Effects Of Microplastics On Soil Properties:Laboratory Experiments And Meta-Analysis

As a new type of environmental pollutants,microplastics come from a wide range of sources,have small particle size and are difficult to degrade.They will exist in soil for a long time and affect the physical and chemical properties and biological properties of soil,such as changing soil porosity,soil aggregate structure and nutrient cycling,and affecting soil enzyme activity and microbial community structure.The research on the effect of microplastics on soil system is still advancing,so it is of great significance to explore the effect of microplastics on soil properties.This study consists of two parts.The first part takes Jiangxi red soil as test soil sample,through two months of culture experiment,The concentration（1%,5%）and particle size（150 mesh,500 mesh）of two kinds of microplastics polyethylene（PE）and polyvinyl chloride（PVC）were compared on the main physical and chemical available nutrient indexes of soil（soluble organic carbon（DOC）,ammonium nitrogen（NH₄⁺-N）,nitrate nitrogen（NO₃^--N）,and available phosphorus）.And soil biological indicators（soil acid phosphatase,invertase and urease activity;Using PLFA to characterize microbial community structure）.In the second part,Meta analysis was conducted to explore the effects of different microplastics characteristics and soil properties on the stability of soil aggregates.The results showed that:（1）PE and PVC could significantly reduce soil available phosphorus content（p<0.05）,and the reduction effect of PE was stronger（p<0.05）.For PE,the effect of high supplemental level（5%）was more significant（p<0.05）,while there was no significant difference between the two supplemental doses of PVC（1%and 5%）（p>0.05）.The effect of different particle sizes on the content of available phosphorus was not obvious under the same additive amount.The content of DOC in soil was not affected by the type,addition amount and particle size of microplastics.（2）PE and PVC can reduce the contents of NH₄⁺-N and NO₃^--N in soil（p<0.05）.The decreasing effect of PE was enhanced with the increase of the added amount（p<0.05）.At low dose（1%）,the larger the PE particle size was,the more obvious the inhibiting effect on NO₃^--N was;at high dose,there was no significant difference between the treatments of different PE particle size.There were no significant differences in the reduction effect of PVC among different addition levels and different particle sizes（p>0.05）.（3）Both PE and PVC could activate soil acid phosphatase activity（p<0.05）,and the activation effect of PE was stronger than that of PVC（p<0.05）.There were no significant differences in the activation effect of PE on phosphatase among different dosage and particle size（p>0.05）.For PVC,at large particle size（150 mesh）,there was no significant difference in the effect of supplemental level on phosphatase activity（p>0.05）,while at small particle size（500 mesh）,the activation effect of PVC on phosphatase decreased with the increase of supplemental level（p<0.05）.At the same dose,the activation effect of small particle size was weaker（p<0.05）.Except for PE500 treatment,urease activity was slightly increased in other treatments.The invertase activity was not affected by the addition amount and particle size of PE,but was only affected by high dose（5%）PVC（p<0.05）.（4）The addition of microplastics could change the relative abundance of different bacterial species in soil.Except for the two treatments of high dose（5%）PE500 and PVC500,the other treatments significantly reduced the ratio of fungi to bacteria（p<0.05）,and promoted the transformation of fungi to bacteria in soil.The effect of microplastics on Gram-positive/Gram-negative（G⁺/G^-）was related to the type,particle size and dose of microplastics.For the two treatments of PE（PE150 and PE500）,the ratio of G⁺/G^-was not significantly different from that of the control at low dose（1%）,but was significantly lower at high dose（5%）,regardless of particle size.For the two treatments of PVC（PVC150 and PVC500）,the effect of PVC150 on G⁺/G^-was not significant compared with the control（P>0.05）,and was not dose-dependent,while the low-dose treatment of PVC500 increased G⁺/G^-,while the high-dose treatment of PVC500 decreased G⁺/G^-.It can be seen that the addition of microplastics can change soil microbial community structure and stimulate soil cycling.（5）On the whole,Exposure to microplastics had a negative effect on the stability of soil aggregates（p>0.05）,which was related to soil properties,utilization methods and microplastics characteristics.The shape and type of microplastics can inhibit the stability of soil aggregates.The stability of aggregates in sandy loam is lower than that in clay,and the stability of aggregates in farmland is lower than that in woodland and orchard.The response value of soil aggregate stability had significant positive correlation with the dosage of microplastics and soil SOC content,but had no significant correlation with soil p H.Under different experimental conditions,the aggregate stability tends to be inhibited.