Contribution of a mixed inoculant containing strains of Burkholderia spp. and Herbaspirillum ssp. to the growth of three sorghum genotypes under increased nitrogen fertilization levels

Sorghum bicolor is a C4 plant that is normally associated with diazotrophs. Inoculation with selected bacteria can improve plant growth, biological nitrogen fixation (BNF) and nutrient uptake. Therefore, the objective of the current study was to evaluate the ability of twenty-two diazotroph strains to improve sorghum growth in a greenhouse. Then, a mixture of the best performing strains was applied in the field under different levels of N-fertilizer applied to three sorghum genotypes to evaluate crop productivity, N content, BNF contribution and the efficiency of the N fertilizer used under field conditions. Two Herbaspirillum and two Burkholderia species were selected under greenhouse conditions using a sterile substrate of three sorghum genotypes: grain sorghum lineage 9920044, grass BRS655 and sweet sorghum BR501. Four diazotrophs were selected for the field trial. Burkholderia kururiensis (16 and 109), B. tropica (PPe6) and Herbaspirillum seropedicae (ZMS176) were cultivated and mixed at planting in a Typic Hapladult soil using peat inoculant. Crop yield, N-content and BNF quantification were determined using the natural abundance of 15N, and the use efficiency of the applied N was assessed using urea labelled with 15N. The grain yield showed a 24% enhancement in the inoculated grain sorghum compared with the control without nitrogen. In addition, the aerial biomass was 13% higher in the grass genotype compared with the control. The sweet sorghum did not respond to any treatment. BNF quantification reached 31.4% in the grain, 22.7% in the grass and 19.4% in the sweet sorghum genotypes. Grain sorghum inoculation increased BNF by 4% (9.5kgha−1N fixed). Inoculation increased fertilizer efficiency recovery by 28.9%. Inoculation using a mixture containing the four diazotrophs improved the sorghum productivity by enhancing the dry mass, N-content, N from BNF and N use efficiency in the grain and grass genotypes. Under zero nitrogen fertilizer application, 22% of the N that accumulated in the biomass was derived from the air. New inoculants using a mixture of different bacterial species/strains can improve the knowledge and contribution of BNF in agricultural systems.