Adsorption Behavior, Residual Phytotoxicity And Pollution Control Of Herbicides Flufenacet And Imazaquin In Soil

The injury to sensitive rotational crops caused by residual herbicides from then-applications has happened frequently. The assessment of activity of residual herbicide in soil has been of agronomic concern. Bioavailability plays an important role in the risk assessment of residual herbicides for it was affected by physico-chemical properties of soil and coexisting pollutants. Thus, total amount of herbicide can’t be used to estimate the toxicity accurately. It is important to find a method basen on bioavalability for risk assessment of herbicides in soil. Experiments of adsorption and toxicity of flufenacet and imazaquin in soil were conducted according to OECD guideline106and227. The results were summarized as:The batch equilibrium method was adopted for the study of herbicide adsorption. The adsorptions of herbicides were varying in test soils and were depended on soil properties. The adsorptions of flufenacet in5test soils were different with Kf^values of0.78-6.24, and the Kf**values of imazaquin were0.01-0.65. The results indicated that the adsorption affinity of flufenacet in test soil were more than that of imazaquin. The results of correlation analysis revealed that soil organic matter content was predominant in adsorption of herbicides. The adsorption of flufenacet and imazaquin were all declined by the presence of the co-existing substance and the reduction in Kfads value increases with the concentration of the co-solute. The Kf**value of flufenacet was reduced by49.4%,51.3%and51.4%in the presence of imazaquin at concentration of3.21,32.1and128.5μM, respectively. Similary, the reduction of14.4%,18.8%and25.3%in Kfads value were observed in the presence of flufenacet at13.76,41.28and68.81μM, respectively. Biochar displayed great ability in flufenacet adsorption with the Kfads value of583.74, and improved soil adsorption ability. The improvement was positive correlated with biochar application rate. To accurately determine the toxicity and obtain a comparable concentration that inhibits growth by50%(IC50) in different soils, the concentrations of flufenacet and imazaquin in CaCl2(CCaCl2) and H2O (CH2O) extraction, and in in situ pore water (Cpw) were adopted for the estimation of IC50to sorghum. The results of flufenacet toxicity experiment indicating that the IC50values based on CCaCl2and CH2Owere0.08-0.33pM and0.19-0.69μM, respectively, with variation coefficients of46.39%and39.67%. However, Cpw based IC50values ranged from0.30to0.72μM with a variation coefficient of31.13%. The results of imazaquin toxicity experiment indicating that the IC50values based on CCaCl2and CH2O were0.19-0.42μM and1.03-2.28μM, respectively, with variation coefficients of38.32%and34.93%. The Cpw based IC50values were2.89-3.83μM with a variation coefficient of10.42%. This implies that the IC50based on imazaquin concentration in in situ pore water is almost identical in the five soils. Thus, herbicides concentration in pore water could be used to estimate the residual phytotoxicity of herbicides.Toxic unit (TU) was employed as a useful approach for the assessment of combined toxicity. The present study was conducted to assess the combined toxicity of flufenacet and imazaquin to sorghum with their total concentration and concentration in soil pore water. The IC50values of flufenacet and imazaquin individuals, and their combination estimated with the concentration in soil pore water notable differed from those based on the amended concentration due to the decline in bioavailability resulted from adsorption of the herbicides onto soil. According to the amended concentration, the combined effect of flufenacet and imazaquin in soil to sorghum growth was identified as synergy (TU=0.95). With the concentration in soil pore water, however, it was determined as antagonism (TU=2.18with TUo=1.42) which was identical to that observed in the test with culture solution. The results revealed that pore water herbicide concentration might be an effective tool to assess the combined toxicity of herbicides in soil to rotational crops.Biochar application blocked the dissipation of flufenacet in soil, and reduced the in situ porewater content with reductions of26.26%,34.17%,42.63%and73.20%. The phytotoxicity of flufenacet in soil was alleviated by biochar with the IC50values from2.43to25.07μM. The correlation analyses revealed the enhanced adsorption ability and the reduced in situ pore water content controlled the flufenacet phytotoxicity well together.