Degradation Dynamics Of Two Herbicides In Soil And Effect On Soil Enzymes

Imazethapyr and Fomesafen were commonly used herbicides in soybean and peanut field.The analysis methods of imazethapyr and fomesafen in soil were developed in the text. Studiedthe soil moisture content, organic matter content, pH, temperature and microbial changes on theeffects of two herbicides degradation rate, and the influence of two herbicides to four kinds of soilenzymes. Specific results are as follows:(1)Soil imazethapyr was extracted by alkaline methanol; extract was detected by highperformance liquid chromatography (HPLC) after acidification. Soil fomesafen was extracted byacidic acetonitrile, and detected by HPLC after PSA purification. Imazethapyr and fomesafen hadgood linear response in the scope of0.05mg/L~10.0mg/L concentration. The spiked recoveriesof imazethapyr in the soil were in the range of93.9%~104.3%, the relative standard deviationwas2.9%~7.4%. The spiked recoveries of fomesafen in the soil were in the range of93.9%~104.3%, the relative standard deviation was2.3%~6.9%. The limit of quantification (LOQ) ofimazethapyr and fomesafen in the soil were0.021mg/kg and0.029mg/kg respectively. Theaccuracy, precision and sensitivity of the method meet the requirements of pesticide residueanalysis.(2)The degradation rate of fomesafen accelerated along with the increase of soil moisturecontent, the degradation half-lives of fomesafen were177.7d,100.5d and91.2d respectivelywhen soil water contents were40%,60%and80%; But the degradation rate of imazethapy did notchange along with the change of soil moisture content, the degradation half-lives of imazethapyrwere28.7d,32.3d and30.5d when the conditions of soil water content were40%,60%,80%respectively. The degradation rate of imazethapyr and fomesafen augmented when temperatureincreased; the half-lives of imazethapyr and fomesafen were169.1d,75.3d,73.7d and239.0d,121.6d,108.3d under15℃,25℃,35℃respectively. The degradation rates of imazethapyrand fomesafen were increasing along with soil pH; the degradation half-lives in acidic, neutral andalkaline soils were144.4d,82.5d,75.3d and92.4d,74.5d,32.3d respectively. The degradationrate of fomesafen and imazethapyr in the high organic matter content of the soil was faster thanthat in low organic matter content of soil; the Fomesafen and imazethapyr degradation rate insterilized soil slowed down, indicating that microbial plays an important role in the degradation of them. The degradation dynamics of imazethapyr and fomesafen in four different soil types werestudied in the field in2012. The degradation half-lives of fomesafen were30.4d,28.8d,50.6d,and55.0d in fluvo-aquic soil, yellow cinnamon soil, mortar black soil and cinnamon soil; thedegradation half-lives of imazethapyr were26.3d,39.2d,33.5d, and41.2d in fluvo-aquic soil,yellow cinnamon soil, mortar black soil and cinnamon soil.(3) Fomesafen (1.0mg/kg and1.5mg/kg) had no effect on soil urease activity; it was"suppression-Activation-suppression" effect on soil alkaline phosphatase soil; it had asignificant "activation-inhibition" effect on soil invertase activity; it showed inhibition onprotease activity. Fomesafen residues in the soil showed a significant negative correlation (p <0.05)with invertase activity, and showed weak correlation with soil protease, phosphatase, ureaseactivity.High-dose Imazethapyr (1.5mg/kg) showed "activate-inhibiting" effect on soil ureaseactivity. Low-dose imazethapyr (1.0mg/kg) had no effect on1d~7d, after seriously inhibiturease activity on7d~45d. High-dose Imazethapyr (1.5mg/kg) had "Activate-inhibiting"effect on alkaline phosphatase; low dose (1.0mg/kg) imazethapyr showed inhibition on soilinvertase activity; high dose (1.5mg/kg) imazethapyr showed a significant inhibition on soilinvertase activity at1d~21d, showed significant activation at30d~45d. Imazethapyr showedsignificant inhibitory activity within1d~14d on soil protease, the soil protease activity levelswas recovering after21d. Imazethapyr residues in soil had a significant positive correlation (p<0.01) with soil urease activity, and negatively correlated with soil protease and invertase activity,and positive correlation with phosphatase, they did not reach significant levels.