Degradation Dynamics Of Three Herbicide Residues In Oat Field Soil And Their Effects On Soil Microorganisms

In order to investigate the dissipation and residue dynamics of pendimethalin,quinclorac and bensulfuron-methyl in oat field soil and their effect on soil microorganisms and enzyme activities,a field experiment was conducted at the experimental field station of Gansu Agriculture University in Yuzhong County,Lanzhou（Gansu,China）.Three herbicides were sprayed with recommended dosage and 1.5 times of recommended dosage,respectively.Using a combination of field experiment and laboratory analysis to investigate the natural degradation characteristics and residual levels of three herbicides within 0 d⁶0 d in oat field soil.The effects of three herbicides on soil bacteria,fungi,actinomycetes and soil urease,invertase,catalase activities were also determined.The main results were as follows:（1）Optimized residue analysis method（high performance liquid chromatographic（HPLC））for pendimethalin in oat field soil.Under this method,the average recovery rate was90.72%⁹6.59%as the addition level of pendimethalin in soil were 0.05,0.5 and 1.0 mg·kg^-1,respectively,and the relative standard deviation was 3.21%⁵.46%,the minimum detection mass fraction was 0.036 mg·kg^-1.The residue degradation dynamics and residual levels of pendimethalin in oat field soil were ascertained.Pendimethalin was applied with recommended dosage and high dosage（1.5 times of recommended dosage）3 days after oat sowing.The digestion of pendimethalin in soil was in accordance with the first-order kinetic model,the reaction equations can be fitted as Ct=0.4376e^-0.0187t（R²=0.8079）and Ct=0.6670e^-0.0179t（R²=0.8222）,the half-life values were 37.1 d³8.7 d.Recommended that pendimethalin belongs to moderately degraded pesticide.After 60 days of pesticide application,the degradation rates in the soil were 77.57%and 75.07%,respectively.（2）Two herbicides,quinclorac and bensulfuron-methyl,were applied at the recommended dosage and high dosage（1.5 times of recommended dosage）in the 3 to 4 leaf stage of oat.The degradation in soil was in accordance with the first-order kinetic model.The degradation reaction equations of quinclorac can be fitted as Ct=0.1086e^-0.0260t and Ct=0.1789e^-0.0294t,the half-life was between 23.6 d²6.7 d,recommended that quinclorac belongs to easily degradable pesticide,and the degradation rates in soil after 60 d were 83.53%and 84.92%under 2 dosages.The degradation equations of bensulfuron-methyl can be fitted as Ct=0.0115e^-0.0388t and Ct=0.0221e^-0.0344t,and the half-life was between 17.9 d²0.1 d,also recommended as an easily degradable pesticide.After 60 days of pesticide application,the degradation rate was 95.04%and 92.13%,respectively.（3）The effects of three herbicides on the amount of soil microorganisms were varied.pendimethalin had an active effect on soil bacteria and fungi,with the highest activation rate of 77.54%and 85.08%,respectively,and had little effect on soil actinomycetes.Quinclorac inhibited the bacteria in the soil,and the inhibitory effect enhanced with the increase of time after treatment,and alternating activation and inhibition against fungi and actinomycetes.The effect of Bensulfuron-methyl on soil microorganisms didn’t have obvious regular pattern.It showed greater disturbance to soil microorganisms by pendimethalin.The significant impact on the amount of soil microorganisms mostly occurred in the middle of the experiment process,and at the end of the experiment,most of them recovered to the control level.There was no obvious concentration-effect relationship between the high and low dosage of the three pesticides.（4）There were significant differences in the effect of different herbicides on soil enzyme activity.The effect of pendimethalin on soil urease activity was activated and then inhibited.The highest activation rate was 17.3%after 3 d of pendimethalin application,and the highest inhibition rate was 9.38%at 45 d,and the inhibition to invertase and catalase was mainly.Quinclorac inhibited the activity of soil urease and catalase,the highest inhibition rates were9.86%and 6.07%,respectively,and the activity of soil invertase was activated and then inhibited,the highest activation rate was 11.19%（15 d）,the highest inhibition rate was 6.08%（60 d）.Bensulfuron-methyl showed inhibitory effect on soil catalase,mostly,and the effect on soil urease and invertase didn’t have obvious regular pattern.