyrhizobium isolated from root nodules of pigeonpea in the Dominican Republic, we formulated them using innovative carriers, and finally we tested the formulated inoculant on eight farms located in the Dominican Republic. Three different carriers (sewage sludge, pine bark biochar and poultry litter) plus perlite as control were tested for the formulation. The first step was to test bacterial survival in the different carriers at different times up to one year (shelf-life). Pine bark biochar and the control resulted in the best shelf-life and were selected for the field experiment. The experimental design comprised two pigeonpea cultivars and five fertilisation strategies: Inoculation with three Bradyrhizobium strains without nitrogen (N) fertilisation, plus two non-inoculated controls one of them fertilised with 125 and 210 kg N ha−1 in non-irrigated and irrigated fields respectively. Nodule occupancy by the inoculated bacterial strain depended solely on the level of soil native nodulating bacteria and not on the bacterial strain or the carrier. Inoculation produced, on average, a significant yield increase compared to the non-inoculated non-N-fertilised control. Furthermore, the yield with the inoculated treatments did not significantly differ from the yield of the N-fertilised control. However, either inoculation or N-fertilisation were ineffective in two out of the eight fields with more than 104 nodulating bacteria g soil−1, compared to the non-inoculated non-N-fertilised control. Yield was neither significantly affected by the bacterial strain nor by the cultivar-strain interaction, and therefore, the two cultivars can be considered promiscuous. Neither carrier had a significant effect on yield. Thus, inoculants based on strains of Bradyrhizobia isolated from the root nodules of pigeonpea, formulated with pine bark biochar or perlite as carrier, are recommended as a good strategy to ensure food security in tropical agroecosystems with pigeonpea.