Foliar Nitrogen Fixers on Knol Khol: Differential Isolation, Characterization and Biofertilizer Potential of Azotobacter and Azospirillum

The rising need for sustainable agricultural practices, especially in non-leguminous vegetable production, necessitates the exploration of alternative nitrogen sources like plant-associated nitrogen-fixing bacteria. This study aimed to isolate, characterize and evaluate native foliar nitrogen-fixing bacteria, Azotobacter and Azospirillum, from knol khol (Brassica oleracea var. gongylodes) leaves collected across diverse farming regions of Chhattisgarh, India, to assess their potential as foliar biofertilizers. Twelve leaf samples were collected from fields representing varied cultivation practices, including chemical application. Foliar Azotobacter was isolated from 10 samples but could not be recovered from fields associated with heavy pesticide use, whereas Azospirillum was successfully isolated from all 12 samples, suggesting its potential endophytic nature and greater resilience to chemical stress. All isolates were confirmed as Gram-negative with variable biochemical profiles. Nitrogen-fixing efficiency varied significantly, with the most efficient Azotobacter isolate (Kk-Azot-6) fixing 10.58 mg N g-1 sucrose and the top Azospirillum isolate (Kk-Azos-10) exhibiting a higher efficiency of 15.56 mg N g-1 malate. The superior nitrogen-fixing potential and differential stress tolerance observed, particularly in Azospirillum, emphasize the significant ecological role of these bacteria on aerial plant surfaces and their strong candidacy for developing effective foliar biofertilizers for sustainable and eco-friendly crop production.