Study On Residue And Degradation Characteristics Of Thiacloprid In Water-soil-plant System Under Alternate Drying And Wetting Irrigation

Neonicotinoids are currently the most widely used insecticides in the world,which occupied more than 25%of the insecticide market.In the production of rice and other crops,it is necessary to apply a large amount of neonicotinoids to control pests and diseases.Neonicotinoids have been detected in a variety of environmental media,posing a certain threat to the ecological environment.Alternating wet and dry irrigation(AWD)is a widely used water-saving irrigation technology,but the current research on the residues and degradation of neonicotinoids under AWD is still unclear.In this paper,thiacloprid,a typical neonicotinoid pesticide,was used as the research object.A self-developed pollutant migration and transformation simulation experimental device was used to systematically explore the residue and degradation characteristics of thiacloprid in water-soil system and water-soil-plant system under AWD.The main contents and conclusions are as follows:(1)This paper systematically reviewed the material transport in paddy fields under AWD and quantitatively analyzed the main controlling factors through the multiple linear regression(MLR)analysis.It was found that the of thresholds of AWD and soil type were the important control factors that affect the amount of irrigation,environmental factors,and material transportation.(2)Environmental factors and multimedia partitioning and degradation characteristics of thiacloprid were explored using the self-developed pollutant migration and transformation simulation system.Four system were established:soil aerobic system(SC),soil-flooding irrigation system(SCF),soil-mild AWD system(SMA)and soil-heavy AWD system(SSA).The main results were as follows:compared with SCF,SMA and SSA treatments with alternating aerobic-anaerobic states caused periodic changes in soil pH,increased overlying water pH and soil oxygen content,and decreased seepage water pH and soil redox potential(ORP).Except for soil pH,which decreased with time,SC had no significant effect on other environmental factors.The aerobic state of the soil-water system can significantly increase the degradation rate of thiacloprid,and its half-life(t1/2)in each treatment was:SC(9.5d)<SSA(10.5d)<SCF(12.9d)<SMA(14.6d).At the same time,compared with SCF,SMA and SSA significantly increased the distribution ratio of thiacloprid in soil(by 3.1%and 2.0%),and reduced its distribution in overlying water(by 2.1%and 1.4%)and distribution in interstitial water(reduced by 1.5%and 1.0%).(3)In water-soil-plant system,there were three systems:rice-soil-flooding irrigation system(R_SCF),rice-soil-mile AWD system(R_SMA)and the rice-soil-heavy AWD system(R_SSA).The results in water-soil-plant system were consistent with the water-soil system.Compared with R_SCF,soil pH in R_SMA and R_SSA showed periodic changes,increased overlying water pH and soil oxygen content,and decreased seepage water pH and ORP.t1/2 of thiacloprid were 11.2d(R_SCF),9.3d(R_SMA)and 7.6d(R_SSA).At the same time,compared with R_SCF,R_SSA and R_SMA increased the distribution ratio of thiacloprid in soil by 1.7%and 0.3%,respectively;decreased the distribution ratio in interstitial water by 1.0%and 0.5%,respectively.(4)Comparing the experimental results of water-soil system and water-soil-plant system,it was found that the presence of plants accelerated the degradation of thiacloprid(viz.t1/2)in the system due to the absorption of plants(rice)and their root microbial activity.Specifically,R_SCF<SCF(shortened by 3.4d),R_SMA<SMA(shortened by 3.6d)and R_SSA<SSA(shortened by 2.9d).Meanwhile,rice increased the distribution ratio of thiacloprid in soil and decreased its distribution ratio in overlying water and interstitial water.Compared with the corresponding soil-water-plant system,the distribution ratio of thiacloprid in soil increased by 7.1%(R_SCF),5.4%(R_SMA)and 6.5%(R_SSA),respectively.The distribution ratios in the overlying water were reduced by 3.7%(R_SCF)and 2.6%(R_SMA),respectively;the distribution ratios in the interstitial water were reduced by 3.2%(R_SCF),2.9%(R_SMA)and 3.9%(R_SSA),respectively.In conclusion,AWD changed the environmental factors in the system,which in turn affected the residue and degradation characteristics of thiacloprid.Rice could also affect the residue and degradation characteristics of thiacloprid.This study can provide theoretical guidance for the research of thiacloprid in this area and reducing its environmental harm.