Stress Mechanisms Of Microplastics On Lettuce Growth Under Co-pollution Of Cd/Pb

As an emerging pollutant with a diameter of less than 5 mm,microplastics widely exist in soil environments.It is noteworthy that microplastics can serve as carrier molecules of heavy metals in heavy metal-contaminated soils,resulting in combined pollution of microplastics and heavy metals,which poses significant risks to the soil-plant system,agricultural production,and even human health.Currently,it is advocated to replace traditional plastics with biodegradable plastics to alleviate microplastics pollution.However,the risks of biodegradable microplastics in the soil environment have not been well explored.In addition,the aging of microplastics leads to the changes in their characteristics with exposure time elapsing.The interaction mechanisms between different types of aging microplastics and heavy metals,as well as their effects on plant toxicity and growth remain unclear.Therefore,this work took lettuce and Pb/Cd-contaminated soil as the test plant and test soil,respectively.A pot experiment was conducted with the addition of traditional microplastics polyethylene（PE）and biodegradable microplastics polylactic acid（PLA）at four concentration gradients（0,0.1%,1.0%,and 5.0%,w/w）to explore the impacts of the combined pollution of different microplastics and heavy metals on soil-plant system.The changes in the physicochemical properties of Cd/Pb-contaminated soil,plant growth,and plant physiological parameters were analyzed under the influence of microplastics.The mechanisms of microplastics affecting plant growth and heavy metal absorption were elucidated.The main results are as follows:（1）The presence of microplastics significantly altered the physicochemical properties and enzyme activity in Cd/Pb-contaminated soil.Low concentrations of microplastics reduced the soil p H,whereas moderate and high concentrations of microplastics increased the soil organic carbon（SOC）content by 10.9%–49.0%.PLA significantly increased soil dissolved organic carbon（DOC）and soil total nitrogen（TN）contents and decreased the soil nitrate nitrogen（NO₃^--N）.Both original and aged PE significantly reduced soil available phosphorus（AP）content.In general,microplastics stimulated the activities of soil peroxidase,urease,and phosphatase,although high concentrations of microplastics hindered the activity of soil sucrase.A significant interaction effect existed between soil enzyme activity and the type and concentration of microplastics.（2）In comparison to PE,PLA exhibited greater toxicity towards plants.The addition of PLA resulted in the decrease of biomass,plant height,chlorophyll content,and lettuce N content.Original and aged PE promoted the growth of lettuce,however,showed the hindrance of N and P contents in lettuce tissues at high concentrations.Exposure of lettuce to microplastics resulted in a decrease in the net photosynthesis rate by 4.0%–26.7%.The stress of microplastics intensified the oxidative damage to lettuce tissues,as well as significantly increased the content of antioxidant enzymes SOD and POD.（3）Microplastic exposure enhanced the bioavailability and bioaccessibility of Pb/Cd in Cd/Pb-contaminated soils.The soil DTPA-Cd/Pb content was significantly increased by 13.1%–74.0%and 8.7%–76.0%,respectively under the influence of microplastics.Moreover,the content of Cd/Pb in lettuce tissues exhibited a positive correlation with the availability of Cd/Pb in soil.Original PE treatment resulted in a marked decrease in Cd absorption in shoot and root of lettuce and Cd transfer factor.Conversely,the uptake of Cd in lettuce tissues was higher in aged PE and PLA treatments compared to original microplastics.The addition of all types of microplastics significantly elevated the level of Pb present in the root of lettuce.However,it substantially diminished the transfer factor,resulting in a decrease in the amount of Pb absorbed by the shoot portion.Overall,changes in soil physicochemical properties and the increased availability of heavy metals induced by microplastics have affected soil system functions,further influencing plant growth physiology and the accumulation and distribution of heavy metals in plant tissues.The impact of microplastics on the soil-plant system depends on their types and concentrations.Generally,biodegradable microplastics have a higher impact on soil properties and plant growth than traditional microplastics.The increasing degree of environmental risk to Cd/Pb caused by aged microplastics is higher than that caused by original microplastics.These results can provide some bases for the potential ecological risks of microplastics to the soil-plant system,as well as the effects of co-pollution of microplastics and heavy metals on plant growth.