Toxic Effects Of Cloransulam-methyl And Diclosulam On Soil Microorganisms

Cloransulam-methyl and diclosulam belong to the triazolopyrimidine sulfonanilide herbicides.These herbicides have the advantages of low application rate,high herbicidal efficiency,and the broad weeding spectrum,so they have a broad application prospect.Both cloransulam-methyl and diclosulam can be used to control broad-leaved weeds in soybean fields and have good weed control effects.With the continuous use of the two herbicides,they would produce residues in the soil environment through adsorption or sedimentation,which would cause a certain interference to soil microorganisms.At present,the research on cloransulam-methyl and diclosulam is mostly focused on the control effects of herbicides and their effects on crops,but there are few reports on the research of the two herbicides on soil microorganisms.The soil microbial system plays an important role in maintaining the stability of the ecosystem and the growth or yield of crops.Therefore,it is significant to study the toxic effects of cloransulam-methyl and diclosulam on soil microorganisms.In the present study,the method of indoor toxicity test was used to expose the tested brown soil samples to different doses of cloransulam-methyl and diclosulam to study the toxicity mechanism of the two herbicides to soil microorganisms.Three concentrations of 0.05,0.5,2.5mg kg^-1 corresponding to low,medium,and high concentrations were set for the two herbicides,and the control group was the solvent(acetonitrile)treatment group.The soil samples at the 7,14,28,42,56 days after exposed to the cloransulam-methyl or diclosulam were measured at the levels of bacteria,gene,and enzyme.High-throughput sequencing technology was used to determine the effects of cloransulam-methyl and diclosulam on soil microbial communities,and fluorescence quantitative polymerase chain reaction technology was used to analyze the effects of herbicides application on the abundances of functional genes include the soil nitrogen cycle-related genes(nifH,AOA,AOB,nirK,nirS)and the genes related to Calvin cycle(cbbLG,cbbLR).The colorimetric method was used to determine the effects of two herbicides on the activities of ?-glucosidase and urease in the soil.The integrated biomarker index(IBR) was used to evaluate the comprehensive toxicity of the two herbicides to soil microorganisms.The present study would enrich the toxicological data of cloransulam-methyl and diclosulam on the soil microorganisms,and provided scientific suggestions for the selection of herbicides in soybean fields.The main results of this study are as follows:(1)The effects of two herbicides on the abundance of functional bacteria in the soil were analyzed by using high-throughput sequencing technology.The results showed that the application of the two herbicides would cause changes in the abundance of bacteria related to the nitrogen cycle in the soil.Besides,the two herbicides increased the abundances of some functional bacteria that can degrade pollutants and pesticides,such as Nocardioides,Serratia,Sphingomonas,etc.(2)Based on the discussion of the effects of the two herbicides on the abundances of functional genes related to the nitrogen cycle,we speculate that the two herbicides affected the expression of functional genes AOA and AOB and then inhibited soil nitrification.Cloransulam-methyl at a low concentration dose and diclosulam at a low or medium concentration dose might promote the progress of soil denitrification by affecting the expression of nirK and nirS genes.(3)Cloransulam-methyl has no significant effect on the abundances of cbbLG and cbbLR genes involved in the Calvin cycle.Diclosulam promoted the expression of cbbLG and cbbLR genes significantly at a dose of 0.5 mg kg^-1 except for the days 14 and 42 of the experiment period,which indicates that the application of diclosulam may enhance the ability to fix carbon for microbe at 0.5 mg kg^-1.(4)The two herbicides promoted the activity of ?-glucosidase,but the effect of promotion was significant only at the dose of 2.5 mg kg^-1.Diclosulam activated urease activity from the day 14 while the effect of cloransulam-methyl on urease activity has fluctuated to a certain extent.At the end of the experiment period,cloransulam-methyl promoted the urease activity at low-medium concentration doses but the high concentrations dose has no significant effect on urease activity.(5)According to the IBR evaluation,we concluded that the IBR index of diclosulam was greater than that of cloransulam-methyl in the middle and the end of the experiment,which means that the comprehensive toxicity of cloransulam-methyl to soil microorganisms was less than that of diclosulam.We recommend the use of cloransulam-methyl to control weeds in the soybean fields for the selection of herbicides in the soybean field.