Isolation and Assessment of Nitrogen-Fixing and Phosphate-Solubilizing Bacteria for Use as Biofertilizers

Abstract

Nitrogen and phosphorus are essential macronutrients for plant growth. The major sources of N and P in agriculture are chemical fertilizers. Nitrogen in the soil can be lost due to denitrification, leaching, and volatilization. The only phosphate source in the world, rock phosphate, is being depleted. When it is applied to soil, phosphate is quickly immobilized and cannot be absorbed by the plants. In addition, chemical fertilizers are not affordable for many of the world’s farmers. Thus, there is a growing opportunity for efficient biofertilizers. N-fixing and P-solubilizing microorganisms are the two major groups of biofertilizers. The overall aims of this study were i) to conduct comprehensive analyses of N-fixing and P-solubilizing bacteria in specific crop situations in Alabama; and ii) to characterize and select effective strains for use as biofertilizers. Two main strategies for isolating strains were conducted. One was to isolate them from selected perennial Gramineae (giant reed and switchgrass); the other was to isolate from specific long-term rotations where nutrient application had been controlled (the Old Rotation and Cullars Rotation experiments). A total of 354 N-fixing and P-solubilizing bacterial strains were isolated. The N-fixing activity of all the putative N-fixing strains was confirmed using acetylene reduction assay, and the P-solubilizing activity was confirmed by both qualitative and quantitative evaluations. Auxin and siderophore production were also evaluated in vitro, and 16S rDNA sequencing was used for molecular identification. The results showed a wide diversity of bacterial genera. Five new genera were first reported to have N-fixing activity. Selected N-fixing and P-solubilizing strains that had shown outstanding performance during in vitro tests were inoculated on corn, wheat, and pepper under greenhouse conditions. Interestingly, the results showed that the selected N-fixing and P-solubilizing bacteria could promote corn, wheat, and pepper growth by increasing plant growth parameters, such as root/shoot dry weight, height, and SPAD, and the root morphology parameters. Among corn and wheat inoculated with selected strains, an improvement of nutrient uptake rate was found in the analysis of aboveground tissues. But the results showed plant growth promotion and nutrient uptake are not linked.