Isolation, identification and characterization of efficient free-living nitrogen-fixing bacteria from rice rhizosphere ecosystem

Abstract

Biological nitrogen fixation (BNF) is a natural bacterial process of utmost importance due to its potential to supply alternative source of plant available nitrogen for the crops to enable sustainable and economical production. The aim of the present study is to isolate efficient nitrogen fixing bacteria with a potential to be developed into bio-fertilizer for supplying nitrogen and for improving growth and yield of rice. With this background, rice (Oryza sativa L.) rhizosphere soil samples were collected from different locations and 197 nitrogen-fixing bacterial strains were isolated. Among these, eight promising bacterial strains that showed highest nitrogen fixing ability based on Acetylene Reduction Assay (ARA) were isolated and identified as Stenotrophomonas sp., Paenibacillus sp. (isolate 1), Ochrobactrum sp., Paenibacillus sp. (isolate 2), Burkholderia cepacia (isolate 1), Burkholderia cepacia, Xanthomonas sacchari and Rhizobium sp. The reduction rates of the eight potential isolates indicating the nitrogen-fixing ability ranged from 115 to 490 nm of C2 H4 produced h-1 mg-1 protein. These bacterial isolates were further characterized for their morphological features and for their plant growth-promoting (PGP) activity like indole acetic acid (IAA) production and insoluble mineral (P, K, Zn) solubilization. All isolates exhibited IAA production ranging from 11.90 to 47.40 µg/ml while two isolates possessed phosphate and zinc solubilization activity.