Ecological Effects Of Microplastics And Heavy Metals On Soil-plant System

With the development of modern industrial technology,the types and applications of plastics are becoming extensively.Common traditional plastics include polyethylene（PE）,Polystyrene（PS）,and Polyamide（PA）.As alternative to traditional plastics,bio-based degradable polymers,such as polylactic acid（PLA）and poly-β-hydroxybutyrate（PHB）,and petro-based degradable plastics,polybutylene succinate（PBS）,are also increasing year by year.These plastics can be broken and degraded by external forces in the environment to form microplastics（MPs）,and enter soil environment through a variety of ways,threatening the soil ecosystem.At present,the ecological effects and environmental behavior of different kinds of MPs are still in its infancy,especially the compound effects with other pollutants.Therefore,the above six kinds of plastics were selected in this study,with two additions（0.2% and 2%,w/w）,and the average particle size（30～80 μm）.Taking farmland soil as the test soil,the Pb and Zn were added to simulate heavy metal pollution.The response mechanism of ecological effects on "soil-plant system" was discussed.By studying the soil culture experiment and plant pot experiment of MPs in Pb/Zn-contaminated soil,the ecological response mechanism to "soil-plant system" was discussed in this paper.After the 120-day soil culture experiment,it was found that the basic physicochemical properties and soil enzyme activity of Pb/Zn-polluted soil changed significantly with the addition of MPs.PLA caused a significant increase in p H and DOC content.Three kinds of degradable MPs all reduced the content of soil available nitrogen.The content of available phosphorus decreased only under the treatment of PE,PS and PHB with the dose of 2%.The bioavailability of Pb/Zn can be changed significantly.High-dose of MPs increased the content of DTPA-Pb,while PLA inhibited the content of DTPA-Zn,indicating that the concentration of MPs had a significant interaction with the boavailability of Pb/Zn.Six types of MPs have significant interaction with urease,sucrase,phosphatase and catalase.High-dose of PLA and PHB promoted the activities of urease,sucrase and catalase,while high-dose of PS and lowdose of PBS and PHB inhibited the activities of sucrase.In general,bio-based degradable MPs（PLA and PHB）are more likely to stimulate soil enzyme activities.Through a 9-week plant pot experiment,it was found that different MPs had great differences on plant growth parameters and heavy metal adsorption capacity.Degradable plastics were more conducive to the growth of plant roots and promoted the elongation of aboveground parts.Most types of MPs reduced the enrichment of Zn in the underground,and PE,PA,PLA and PHB effectively promoted the transport of Zn to the aboveground.Only 2%PHB treatment promoted the enrichment of Pb in sweet sorghum roots and exacerbated the toxicity to sweet sorghum.The addition of MPs had no significant effect on chlorophyll a,chlorophyll b and carotenoids in sweet sorghum leaves,but degradable MPs had some differences in the ratio of chlorophyll a/b.The soluble sugar content decreased significantly under all MPs treatments.With the participation of plants,the soil p H and DOC content of PLA and PHB treatments also increased accordingly.Inorganic nitrogen was further reduced under the treatment of degradable MPs.Soil available potassium decreased only at 0.2% PS and 2% PLA.For the Pb/Zn polluted soil after plant stabilization,the difference of enzyme activity related to the type of MPs.Three types of degradable MPs significantly increased the activities of urease and phosphatase,and also increased the activity of sucrase to a certain extent.MPs changed the structure of soil microbial community,reduced microbial diversity,and further led to the variation of soil functional genes.In the long-term cultivation process,a bacterial community dominated by Actinobacteria,Chloroflexi,Firmicutes,Proteobacteria,Acidobacteria,Gemmatimonadota and Bacteroidota gradually formed in the soil.Through the prediction of KEGG gene metabolic pathway,Biosynthesis of other secondary metals,Nucleoside metabolism,Metabolism of terpenoids and polyketides,Lipid metabolism and Carbohydrate metabolism were significantly up-regulated.The change of these functional genes by MPs might be the main reason for its influence on soil C and N cycle and plant absorption of C,N and other nutrients.In conclusion,MPs directly or indirectly changed the microbial community and function by altered the soil physicochemical properties and the availability of heavy metals,then the growth parameters and the heavy metals adsorption capacity of sweet sorghum were changed significantly.In all types of MPs,degradable MPs had a greater disturbance to the soil.Biobased degradable MPs（PLA and PHB）had a greater impact on the environment than petrobased degradable MPs（PBS）.