| With the accelerated industrialization and urbanization in China in recent years,cadmium（Cd）pollution in agricultural soils has been intensified due to factors such as sewage irrigation and unreasonable fertilization.At the same time,the emergence of an emerging pollutant"microplastics"（MPs）has made the soil environment more complex.It has been proved that MPs and heavy metal pollution in soil are not single,but complex and continuous.However,current studies have mainly focused on the ecological effects on soil and plants when one of the two contaminants is present singly,and no comprehensive study has been conducted on the combined contamination of MPs and Cd and their toxicity to soil,microorganisms,and plants.In this study,we set up pot experiments with maize（Zea mays L）seedlings,and conducted single Cd,single PP and PP+Cd combined pollution experiments with original and aged（60 d）polypropylene microplastics（PP,1 mm）and Cd(Cd,20 mg·kg-1)as pollutants,and set up two different PP dosing levels（0.1%,1%）,by comparing soil physicochemical properties,maize rhizosphere microbial community and maize growth physiological indexes among different treatment groups,the effects of single and combined contamination with PP microplastics and Cd on soil physicochemical properties and maize seedling growth at different dosing levels and aging levels were investigated,with a view to providing basic data on the ecological effects of combined contamination with microplastics and heavy metals on soil.The main findings are as follows:（1）The surface of PP after UV aging for 60 d produced a large number of pores and cracks.The surface increased its roughness,and the specific surface area increased by 0.5%compared with the original PP;the crystallinity of PP decreased by 7.17%after aging;the types of functional groups on the surface of PP before and after aging were approximately the same,but the intensity of peaks changed significantly,and a new peak representing carbonyl group（C=O）was generated at 1760～1600 cm-1 in aged PP and the oxygen content increased from 13.40%to 31.02%.（2）Soil p H increased significantly in all experimental groups,especially at higher PP single contamination（7.88→8.12）;in addition,cation exchange capacity（CEC）and soil enzyme activity decreased significantly.The effect of PP dosing on soil physicochemical properties（p H,EC,CEC,organic matter）was mainly antagonistic,but synergistic on soil enzyme activity.Catalase activity and urease activity decreased with the increase of PP addition.Aging PP caused changes in soil physicochemical properties and enzyme activities but the effects were not significant.Compared with the Cd group,the weak acid extractable fraction（F1）decreased with the increase of PP microplastic addition in the PP and Cd group;the reducible fraction（F2）decreased slightly compared with the Cd group（P>0.05）;the oxidizable fraction（F3）increased significantly;and the residue fraction（F4）decreased slightly.The correlation analysis of Cd form in soil with soil properties and enzyme activities showed that the reducible fraction（F2）was significantly positively correlated with EC,and the residue fraction（F4）was significantly negatively correlated with sucrase activity and urease activity.（3）Single and combined contamination PP or Cd had toxic effects on maize rhizosphere soil microorganisms and showed a synergistic effect significantly reducing the microbial community number when both coexisted.The most and second most dominant phyla in each treatment group at the phylum level were Amoebacteria and Actinomycetes and were reduced relative to the CK group,especially the addition of PP under combined pollution reduced the abundance of heavy metal tolerant phyla to reduce the resistance of maize seedlings to heavy metals;in addition,the abundance of Amoebacteria was not significantly correlated with soil properties and the abundance of Actinomycetes was significantly and positively correlated with peroxidase activity in soil.In addition,the abundance of Aphanomyces phylum was not significantly correlated with soil properties,and the abundance of Actinomycetes phylum was significantly and positively correlated with soil catalase activity.The microbial community diversity index and rank clustering curves showed that the CK group had the lowest microbial community abundance and the 1%aging PP group had the highest microbial community abundance.In terms of the effect on microbial community structure,the greatest variability in species diversity existed between the Cd and CK groups,followed by the PP and Cd coexistence treatment groups,and the smallest was the single PP treatment group.（4）Under single contamination,PP and Cd had no significant effect on photosynthetic pigments of maize seedlings,and significantly inhibited their plant height,stem thickness,root length,and above-ground and below-ground biomass,and promoted the root-to-crown ratio.The coexistence of PP and Cd showed a synergistic effect on plant height,root length and photosynthetic pigmentation under compound pollution.In addition,the coexistence of PP and Cd showed antagonistic effects on the above-and below-ground biomass and root-to-crown ratio.In single contamination,1%PP microplastics had a stronger inhibitory effect on maize growth,but there was no significant effect in compound contamination;aging could cause changes in growth indicators of maize seedlings,but none of them was significant.The results of the three-way ANOVA also showed that Cd addition was the main factor affecting the growth of maize seedlings,and there was no interaction between microplastic addition,aging and Cd addition.1%PP microplastic and Cd single and combined pollution both caused oxidative stress in maize seedlings,thus increasing the content of antioxidant enzymes（POD,SOD,CAT）and malondialdehyde,especially 1%aging PP had a stronger promotion effect on antioxidant enzymes;in particular,when PP and Cd coexisted,the environmental stress exceeded the regulatory capacity of the antioxidant defense system,and the defense capacity might be lost,resulting in a significant decrease in CAT content in the 1%PP and Cd group.The three-way ANOVA showed that PP aging,dosage and Cd addition significantly affected the antioxidant enzyme content in the stems and leaves of maize seedlings,and PP addition had an inhibitory effect on the Cd content in the aboveground part of maize seedlings,and this alleviating effect was more pronounced with 1%PP addition,while PP addition had less effect on the Cd content in the below-ground part.In addition,correlation analysis indicated that only above-ground Cd content was negatively correlated with oxidizable state.Meanwhile,the growth of maize seedlings was related to organic matter,soil enzyme activity and rhizosphere microbial community,and the dominant strains in maize inter-root soil significantly influenced the growth of maize seedlings. |
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