Toxic Effects Of Single And Combined Exposure To Flumetsulam And Microplastics On Soil Microorganisms

Flumetsulam is a typical triazolopyrimidine sulfonamide herbicide,which is widely used to control broad-leaved weeds in soybean,corn,and wheat fields because of its high efficiency and broad spectrum.With the wide application and environmental residue of flumetsulam,it will inevitably have an impact on non-target organisms in the soil environment and then bring ecological risks.At present,the research on flumetsulam is mainly focused on residue analysis,field weed control effect,and crop safety evaluation,but the eco-toxic effect on soil microorganisms is not clear.Therefore,the study on the toxic effect of flumetsulam on soil microorganisms is of great significance for the scientific understanding and rational use of flumetsulam,and to determine the safety of flumetsulam to the soil ecological environment.Plastic products are widely used in agricultural production,industrial manufacturing,and packaging fields because of their strong plasticity,low manufacturing cost,durability,and other advantages.After entering the environment,these widely used plastics are transformed into microplastics through the processes of physical crushing,chemical aging,and biodegradation.As an emerging persistent organic pollutant,microplastics have attracted the wide attention of scholars at home and abroad because of their small particles and potential risks to the ecological environment.In addition to its own toxicity,microplastics can absorb coexisting pollutants in the environment and change the environmental behavior and ecological risks of microplastics and their adsorbed pollutants.Therefore,on the basis of studying the toxic effect of a single exposure to flumetsulam on soil microorganisms,this paper further explored the difference in the toxic effect of combined exposure of flumetsulam and microplastics on soil microorganisms.In this study,flumetsulam and polyethylene microplastics were used as research objects.The changes in soil bacterial community structure were studied by high-throughput sequencing technology.The number of soil microorganisms(16S and ITS),the abundance of functional genes related to the soil nitrogen cycle(AOA-amo A,AOB-amo A,nir K,nir S,and nif H)and carbon cycle(cbb LR and cbb LG)were determined by q PCR technology.The activities of urease and β-glucosidase were determined by colorimetry.The toxic effects of single and combined exposure of different concentrations of flumetsulam(0.05,0.5,and 2.5 mg/kg)and polyethylene microplastics(500～1000 μm,1%)on soil microorganisms for 0,28,and 56 days were systematically studied.Finally,the sensitivity of the above microbial indexes(soil microorganisms number,functional genes abundance,and soil enzyme activity)to single and combined exposure to flumetsulam and microplastics was evaluated by calculating the integrated biomarker response(IBR)index,and the toxicity and difference of single and combined exposure to soil microorganisms were evaluated.The main results are as follows:(1)Single exposure to flumetsulam significantly reduced the number of soil bacteria and increased the number of soil fungi.Compared with the single exposure of flumetsulam,the combined exposure of flumetsulam and microplastics had no significant effect on the number of soil bacteria,but significantly reduced the number of soil fungi.(2)Single and combined exposure to flumetsulam and microplastics induced changes in the abundance of bacteria genera associated with nitrogen and carbon cycles in soil.The single exposure to flumetsulam reduced the abundance of bacteria genera with nitrification function,such as Nitrospira,and increased the abundance of bacteria genera with denitrification,nitrogen fixation,and carbon transformation functions,such as Haliangium,MND1,RB41,Candidatus Udaeobacter,Gemmatimonas,and Bryobacter.Compared with the single exposure to flumetsulam,the combined exposure to flumetsulam and microplastics reduced the abundance of Nitrospira,MND1,and Bryobacter,and inhibited the nitrification,nitrogen fixation,and carbon transformation of soil microorganisms.(3)The results of soil bacterial interaction network showed that a single exposure to flumetsulam increased the number of negative edges among soil bacteria,and the competition and mutual exclusion among bacteria were increased.Compared with a single exposure,combined exposure to flumetsulam and microplastics increased the number of network nodes and positive edges among soil bacteria and promoted synergistic symbiosis among soil microorganisms.(4)The single exposure to flumetsulam inhibited the expression of the nitrification functional gene(AOA-amo A)and promoted the expression of the denitrification functional gene(nir K),nitrogen fixation functional gene(nif H),and carbon sequestration functional gene(cbb LR).Compared with the single exposure to flumetsulam,the combined exposure to flumetsulam and microplastics inhibited the expression of the nitrification functional gene(AOB-amo A),denitrification functional gene(nir K),and carbon sequestration functional gene(cbb LR).The activities of urease and β-glucosidase were inhibited by the single exposure to flumetsulam.Compared with the single exposure to flumetsulam,the combined exposure to flumetsulam and microplastics promoted urease activity,but β-glucosidase activity was not significantly affected.(5)The IBR index calculated by the number of soil microorganisms,functional genes,and soil enzymes showed that the comprehensive toxicity of single exposure of flumetsulam to soil microorganisms increased with the increase of the concentration of flumetsulam,and there was an obvious dose-effect relationship.Compared with the single exposure to flumetsulam,the combined exposure to low concentration(0.05 mg/kg)of flumetsulam and microplastics increased the toxicity to soil microorganisms,while the combined exposure to medium and high concentration(0.5 and 2.5 mg/kg)of flumetsulam and microplastics decreased the toxicity to soil microorganisms.In addition,the nir K gene associated with denitrification and the cbb LR and cbb LG genes associated with carbon sequestration were the most sensitive biomarkers to the single and combined exposure of flumetsulam and microplastics,and they could be used as early warning indicators of soil pollution caused by single and combined exposure of flumetsulam and microplastics.