Effect Of Microplastics On Growth And Cadmium Uptake Of Alfalfa

Microplastics as persistent pollutants have become a growing global concern.However,there are few studies on the effects of microplastics on plant growth physiology,and the interaction mechanism of microplastics on soil properties and the heavy metal cadmium is still unclear.In this study,alfalfa（Medicago sativa L.）was used as the research object to carry out pot experiments to explore the effects of microplastics on soil physicochemical properties and plant growth physiology,as well as the interaction effects under the coexistence of microplastics and Cd.In this study,microplastics alone and microplastics+Cd contamination experiments were carried out,in which the microplastics alone experiments set up 2 types of microplastics（low density polyethylene（LDPE）and polylactic acid（PLA））and 6 concentration gradients（0,0.1%,,0.5%,1%,5%,10%,w/w）,the composite pollution test set up 2 types of microplastics,3 concentration gradients（0.5%,1%and 5%,w/w）and 2 Cd concentration gradients(5 mg·kg^-1 and 50 mg·kg^-1),to explore the growth characteristics and physiological responses of microplastics to plants,their effects on soil nutrients and microorganisms,and the absorption of plant Cd.The main conclusions are as follows:（1）High concentrations of microplastics have an inhibitory effect on plant growth.10%LDPE and PLA reduced aboveground biomass by 20.7%and 61.9%,while medium and high concentrations of PLA（1%,5%,and 10%）significantly reduced belowground biomass.The reductions were 25.6%,33.7%and 59.3%,and the same results were also reflected in the root morphology,which indicated that the addition of PLA had a certain toxic effect on the roots of alfalfa,while the effect of LDPE on the roots was not obvious.The addition of high-concentration PLA significantly reduced the chlorophyll content of plants by 22.6%.From the results that the activity of defense enzymes increased with the increase of its concentration,PLA stimulated plants to produce corresponding physiological responses,while the effect of LDPE was not obvious.（2）LDPE increases soil p H,but PLA does the opposite.Soil total organic matter content increases with the increase of microplastic concentration,but soil DOC content has the opposite trend.Soil total nitrogen and nitrate nitrogen were significantly decreased by 37.9%and 29.9%,respectively,by high concentration of LDPE（10%）,while soil amine nitrogen increased with the increase of PLA concentration.The high concentration of microplastics significantly decreased the content of total phosphorus and available phosphorus in soil.On the whole,the activities of the three soil enzymes showed a downward trend with the increase of the concentration of microplastics.The addition of high concentrations of microplastics made the soilβ-glucosidase,urease and alkaline phosphatase significantly lower than those of CK,while the soil enzyme activities were significantly lower than those of CK.There was a significant positive correlation between soil available nutrients.（3）On the whole,microplastics can alleviate the growth inhibition effect of high concentration of Cd.Compared with the addition of high concentration of Cd alone,the addition of high concentration of LDPE significantly improves the biological properties of alfalfa,while the alleviation effect of high concentration of PLA does not.Significantly.When treated with high concentration of Cd,both high concentrations of microplastics reduced the absorption of Cd in alfalfa plants,and the effect on the root system was much higher than that of the shoots.The enrichment factor and transport factor for Cd decreased.Both types of microplastics decreased the soil available Cd content,and this effect was more pronounced with the increase in the concentration of microplastics.Soil p H and DOC were simultaneously affected by microplastics and Cd,and there was a significant positive correlation between DOC content and available Cd content.This paper can provide a certain scientific basis for the ecological risk of microplastics to the soil environment and the harm of plant growth,and also provide a reference value for the impact of microplastics on heavy metal pollutants.