Identification of ten N2-fixing bacteria using 16S rRNA and their response to various zinc concentrations

Zinc is presently being used as a micronutrient essential for plant growth in agriculture in the form of zinc sulphate in fertilizers. Considering the importance of nitrogen-fixing microorganisms in soil fertility and plant growth, and the toxic effects of zinc, this study aimed at isolating nitrogen-fixing microorganisms from the asparagus rhizosphere arable soil and examining the sensitivity of the isolated strains to different zinc concentrations. The asparagus rhizosphere soil was cultivated in a nitrogen-free environment at 30 °C for 48 hours. Strains were identified by both biochemical and molecular methods. The presence of the nitrogenase enzyme system was confirmed by testing for the presence of the nifH gene through PCR analysis. The minimum inhibitory concentration (MIC) and optimal zinc concentration for the growth of each strain were also determined. A total of 10 different bacterial species were identified in three different soil samples. The presence of the nifH gene was confirmed in seven different strains. MIC and the average optimal zinc concentration for bacterial growth were 130.7 ppm and 13 ppm, respectively. The asparagus rhizosphere soil contains different species of nitrogen stabilizing microorganisms. An optimal zinc concentration in soil of 20 ppm is suggested.