Abstract: The nicosulfuron residues in soil can cause different degrees of phytotoxicity to succession crops.In order to solve the problem that nicosulfuron residues in cornfield are phytotoxic to the succession crop sorghum,and to reduce their phytotoxicity on sorghum,the high-efficiency nicosulfuron-degrading strain,Trichoderma virens DT-4,which was screened out in a previous stage,was used to detect the degradation rate by high performance liquid chromatography-mass spectrometry.Degradation effect of degrading strain on nicosulfuron in soil was determined by the indoor potted plant bioassay combined with field plot trials method.The results showed that when the residual concentration of nicosulfuron was in the range of 0.0025-0.1 ug/mL,the standard curve was of good linearity,indicating that the requirements for pesticide residue analysis were met.The degradation rate of nicosulfuron in PDB liquid medium reached as high as 93.00% when treated with DT-4 degrading bacteria for 100 h.The optimal degradation conditions for the strain were culture temperature 35℃,pH 5,optimal inoculum amount 5%,and the initial mass concentration of nicosulfuron 200 mg/L.When the residual concentration of nicosulfuron in the soil for pot experiment was 0.075 mg/kg,the addition of degrading bacteria DT-4 could significantly alleviate the growth and development of sorghum as shown in the indexes such as plant height,root length,fresh weight,chlorophyll content,root activity and net photosynthetic rate,indicating that it obviously promoted the growth of sorghum.In the field plot trials,after being treated with pesticide-degrading bacteria of different formulations,the most significant effect was observed at the heading stage of sorghum.The degrading bacteria could restore the sorghum in soil contaminated with nicosulfuron of less than 60 g a.i./hm² to a level without any obvious phytotoxicity.Among all the pesticide-degrading bacteria,the application effect of powder was better than that of bacteria solution.This study shows that the high-efficiency degrading strain T.virens DT-4 can effectively alleviate the phytotoxicity of nicosulfuron residues on succession crop sorghum,thus provide a scientific reference for the microbial remediation of soil contaminated with nicosulfuron residues under the corn-sorghum crop rotation system.When concentration of nicosulfuron in pot experiment soil was 0.075 mg/kg,the addition of degrading bacteria DT-4 could significantly alleviate the growth and development of sorghum plant height,root length,fresh weight,chlorophyll content,root activity and net photosynthetic rate,indicating that it had obvious promotion effect on the growth of sorghum.In the field plot test,different formulations of degrading inoculants had the most significant effect at the heading stage of sorghum.The degrading bacteria could restore the sorghum with less than 60 g a.i./hm² of nicosulfuron-contaminated soil to a level without obvious phytotoxicity,and the application effect of the degrading bacterial powder was better than that of bacteria liquid.This study showed that the high-efficiency degrading strain T. viridans DT-4 could effectively alleviate the toxic effect of nicosulfuron residues on subsequent crop sorghum,and provide a scientific reference for the microbial remediation of soil polluted by nicosulfuron-methyl residues under the corn sorghum crop rotation system.

Key words: nicosulfuron, residual phytotoxicity, Trichoderma virens, bioremediation, sorghum