Residues Of Pesticides In Corn Seed Treatment And Their Effects On Bacteria And Enzyme Activities In Soil

Systemic pesticides(including neonicotinoids and triazole fungicides)are widely used in agriculture and public health all over the world,especially in seed treatment,because of their broad-spectrum and highly effective insecticidal and anti-microbial effects.However,its high frequency detection in environmental media and agricultural by-products,as well as its known negative effects in the ecosystem,all indicate that systemic pesticide is unsafe to some extent.However,at present,there is not enough attention on the ecotoxicity of systemic pesticides at home and abroad,especially on the dynamic migration and residues in soil and its impact on soil ecosystem.Therefore,this study used thiamethoxam and difenoconazole as the target systemic pesticides,to investigate their effects on soil microbials via the planting the corn seeds treated with those two pesticides.The results were revealed by analyzing the number of culturable microorganisms in soil,the structure and diversity of soil bacterial community and the state of soil related enzyme activities.The main results are as follows:(1)The thiamethoxam residues in the soil were mainly concentrated in the 0-10 cm soil layer when the thiamethoxam and difenoconazole·thiamethoxam were applied to the soil at labelled recommended dosage(6ml/1kg seeds).Difenoconazole was not detected in different depths of soil.Although thiamethoxam migrated downward in soil,the migration ability of thiamethoxam was low.The maximum concentration of thiamethoxam in soil was 0.068(μg/g)and 0.036(μg/g)respectively.The residue of thiamethoxam in soil was lower than the detection limit(0.004μg/g)during corn harvest.Difenoconazole was not detected in soil at different growth stages of maize.And only about 1% of the pesticides in the seed coating agent are absorbed by corn plants.Most of the pesticides remain in the soil,which may cause soil pollution.(2)Thiamethoxam and difenoconazole·Thiamethoxam could inhibit the number of culturable bacteria in the rhizosphere and non-rhizosphere soil.With the growth of maize,the inhibitory effect of thiamethoxam and difenoconazole·thiamethoxam on culturable bacteria gradually weakened until it recovered.There was significant difference in the number of culturable bacteria in the rhizosphere soil between the two treatments only in the first sampling period(early stage of maize plant growth).The numbers of culturable bacteria in the rhizosphere soil were always higher than those in the non-rhizosphere soil.(3)The effects of thiamethoxam and difenoconazole·thiamethoxam on the diversity of bacterial community in the rhizosphere soil were small,and the abundance of bacterial community increased.With the growth of maize,the abundance of bacterial community returned to the control level.High throughput sequencing analysis for the rhizosphere soil samples at the early growth stage showed that thiamethoxam and difenoconazole·thiamethoxam changed the bacterial community structure,increased the relative abundance of Bacillaceae,Chitinophagaceae and Xanthomonasdaceae,and decreased the relative abundance of norank＿o＿Vicinamibacterales,Vicinamibacteraceae,Gemmataceae and WD2101＿soil＿group.The relative abundance of Nitrososphaeraceae decreased slightly in the treatment of thiamethoxam,and increased significantly in the treatment of difenoconazole·thiamethoxam.The bacterial community structure returned to the control level toward the end of corn growing period.(4)Thiamethoxam and difenoconazole·thiamethoxam can activate the activities of catalase and dehydrogenase in the rhizosphere and non-rhizosphere soil,and there was no significant difference between the two treatments.The activities of catalase and dehydrogenase in the rhizosphere and non-rhizosphere soil were restored with time.The enzyme activity of rhizosphere soil was higher than that of non-rhizosphere soil.In conclusion,the application of thiamethoxam and difenoconazole·thiamethoxam in corn seed treatment according to the recommended dosage has a certain impact on soil microbial ecology at the early growth stage of maize.However,in the actual agricultural production,it is still necessary to control the application concentration and frequency of thiamethoxam and difenoconazole·thiamethoxam,so as to avoid permanent damage to the soil and further affect the crop yield and environmental safety.In order to better understand the residual dynamics of insecticides and fungicides in soil and their impact to soil micro ecological environment under the actual agricultural production situation,it is suggested that:(1)field experiments should be carried out in consecutive-year manner to understand the long-term accumulation of thiamethoxam and difenoconazole·thiamethoxam in soil and its impact on soil microorganisms;(2)the research methods of microbial ecology should be improved and enriched to establish more efficient and universally applicable biomarkers to provide basis for monitoring soil pollution;(3)Combined with the results of high-throughput sequencing,isolates and purifies the strains that can efficiently degradation thiamethoxam and difenoconazole·thiamethoxam,so as to develop the soil pesticide pollution remediation work in a targeted way,and provide a strong scientific and technological support for the guarantee of food security and environmental health in China.