Study On Environmental Fate And Behavior Of Three Hydrogel Encapsulated Herbicides In Soil By Isotope Tracing

Concerns about the environmental impact of the growing amount of pesticide used,have arisen worldwide.Aside from the development of novel pesticides,research works have been engaged on the improvement of pesticide formulations and application method to increase pesticide efticiency and decrease pesticide contamination.As a common slow-release substance,starch grafted hydrogels could have unpredictable influence on environmental fate and behavior of pesticides by changing pesticide concentration distribution,affecting soil adsorption and soil microbial activity.In order to evaluate the environmental behavior and potential risks of such slow-release formulations,this study has engaged isotope tracing to research into the transformation of three hydrogel encapeulated(HE)herbicides,metsulfuron-methyl(MSM),acetochlor(ACE)and atrazine(ATZ)in aerobic soils.By combining liquid scintillation measurement technology and spectrometric analysis of chemicals,the degradation products,transformation of metabolites and degradation pathways of the three herbicides were studies as well.In the meanwhile,soil bacterial community and diversity after the application of different herbicide formulations were studied by high-throughput sequencing.By comparing with the environmental fate and behavior of each conventional herbicide,this study has come to conclusions as follows:(1)Bacterial community and diversity of soils treated with hydrogel encapeulated metsulfuron-methyl,acetochlor and atrazine differ from that of conventional herbicide applied soils.HE-MSM and HE-ATZ have enhanced soil bacterial diversity in acidic soil SI and alkaline soil S3,while Shannon indexes of genetic diversity in neutral soil S2 were lower compared to conventional MSM and ATZ treated soils.Soil bacterial diversity of HE-ACE treated S1 was reduced and that of S2 and S3 were enhanced.The effect of HE herbicides and conventional ones on soil bacterial community varied,abundances of soil native microflora such as Firmicutes,Proteobacteria,Actinobacteria,Acidobacteria,Bacteroidetes,Gemmatimonadetes,Chloroflexi,and Planctomycetes were changed to some extent.(2)After incubation experiments under aerobic condition,HE acetochlor have produced 4.13%,11.70%,and 7.54%mineralized 14CO2 of applied radioactivity,while that of conventional acetochlor were 1.87%,9.33%,and 4.81%.By increasing mineralization of acetochlor,HE treatments have significantly reduced acetochlor residue in soils.Extractable residues in particular,were relatively reduced by 8.54%,4.82%,and 6.90%in 3 soils after hydrogel encapsulation.Half-lives of HE acetochlor in S1-S3 were 43.48 d,5.13 d,and 9.37 d,respectively,which were significantly shortened compared with conventional acetochlor.Three degradation productes were detected and identified as ACE-M1:N-(ethoxymethyl)-N-(2-methyl-6-vinylphenyl)formamide;ACE-M2:2-ethyl-6-methyl-N-methyleneaniline;ACE-M3:2-chloro-N-(2-ethyl-6-methylphenyl)acetamide.Degradation products of HE acetochlor were consistent with conventional acetochlor,however,total proportion of metabolites degraded by mother compound was increased,and the concentration of final product ACE-M2 was especially increased in this study.In conclusion,HE treatment has promoted the degradation of acetochlor,enhanced acetochlor mineralization and deduced acetochlor residues in soil.The result provides a new approach for acetochlor formulation development.(3)In acidic soil S1,HE treatment has surpressed hydrolysis of metsulfuron-methyl(MSM),resulting in longer half-life,less mineralization and less bound residue.In neutral soil S2 and alkaline soil S3,HE-MSM have increased mineralization of MSM,but no significant effect on reducing residue of metsulfuron-methyl was observed.Degradation products MSM-M2:methyl 2-(N-((4-hydroxy-6-methyl-1,3,5-triazin-2-yl)carbamoyl)sulfamoyl)benzoate,MSM-M3:methyl 2-(N-((N-carbamoylcarbamimidoyI)carbamoyl)sulfamoyl)benzoate,MSM-M4:4-methoxy-6-methyl-l,3,5-triazin-2-amine were detected in all 3 soils,and MSM-M1:E)-methyl N-((E)-((l,l-dioxido-3-oxo-2,3-dihydrobenzo[d]isothiazole-2-carbonyl)imino)methyl)formimidate,MSM-M5:4-amino-6-methyI-l,3,5-triazin-2-ol,and MSM-M6:l-(4-methoxy-6-methyl-l,3,5-triazin-2-yl)urea were detected in S1.HE treatment has reduced production of MSM-M4 by surpressing sulfonylurea cleavage;in contrast,MSM-M3 production was increased by HE treatment.(4)In this study,HE atrazine and conventional atrazine barely mineralize in S1 and S3.Small amount of atrazine was mineralized in S2,but HE treatment has reduced atrazine mineralization and have increased HE atrazine bound residue in soil.HE treatment has shortened atrazine half-lives from 73.15 d,38.25 d,and 62.75 d to 66.60 d,25.48 d,and 49.13 d in 3 soils,respevtively.Five degradation products were detected and identified as ATZ-M1:4,6-bis(isopropylamino)-1,3,5-triazin-2-ol;ATZ-M2:6-chloro-N2-isopropyl-1,3,5-triazine-2,4-diamine;ATZ-M3:6-chloro-N2-ethyl-1,3,5-triazine-2,4-diamine;ATZ-M4:4-(ethylamino)-6-(isopropylamino)-1,3,5-triazin-2-ol,and ATZ-M5:6-chloro-1,3,5-triazine-2,4-diamine,among which ATZ-M3 and ATZ-M5 were only detected in S2 and S3.HE treatment has increased production of ATZ-M4 and ATZ-M5.The result reveals that application of formulated pesticides should be selective.Though HE treatment has promoted degradation of atrazine,the higher polarity metabolites with biological activity could be a treat to the environment.Atrazine degradation was accelerated after hydrogel encapsulation and proportions of degradation products were increased in three soils,while mineralization of HE-ATZ was surpressed in neutral soil.Microbial degradation of HE herbicides in soils may be interfered as soil bacterial community and diversity were changed to some extent after HE herbicide treatments.Comparing to conventional formulations,HE-ACE had higher mineralization and lower residues in all three tested soils;mineralization of HE-MSM in neutral and alkaline soils were also enhanced but residues of HE-MSM were not significantly changed since the total mineralization rates were relatively low;hydrogel encapsulation had not significantly affected residues of atrazine in acidic and alkaline soils and mineralization of HE-ATZ in neutral soil was reduced.Degradation of three hydrogel encapsulated herbidides were accelerated and dynamic formation of degradation products were different compared with conventional formulations,in the meanwhile,degradation products of hydrogel encapsulated herbicides remain the same.