Effects Of Combined Exposure Of Microplastics And Cadmium On Vegetable Growth And Cadmium Accumulation

Microplastics and Cadmium are two common pollutants in farmland,and their effects on farmland soil ecosystem have been widely concerned by researchers throughout the world.However,it is not clear about the effect of combined exposure of Microplastics and Cadmium on crop growth characteristics and the response of plant-soil system to combined exposure.Thus,the study on the combined exposure of these two kinds of pollutants is of great significance to understand the physiological and ecological characteristics of plants and the accumulation of soil pollutants under the coexistence of the two pollutants.In this study,pakchoi was selected as the research object to carry out pot control experiment by adding different kinds（refractory polyethylene PE,biodegradable polylactic acid PLA,petrified degradable poly adipic acid/butylene terephthalate PBAT）,different mass concentrations（0,1,5%）of microplastic particles and heavy metal Cadmium(concentration 5 mg·kg^-1).The purpose of this study is to reveal the effects of combined exposure of Microplastics and Cadmium on crop-soil system.It will provide practical guidance for pollutant management in agricultural soil.The main results and conclusions are as follows:（1）Three kinds of Microplastics can significantly affect the content of carbon and nitrogen in soil,resulting in the increase of carbon and nitrogen content.The possible reason is that the addition of Microplastics to stimulate soil enzyme activity and then activate soil organic carbon and nitrogen pool.The single treatment of three kind of Microplastics promote the accumulation of soluble organic carbon and nitrogen.Three kinds of Microplastics can affect soil p H,electrical conductivity and cation exchange capacity.Compared with refractory Microplastics,degradable Microplastics PLA and PBAT have more significant effects on soil p H,electrical conductivity and cation exchange capacity.This phenomenon is related to the degradability and degradation products of Microplastics.At the same time,it was also related to the changes of soil microbial community structure,the composition and structure of soil colloid and particlesand metal forms caused by Microplastics,and the results of two-way ANOVA showed that the types and concentrations of Microplastics had obvious interactive effects on soil p H,electrical conductivity,total nitrogen,organic carbon and available potassium.（2）The addition of Microplastics significantly reduced the soil microbial diversity,and order of the toxicity was degradable Microplastics>refractory Microplastics,and bio-based Microplastics>petrochemical-based Microplastics;Degradable Microplastics can significantly affect the species richness of soil microorganisms and the diversity of bacterial communities in soil,and significantly reduce the species richness of soil microorganisms and the diversity of bacterial communities.The results of two-way ANOVA showed that the type and concentration of Microplastics had significant interactive effects（P<0.01）on Shannon,Simpson and Chao1 under combined exposure with Cadmium.Degradable Microplastics can affect the relative abundances of dominant bacteria in soil and have a significant effect on the structure of soil bacterial community.Compared with the single treatments of Microplastics,high concentration（5%）degradable Microplastics and Cadmium exposure had a greater effect on the relative abundance of dominant microorganisms.Soil microbial community diversity and abundance were positively correlated with soil p H,electrical conductivity（EC）and total organic carbon（TOC）,and negatively correlated with cation exchange capacity,total nitrogen,available phosphorus and available potassium content.The changes of soil physical and chemical properties caused by combined exposure of the two pollutants were significant factors which affect the Alpha diversity of the biological community.（3）Biodegradable Microplastics can significantly inhibit the growth of pakchoi.Combined exposure of PE and Cadmium can significantly inhibit the net photosynthetic rate of pakchoi,mainly caused by non-stomatal restriction.Microplastics can significantly inhibit the accumulation of Cadmium in pakchoi,and the stronger inhibitory effect increased with the increasing concentration of Microplastics.At the same concentration,the biodegradable Microplastics had more significant inhibitory effect on the absorption of Cadmium by pakchoi.Compared with different types of degradable Microplastics,the inhibitory effect of PLA on the absorption of Cadmium by pakchoi was greater than that of low concentration of PBAT.The effect of PE on the antioxidant system of pakchoi was greater,which showed that low concentration of（1%）PE alone or combined with Cadmium could decrease the activities of SOD and CAT in pakchoi.The combined effect of PE and Cadmium at high concentration had a greater effect on the osmotic regulation system of pakchoi,which showed that the combined exposure of high concentration of PE and Cadmium would significantly increase the content of soluble protein in pakchoi.（4）Three kinds of Microplastics could significantly change the content of bioavailable Cadmium in soil,and the effect of biodegradable Microplastics（PLA,PBAT）was stronger than that of refractory Microplastics（PE）.The effects of three kinds of Microplastics on the content of potentially biologically available Cadmium were as follows:all the other treatments could increase the content of potentially biologically available Cadmium except for high concentration of PBAT.in which the content of potentially available Cadmium increased with the concentration of PE.While PBAT and PLA showed relatively higher content of potentially available Cadmium in low concentration treatment.On the one hand,the reason why Microplastics affect the morphological change of Cadmium may be that it indirectly changes the form of Cadmium by changing the chemical properties due to the changes of soil organic matter resulted from the change of physicochemical properties,on soil soil microorganisms and crops effects of soil,such as soil p H and electrical conductivity which was affected by soil microorganisms and crops.On the other hand,it may be caused by the direct influence of the characteristics of microplastic particles on Cadmium.