Abstract:

Objective Studies on the diversity, resistance and growth-promotion characteristics of nitrogen-fixing microorganisms in the rhizosphere soil of desert shrubs may provide a basis for exploring desert microbial resources. Method Nitrogen-fixing microorganisms were isolated and screened from the rhizosphere soil of four desert scrubs using isolation and culture methods and PCR amplification of the nitrogen-fixing enzyme gene nifH. Nitrogen-fixing strains were identified through observation of colony morphology, determination of physiological and biochemical characteristics, and analysis of the 16S rRNA gene sequences. The tolerance to salt alkali and the resistances of the strains to drought were tested by NaCl, NaOH and PEG6000 stresses. The growth-promoting characteristics of strains such as phosphorus-solubilizing ability, IAA production, siderophore production, and ACC deaminase were detected by colorimetric method. The growth-promoting effects of the strains were verified through the pot inoculation experiment of alfalfa. Result A total of 61 strains of nitrogen-fixing microorganisms were screened from the rhizosphere soil of four shrubs. Most of the strains were gram-negative bacteria, and the H₂O₂ enzyme reaction were positive. The results of 16S rRNA sequence analysis showed that the strains belonged to 38 genera, among which Pseudomonas, Bacillus, Azotobacter and Ensifer had higher isolation frequency, and the diversity of nitrogen-fixing microorganisms in the rhizosphere of Caragana korshinskii was the most abundant. All strains grew normally under the condition of pH10, and most strains tolerated 5% NaCl and 15% PEG6000, showing strong stress resistance. The growth-promoting characteristics of the strains varied, total 17 strains had the ability of dissolving phosphorus, and these strains' maximum amount of phosphorus to be dissolved was 135.84 mg/L; total 15 strains produced siderophores. Eight strains had ACC deaminase activity, the highest enzyme activity was 10.63 U/mg; nineteen strains had the ability to secrete IAA, and these strains' IAA production was between 3.41-56.93 mg/L. There were 6 strains with more than 3 kinds of growth-promoting potential. Among them, the SDQ-1 and MC-20 strains had good comprehensive performance. The alfalfa pot inoculation experiment showed that the inoculation treatment significantly promoted the growth of alfalfa seedlings, and the growth-promoting effect of compound microbial agent group F was the most significant. Compared with the control CK, the seedling height, fresh weight, root length and root area significantly increased by 55.88%, 147.53%, 292.65% and 306.38%, respectively. Conclusion The nitrogen-fixing microbial diversity in the rhizosphere soil of the desert shrub in Ningxia is extremely abundant, and most of the strains have good stress resistance and multiple growth-promoting potentials, which have the value of further development and utilization.

Key words: desert plants, nitrogen-fixing microorganisms, resistance, growth-promoting function, diversity