Diazotrophic Bacterial Response to Herbicide Toxicity: In vitro Analysis

Diazotrophic bacteria play critical role in biological nitrogen fixation (BNF), and the application of herbiced significantly affect growth and activities of these bacteria. To investigate this, 1.0 ml of broth stock culture containing 2.9 x 105 cfu/ml and 2.6 x 106 cfu/ml of Azotobacter and Bradyrhizobium, respectively were inoculated into 10.0 ml of their respective growth medium [ TDC broth, and Yeast extract mannitol broth (YEMB)] containing 0, 0.5, 1.0, 1.5 and 2.0 % v/v of the respective herbicides, atrazine, glyphosate, paraquat and 2,4-D. Thereafter, plate counts of the diazotrophs for each concentration was made at 24, 48, 72 and 120 h intervals using spread plate method on TDC agar and YEMA after incubation at room temperature (30 ± 2 ℃) for 72 h. The LC50 of the respective herbicides for Azotobacter and Bradyrhizobium was determined at 120 h using Probit analysis. Results showed that all tested concentrations except control, retarded diazotrophic bacterial population growth. Growth reduction increased progressively with increased concentrations of herbicides (P < 0.05). In general, herbicides were found to suppress the growth of diazotrophs by 29.7 – 100 %. The LC50 indicated symbiotic Bradyrhizobium displayed greater sensitivity to tested herbicides than free-living Azotobacter (P < 0.05). Conclusively, herbicides suppressed diazotrophic bacterial growth.