Study On Leaching And Dissipation Of Neonicotinoids In Soil-Water System Under The Effects Irrigation Methods And Rice Plants

Neonicotinoids are the most widely used insecticides in the world.The occurrence of neonicotinoids in soil-water system has caused potential threat to ecological environment.The paper used the self-designed simulation device of pesticide migration and transformation in the paddy soil-water system to systematically study that under the effects of different irrigation methods(flooding irrigation,moderate wetting-drying cycles,severe wetting-drying cycles)and whether or not rice planting,the leaching and dissipation rules of 6 typical neonicotinoids(dinotefuran(DIN),nitenpyram(NIT),thiamethoxam(THM),clothianidin(CLO),imidacloprid(IMI)and acetamidine(ACE))in soil-water systems.The research results of this paper can provide scientific theoretical basis for pesticide pollution prevention and remediation of farmland soil-water system and comprehensive analysis of environmental behavior of neonicotinoids.The paper first explored the rule of environmental factors and microbial community succession in soil-water system under the effects of different irrigation methods and rice planting.In water phase,compared with wetting-drying cycles treatment,the flooding irrigation treatment can significantly reduce the pH of leaking water(p<0.05),due to the low dissolved oxygen(DO)of the inundation treatment,which stimulated the production of acidic substances by anaerobic microorganisms.In addition,the pH of leaking water in the treatment with rice planting was significantly higher than that in the treatment without rice planting(p<0.05).The DO of leaking water in the treatment with rice planting was higher than that in the treatment with not rice planting due to radial oxygen loss.In soil phase,rice planting significantly reduced the pH of soil(p<0.05),because rice roots secrete amino acids,organic acids and other substances during the growth process.For soil microorganisms,the Shannon,ACE and Chao of deep soil microorganisms were significantly higher than those of surface soil(p<0.05),which means the diversity of microbial community in deep soil is richer than that in surface soil.The relative abundance of Proteobacteria in the treatment with rice planting(main flora,26.9%-47.0%)was significantly higher than that in the treatment with not rice planting(p<0.05).The total relative abundance of major degrading bacteria corresponding to each neonicotinoid in surface soil was significantly higher than that in deep soil(p<0.05).The relative abundance of aerobic degradation bacteria(Flavobacterium,Pseudomonas,Pseudoxanthomonas,Ensifer,etc.)in surface soil was significantly higher than that in deep soil(p<0.05).The paper explored the the leaching and dissipation rules of neonicotinoids in soil-water system under the effects of different irrigation methods and rice planting.The concentrations of neonicotinoids in each medium of the soil-water system increased and then decreased with time(soil,interstitial water and leaking water),or rapidly decreased(overlying water:90%neonicotinoids were dissipated in the first 9 days of the experiment,and the remaining 10%were basically dissipated until the 61st day).After 61 days experiment,the residual ratios of 6 neonicotinoids in each medium of soil-water system were:<19.8%(soil),<0.7%(interstitial water),<0.4%(leaking water),≈0%(overlying water).The distribution ratios of neonicotinoids in each medium in the soil-water system are as follows:By 23.4%±12.6%(day 1)gradually increased to 82.0%±31.2%(days 61)(soil),from 5.4%±3.7%(day 1)to peak 31.5%±25.4%(days 5)after gradually to 5.1%± 13.9%(days 61)(interstitial water),gradually decreased from 71.1%±11.2%(day 1)to 0.4%±1.1%(day 61)(overlying water),from 0.1%±0.1%(day 1)to 12.5%±27.4%(day 61)(leaking water).In the soil-water system,the total dissipation rule of 6 neonicotinoids was consistent with the first-order reaction kinetics(R2=0.64-0.99).After 61 days experiment,the neonicotinoids dissipation rate>80%.The half-lives(t1/2)of 6 neonicotinoids were NIT(3.53±0.40 days)<ACE(6.84±1.53 days)<DIN(12.68±2.14 days)<THM(13.25±2.99 days)<IMI(15.57±2.18 days)<CLO(17.66±2.18 days).Rice planting can accelerate the dissipation of neonicotinoids in soil-water systems.Based on the Ⅳ fugacity model theory,the multi-media migration and transformation model of neonicotinoids in soil-water system was constructed.Based on the experimental data,the model was calibrated,verified(overlying water,soil and interstitial water phases)and predicted(atmospheric and plant phases).For DIN(high water solubility)and CLO(low water solubility),the main conclusions are as follows:the simulation results of the constructed model fit well with the previous experimental data.The IoA of DIN and CLO were 0.64-0.92 and 0.69-0.94 and the R2 were 0.59-0.99 and 0.68-1.00,respectively.The predicted trend of DIN in atmospheric phase is to gradually increase to the equilibrium state,but the trend of CLO is to continue to increase due to the high vapor pressure,and it does not reach the equilibrium within 61 days.The predicted trend of DIN and CLO in the plant phase is gradually increasing to the equilibrium state.In each environmental phase,the distribution ratios of DIN and CLO content in soil-water system on day 61 are respectively:0.0-18.0%and 0.0-0.2%(in overlying water),71.6-90.9%and 94.3-97.3%(soil),2.1-3.0%and 1.5-4.1%(interstitial water),0.02-0.03%and 0.02-0.07%(atmospheric),6.1-8.4%and 0.7-1.5%(Plants).The main differences of neonicotinoids among different treatments were as follows:in the soil phase,the wetting-drying cycles treatment accelerated the dissipation of neonicotinoids.The irrigation method slowed down the release of neonicotinoids into the atmospheric phase and the absorption of neonicotinoids by plants(after equilibrium,the contents of atmospheric and plant phases were:flooding irrigation>moderate wetting-drying cycles ≈severe wetting-drying cycles).