Effects Of Herbicide Imazethapyr On Soil Microbial Diversity And Environmental Behavior

Imazethapyr, imidazolinone herbicide, is mainly used in soybean fields to control the annual grass weeds, perennial grass weeds and broad-leaved weeds. The long-term use and large amounts use of imazethapyr, long residual herbicide, caused not only succeeding crop injury, forming the so-called "cancer field," but also damaged the soil ecosystem and the sustainable use of resources. In this work, the mechanism of imazethapyr affecting soil microbial diversity were studied by the combination of field surveys and culture experiments in laboratory, and the degradation dynamics, adsorption behavior, and desorption behavior of imazethapyr were studied in soils. The results are as follows:1. A method for the determination of imazethapyr residues in soil was estimated based on solid phase extraction and ultra-performance liquid chromatography-mass spectrometry (UPLC-MS/MS). The samples were extracted with 0.1 mol L-1 NH₄Cl/NH₃·H₂O (pH=10) buffer by ultrasonic wave, and cleaned up by C18 SPE cartridge. The imazethapyr residues were analyzed by UPLC-MS/MS under multiple-reaction monitoring mode. The qualitative results were obtained based on retention time, the precursor ion and two daughter ions, and the quantitative results were on the intention of the characteristic m/z 290>176 ion and m/z 290>245 ion. Average recoveries of imazethapyr in soil samples were found in the range of 83.47%～101.70% at three spiking levels from 0.01 mg kg^-1 to 0.5 mg kg^-1 with relative standard deviations of 4.15%～5.28%. Limit of quantification of imazethapyr was 0.075μg kg^-1. The method is simple and suitable for the routine and confirmation analysis.2. The effects of imazethapyr on soil microbial diversity were studied by the combination of field surveys and culture experiments in laboratory, which provided important scientific evidence for the understanding and prediction of soil quality, soil ecosystem safety and ecological risk assessment of pesticides.In the field surveys, the soils were collected at the same location in different application history soybean field for the determination of imazethapyr residue, soil microbial PLFA content and soil microbial biomass C. The residue in was 1.62μg kg^-1 in the soil (soil 1) with 1 year history of imazethapyr application, and 1.79μg kg^-1 for the soil (soil 2) with 2 years history of imazethapyr application. The microbial biomass C and total PLFAs of soil 2 were much higher than the soil (blank soil) with no imazethapyr application. PLFA profiles showed that fatty acids for Gram-negative (GN) and Gram-positive (GP) bacteria, as well as total bacteria and total fungi in soil 2 were much higher than blank soil. GN/GP and the stress level were much more than the blank soil. Principal component analysis of the PLFAs showed that the microbial community structure differed substantially among the three different soybean field soils. The application of the herbicide imazethapyr to soybean fields clearly changed the soil microbial biomass and shifted the microbial community structure.In the culture experiments, two agricultural soils, a silty loam (HS) and a loamy soil (QL), were spiked with imazethapyr (CK, 0.1, 1 and 10μg g^-1) and incubated for 1, 15, 30, 60, 90 and 120 days. In addition, untreated controls received only water. The soil microbial community structures were characterized by investigating the phospholipid fatty acids (PLFA) and microbial biomass C. The results showed that the dissipation kinetics of imazethapyr in the two soils were described using a first order kinetics model, the half-time were 63.0～111.8d; there was a slow decline in imazethapyr that leveled out at longer incubation times,especially in soil HS with lower pH. The soil microbial biomass C and total concentration of PLFA were reduced by the addition of imazethapyr. Imazethapyr addition also decreased the ratios of GN/GP and fungi/bacteria. A larger stress level, measured as the ratio of PLFA (cyc17:0 + cyc19:0)/(16:1ω7c + 18:1ω7c), was found in the high concentration (1 and 10μg g^-1) herbicide treatment groups. Principal component analysis (PCA) of the PLFA clearly separated the treatments and incubation times. Both soils showed different total PLFA concentrations and ratios of GN/GP and fungi/bacteria, but similar changes in the PLFA pattern upon soil treatment. The soil microbial community structure was shifted by the addition of imazethapyr, which recovered after 60 days. Our results demonstrated that the addition of imazethapyr shifted the microbial community structure, but that it recovered after a period of incubation.3. Adsorption, desorption kinetics and adsorption, desorption isothermal experiments were studied using batch equilibrium method. The results of adsorption, desorption kinetics experiments showed that distribution coefficient (Kd) of imazethapyr in the two soils were 3.42 and 2.01, the adsorption rates were 40.60% and 29.19%, adsorption capacity was soil HS > soil QL. The desorption distribution constant (Kdes) were 19.85 and 12.83, and desorption capacity was soil HS < soil QL. Adsorption, desorption isotherm results showed that adsorption isotherms were described by Freundlich isotherm equation, adsorption coefficient (KFads) were 8.33 and 1.33, adsorption capacity was soil HS > soil QL. Regression coefficient 1/n of imazethapyr in the two soils were less than 1, which indicating Langmuir adsorption isotherm and the adsorption rate reducing with the increase of the concentration of pesticides. Our results demonstrated that imazethapyr adsorption was higher in soil HS than that of soil QL.