| The pollution of microplastics(MPs)in the terrestrial ecosystem is an urgent environmental problem.Small particle size,large specific surface area and hydrophobicity make MPs easily adsorb polycyclic aromatic hydrocarbons(PAHs)in soil.However,little is known about the interaction between microplastics and PAHs in the soil-plant system.In this study,the effects of MPs on the vertical migration of phenanthrene in soil,the phytotoxicity of polyethylene microplastics(PE-MPs)and phenanthrene on wheat in agricultural soil and the effects of microplastics on bacterial flora in wheat rhizosphere soil were studied by leaching and pot experiments.This study provides a scientific basis for the migration of hydrophobic organic compounds such as PAHs affected by MPs in soil and the phytotoxicity of MPs alone and combined with PAHs,which is helpful to evaluate the ecological risk of microplastics alone and in combination with PAHs in the terrestrial ecosystem.The main results are as follows:(1)The effects of polyvinyl chloride microplastics(PVC-MPs),polyethylene microplastics(PE-MPs)and polystyrene microplastics(PS-MPs)on the vertical migration of PAHs in soil were studied by soil column leaching experiment.The adsorption of phenanthrene by three MPs(PVC,PE,PS)increased the content of phenanthrene in soil at all depths.Due to the difference in adsorption capacity,the adsorption strength order of phenanthrene in soil was arranged as PS > PE > PVC.Because of the adsorption of phenanthrene by MPs in soil,the concentration of phenanthrene in leachate treated with MPs was lower than without MPs in the early stage of leaching,and higher than without MPs in the late stage of leaching.The phenanthrene concentration in wheat roots and leaves cultivated in the top soil of polluted column and leachate was positively correlated with that in the cultivation media.In the later stage of leaching experiment,the concentration of phennthrene in roots and leaves of wheat was PS+Phe > PE+Phe > PVC+Phe > Phe.(2)The effects of 0.5%,1%,2%,5% and 8%(W/W)PE-MPs individual and combination with phenanthrene on wheat growth were studied by soil culture experiment.The dose-dependent toxicity of PE-MPs on wheat biomass,plant height and root length were observed.High concentration(8%)of PE-MPs stimulated wheat root elongation,but inhibited wheat seedling growth.Compared with single phenanthrene treatment,the combined pollution of PE-MPs and phenanthrene reduced the accumulation of phenanthrene in wheat roots and leaves.Low concentration of PE-MPs increased the activity of antioxidant enzymes in wheat roots,while phenanthrene and high concentration of PE-MPs(8%)damaged the antioxidant system of wheat roots.No matter whether phenanthrene was involved in the pollution or not,the photosynthetic pigment concentration of wheat leaves under PE-MPs stress showed a double dose concentration effect of "low promotion and high inhibition”.The pollution of MPs and phenanthrene aggravated the damage of photosynthetic system.Therefore,the joint pollution of PE-MPs and phenanthrene enhances the harm to wheat growth.(3)High throughput sequencing technology was used to analyze the effects of PVC,PE and PS on the diversity and structure of bacterial colonies in wheat rhizosphere soil,and Bug Base analysis was employed to predict the changes of functional phenotypes of bacterial colonies in wheat rhizosphere soil.It was found that the three MPs caused different differences in the structure of bacterial flora in wheat rhizosphere,and the dominant bacterial phylum of each treatment group was different too,indicating that MPs have a selective effect on soil microorganisms in plant rhizosphere.All the three MPs had negative effects on bacterial α diversity in wheat rhizosphere soil.In addition,β diversity analysis showed that there were significant differences in bacterial community structure in wheat rhizosphere soil(P < 0.05),implying that MPs could destroy the abundance,diversity,and composition structure of microorganisms in plant rhizosphere soil.Three MPs(PVC,PE,PS)had different effects on the prediction of bacterial colony function in wheat rhizosphere soil,which depends on the difference of their dominant bacterial phylum.MPs would affect the surrounding soil environment through their own properties,and form selective pressure on bacteria in wheat rhizosphere soil,which will lead to the changes in community structure,abundance,and diversity,and eventually exhibit different functional phenotypes.In conclusion,MPs can increase the residual concentration of phenanthrene in soil and stimulate the migration of phenanthrene to deeper soil.MPs and phenanthrene co-pollution in the soil have great phytotoxicity to wheat,and MPs can reduce the abundance,structure,and diversity of bacteria in wheat rhizosphere soil,and then affect the function of bacteria. |
