Siderophore Production Capabilities of Tn5 Mutants from the Nitrogen-Fixing Bacterium (NFB) GXGL-4A Affect the Soil Microecology in Cucumber Rhizosphere

Many nitrogen-fixing bacteria (NFB) can produce siderophores for iron acquisition in soil, but the impact of siderophore producing capabilities of NFB on the rhizosphere soil microecology is not well understood. To explore the effects of root inoculation with NFB strains with different siderophore producing capabilities on the rhizosphere soil microecology, the wild-type (WT) NFB strain Kosakonia radicincitans GXGL-4A and its Tn5 mutants M107 (high siderophore-producing ability) and M246-2 (deficient in siderophore production) were used as biofertilizers in cucumber rhizosphere soil. Iron is important for the growth of NFB bacterial cells, and the mutant M246-2 showed the slowest growth rate compared to the other strains when incubated in A15 nitrogen-free medium supplied with different levels of iron. The mutant M107 had the strongest chelating ability for iron, with the largest yellow halo on the CAS detection plate. There were statistically significant differences in the halo diameters among the three NFB groups. Compared with the control group, the application of NFB significantly increased the activities of soil peroxidase (S-POD) and dehydrogenase (S-DHA) and altered the soil nitrogen contents. Fertilization with the mutant M107 significantly improved cucumber biomass and reduced the abundance and diversity of bacterial communities in the rhizosphere soil compared to the other groups. The contents of soil ammonium nitrogen (NH4+−N) and total nitrogen (TN), and soil dehydrogenase (S-DHA) showed significant correlations with the abundance of most Top 50 dominant genera in the soil. Soil TN content was the essential factor affecting the abundance of Kosakonia bacteria in cucumber rhizosphere.