Identification of free-living nitrogen-fixing bacteria and their impact on growth promotion of non-legume crop plants

As a suitable alternative to chemical fertilizers, nitrogen-fixing microorganisms play a major role in maintaining soil fertility and thereby important for sustainable crop production. In the present investigation, out of 165 distinct bacterial morphotypes isolated from paddy soils, only 32 were positive for both ARA, and nifH gene screening. The ARA of the isolates ranged from 1.8 to 2844.7 nM ethylene h-1 mg protein-1. The 16S rRNA analysis identified the isolates to be members of 13 different genera viz. Bacillus, Pseudomonas, Paenibacillus, Serratia, Ochrobactrum, Lysinibacillus, Burkholderia, Brevundimonas, Herbaspirillum, Novosphingobium, Sphingomonas, Xanthomonas, and Azorhizobium. On the basis of ARA, 17 isolates were screened for multiple PGP traits, and evaluated for their inoculation effects on canola and rice plants. All of the isolates tested positive for ACC deaminase activity and production of IAA, and ammonia. Additionally, four of the isolates were able to solubilize P, five tested positive for Zn solubilization and S oxidation, and eight isolates produced siderophores. Based on the presence of multiple PGP traits, 10 isolates were further selected for inoculation studies. Treatment with Herbaspirillum sp. RFNB26 resulted in maximum root length (54.3%), SVI, and dry biomass in canola, whereas Paenibacillus sp. RFNB4 exhibited the lowest activity under gnotobiotic conditions. However under pot culture conditions, Paenibacillus sp. RFNB4 significantly increased plant height and dry biomass production. Canola plants and rhizosphere soils inoculated with Bacillus sp. RFNB6 exhibited significantly higher ARA. In greenhouse experiments, Serratia sp. RFNB18 increased rice plant height by 35.1%, Xanthomonas sp. RFNB24 enhanced biomass production by 84.6%, and rice rhizosphere soils inoculated with Herbaspirillum sp. RFNB26 exhibited the maximum ARA. Our findings indicate that most of the selected isolates possess multiple PGP properties that significantly improve the growth parameters of the two plants when tested under controlled conditions.