Distribution,Transportation And Transformation Of ¹⁴C-Chlorpyrifos And ¹⁴C-Imidacloprid In Rice-Soil System

Chlorpyrifos and imidacloprid are commonly used in rice pest control in China.In this study,the distribution,transportation and transformation of ¹⁴C-chlorpyrifos and¹⁴C-imidacloprid in simulated rice-soil system were fully demonstrated by ¹⁴C tracing.The influence factors such as pesticide properties,soil types and the addition of exogenous organic matter on the dissipation,distribution,transportation,metabolism/degradation of the two pesticides in rice-soil system were also discussed.The obtained results are going to be helpful for a better understanding about the fates of these two pesticide in faddy field ecosystem,which will be useful for making a more reasonable criteria.The main results are as follows:（1）Chlorpyrifos and imidacloprid were mainly distributed in straw,rice husk and topsoil（0-2.5cm）at initial time.The residues of chlorpyrifos in rice straw,rice husk and topsoil at harvest were 38.88-55.29%,28.91-38.52%and 32.58-85.81%of those at initial time,respectively.The corresponding values for imidacloprid were 18.28-40.60%,7.02-43.83%and 120.58-369.47%,respectively.The concentrations of chlorpyrifos and imidacloprid in rice plants were shown as straw>rice husk>unpolished rice>root.The vertical distribution of chlorpyrifos and imidacloprid residues in soil decreased with the increase of soil depth.（2）The concentration of chlorpyrifos in unpolished rice was 0.021-0.053 mg/kg,which was far lower than the maximum residue limits.At harvest,ratio of maternal chlorpyrifos residues to ¹⁴C-chlorpyrifos residues（Total Radioactive Residues）in rice straw,rice husk and unpolished rice were 16.33²4.93%,32.36³9.38%and8.01¹4.13%,respectively.（3）Soil properties showed a significant effect on the residual distribution of chlorpyrifos and imidacloprid in rice-soil system,and the effect of soil properties on the two pesticides was found to be similar.In general,both of the two pesticides were most easily absorbed by rice in red soil,most easily leached in fluvo-aquic soil,and the adsorption capacity of black soil was the strongest.Soil types can significantly affect the formation of bound residues of chlorpyrifos and imidacloprid.Among the two pesticides,the amount of bound residue in three soils followed as red soil>black soil>fluvo-aquic soil.（4）The addition of biochar into soil could significantly prevent rice from absorbing¹⁴C-chlorpyrifos in red soil and fluvo-aquic soil,and reduce its vertical leaching in soil.Biochar addition in black soil had no significant difference compared with the control.Besides,the addition of biochar had no significant effect on both the formation and distribution of chlorpyrifos bound residues.（5）The effect of livestock manure（chicken manure）addition on the chlorpyrifos residual behavior in rice soil varied with soil types.Adding chicken manure to red soil could inhibit the uptake of ¹⁴C-chlorpyrifos,but had no significant effect on the vertical distribution of ¹⁴C-chlorpyrifos in the soil.Adding chicken manure to black soil and fluvo-aquic soil could not affect the uptake of ¹⁴C-chlorpyrifos,but the addition of chicken manure promoted the leaching of ¹⁴C-chlorpyrifos in black soil and inhibited the leaching of ¹⁴C-chlorpyrifos in fluvo-aquic soil.The addtion of chicken manure can enhence the formation of bound residues of chlorpyrifos in soil and significantly increase its tendency to combine with fulvic acid or humic acid.（6）The leaching ability of ¹⁴C-chlorpyrifos in soil was better than ¹⁴C-imidacloprid,and the residue ratio of ¹⁴C-chlorpyrifos in soil below 5.0 cm was significantly higher than that of ¹⁴C-imidacloprid.Imidacloprid had a stronger ability to form bound residues in soil than chlorpyrifos.The bound residues of the two pesticides were mainly distributed in humin and fulvic acid.Chlorpyrifos was more inclined to combine with humin（47.46-58.24%）,while imidacloprid was more inclined to combine with fulvic acid（51.81-59.94%）.