Rice husk biochar-immobilized bacterial consortium enhances reduction of parent-herbicide residues in wheat-vegetated soil contaminated with a ternary herbicide mixture.

In this study, bacterial consortium AF-3 comprising of four herbicide-degrading bacterial strains was evaluated for the remediation of a ternary mixture of fluroxypyr-meptyl, mesosulfuron-methyl and florasulam. The consortium consisted of Rhodococcus rhodochrous, Bordetella petrii, Achromobacter sp. and Paracoccus pantotrophus. AF-3 showed strong degradation potential against the herbicide mixture in liquid culture. Taguchi L9 screening showed herbicide-specific effects of pH, temperature, medium composition and herbicide concentration. Because optimum conditions differed among the three herbicides, a common practical condition of pH 8.0, 35 °C, MSM and 0.5 mg L⁻1 of each herbicide was selected for a separate ternary-mixture validation experiment. AF-3 degraded 98% fluroxypyr-meptyl, 66% mesosulfuron-methyl and 88% florasulam after 16 days. The remediation potential of AF-3 was further evaluated in wheat-vegetated soil contaminated with the three herbicides at 10:5:5 mg kg⁻1. Free AF-3 reduced parent-herbicide residues by 82, 58 and 82%, respectively. Immobilization of AF-3 on rice husk biochar increased these reductions to 90, 78 and 88%, with apparent half-lives of 35, 66 and 39 days for fluroxypyr-meptyl, mesosulfuron-methyl and florasulam, respectively. HPLC-ESI-MS/MS analysis detected ten transformation products and showed biotransformation of all three herbicides. The rice seed bioassay showed lower phytotoxic effects in soil amended with rice husk biochar-immobilized AF-3. These results show that rice husk biochar-immobilized AF-3 can be used for reducing parent-herbicide residues and associated phytotoxic effects in crop-rooted soil contaminated with multiple herbicides.