Toxic Effects Of Two Pesticides Alone And Co-exposed With Nano-TiO₂ On Soil Microorganisms

Because of its application,pesticides enter the soil and may cause damage to non-target organisms.Strobilurin fungicides are commonly used with broad applications,fluoxastrobin is a representative variety of them.Triazolopyrimidine sulfonamide herbicides are kinds of herbicides with excellent development prospects,florasulam is a usual variety of them.With the large-scale production and use,these two pesticides would be bound to enter and affect the soil environment.Soil microorganisms are essential indicators of soil ecosystem health,and the effects of fluoxastrobin and florasulam on soil microorganisms need to be clearly defined.Nanomaterials have been applied in many fields because of their excellent properties.As one of the most used nanomaterials,nano-Ti O₂ enters and affects the environment.The environmental hazards caused by the composite exposure of nanomaterials and other pollutants attract people's attention.The nano-Ti O₂ in soil may affect the toxic effect of pesticides on soil microorganisms.Therefore,the co-exposed impact of nano-Ti O₂ and pesticides on soil microorganisms still need to be studied and evaluated.This study took fluoxastrobin,herbicide florasulam,and nano-Ti O₂ as the research objects based on the above research status.The effects of recommended dosage and double recommended dosage of these two pesticides on soil microbial indices were determined on day0,day 14,and the crop harvest time to study the toxicity of these two pesticides on soil microorganisms.The effects of these two pesticides(0.1 mg/kg)alone or co-exposed with nano-Ti O₂(0.05 mg/kg)on soil microbial indices were determined at day 0,14 and 50 under laboratory conditions to study the toxicity of co-exposure of nano-Ti O₂ with these two pesticides on the soil microorganisms.The microbial indexes were as follows:the soil microbial populations,the change of soil bacterial community structure,the abundances of functional microorganisms in soil nitrogen and carbon cycling,and the activities of enzymes related to soil nitrogen,carbon,and phosphorus cycles.The toxic effects of two pesticides alone and co-exposed with nano-Ti O₂ on soil microorganisms were systematically studied to provide theoretical support for the risk assessment of pesticides and nanomaterials.The main findings of this paper are as follows:(1)The toxic effects of fluoxastrobin and florasulam alone on farmland soil microorganisms:?)Fluoxastrobin reduced soil microorganisms'populations.The bacterial community structure of the maize field was changed.The synergistic relationships between soil bacteria were increased.More bacteria participated in the soil bacterial network relationship.The nitrification gene(AOA-amo A)abundance was inhibited,and the denitrifying gene(nir K)and nitrogen fixation gene(nif H)abundances were promoted.The carbon sequestration genes(cbb LR and cbb LG)abundances were promoted in wheat field but inhibited in the maize field.The urease and?-glucosidase activities were promoted.?)Florasulam reduced soil fungal population.The bacterial community structure of the maize fields was changed.The synergistic relationships between soil bacteria were increased.More bacteria participated in the soil bacterial network relationship.The nitrification genes(AOB-amo A and AOA-amo A)and nitrogen fixation gene(nif H)abundances were inhibited.The denitrifying gene(nir K)and carbon sequestration gene(cbb LR)abundances in the wheat field was promoted,but the denitrifying genes(nir K and nir S)and carbon sequestration genes(cbb LR and cbb LG)abundances in the maize fields were inhibited.The urease and?-glucosidase activities were promoted.?)The toxicities of double recommended dose fluoxastrobin and florasulam were greater than the recommended dose.?)The most sensitive microbial index in the wheat field after fluoxastrobin and florasulam application is the abundance of the nir K gene associated with denitrification.And the most sensitive microbial indexes in the maize field are the abundances of nitrifying genes:AOB-amo A and AOA-amo A,respectively.The abundances of nir K,AOB-amo A and AOA-amo A genes could be used as early warning indexes of soil contamination by these two pesticides.?)The half-lives of fluoxastrobin at the recommended and double recommended doses were 15.70 d and 22.50 d in wheat field soil,and 18.52 d and 18.04 d in the maize field soil,respectively.The half-lives of florasulam at the recommended and double recommended doses were 5.32 d and 3.89 d in wheat field soil,and 4.07 d and 4.17 d in the maize field,respectively.The final residues of fluoxastrobin and florasulam on wheat and maize grains were lower than the maximum residue limit for these two pesticides.(2)The toxic effects of fluoxastrobin and florasulam co-exposed with nano-Ti O₂ to soil microorganisms:?)Nano-Ti O₂ exposure reduced the number of soil bacteria.The abundances of denitrifying genes(nir K and nir S)were inhibited.However,nano-Ti O₂ exposure did not affect soil bacterial community structure and enzyme activity.?)Fluoxastrobin and florasulam co-exposed with nano-Ti O₂ reduce the number of soil microorganisms.The structure of the soil bacterial community was changed.The synergistic and negative relationships between soil bacteria were increased.More bacteria participated in the soil bacterial network relationship.The abundances of the nitrifying gene(AOA-amo A),denitrifying gene(nir K),nitrogen fixation gene(nif H),and carbon fixation genes(cbb LR and cbb LG)were inhibited.The soil urease,?-Glucosidase,and acid phosphatase activities were inhibited.?)Compared with pesticide exposure and nano-Ti O2 exposure,the co-exposure of the two pesticides with nano-Ti O2 had greater inhibition on the number of soil bacteria and the expression of the denitrifying related gene(nir K).The IBR index analysis showed that the toxicity of co-exposed pesticide with nano-Ti O2 to soil microorganisms was significantly greater than that of pesticide exposure and nano-Ti O2 exposure alone.?)The abundance of the nitrifying gene AOA-amo A was the most sensitive microbial indicator response to the co-exposed pesticide with nano-Ti O₂.?)The metabolic function of bacterial community showed that the co-exposed of pesticide with nano-Ti O₂ activated the membrane transport and signal transduction ability of bacteria,inhibited the lipid metabolism and secretion of bacteria,and increased the toxicity of pesticides to soil microorganisms.