Isolation, Selection, and Biological Evaluation for Bacteria that Fix Nitrogen and Produce Indole-3-Acetic Acid from Paddy Soils in Vietnam

Acidic soils are limiting the production of crops and indirectly harming the environment due to the use of nitrogen (N) chemical fertilizer. Therefore, the current study aims to isolate a promising N₂-fixing candidate to solve this issue in some communes of Phu Luong District, Thai Nguyen Province, Vietnam.

The bacteria were isolated using the Ashby medium. The IAA generation and nitrogen fixation were assessed using spectroscopy. The selected bacteria were tested at temperatures ranging from 28°C to 42°C and pH levels of 4.0 to 9.0. As a result, the Box-Behnken model yielded a growth equation. The API kit test was used to measure the biochemical properties of the selected bacteria.

The result demonstrated that there were ten nitrogen-fixing bacteria (NFB) isolates with capacities of N₂ fixation and IAA production at 2.35–24.61 and 0.00–119.13 µg/ml. Among them, the NL3 strain was the best strain. The optimum condition to grow the NL3 strain was calculated as 5.44 days at pH 7.10 and temperature of 32.18°C to reach a bacterial density of 9.77929×10⁸ CFU/ml. In addition, the NL3 strain was identified as an Azotobacter species. The genomic analysis revealed that the genome of Azotobacter sp. NL3 was nearly 5.4 Mb long and contained 45 N₂ fixation-relating genes and 7 IAA production-relating genes.

Not only can this study provide insight into the features of Azotobacter spp., but it also introduces a potent candidate that can improve soil health and crop yield by fixing N₂ and producing IAA in order to limit the use of N chemical fertilizer for a sustainable agriculture. Therefore, the selected bacterial strain should be further tested under the local field conditions and subsequently commercialized as a biofertilizer.