Study On The Dynamic Changes Of Soil Nutrients And Enzyme Activities In Fluvo-aquic Soil By Exposure Of Polyethylene Microplastics

In recent years,microplastics,as a new type of pollutant,can exist for a long time and cause lasting impact on the environment,which have attracted widespread attention.In order to investigate the long-term effects of microplastics on soil ecosystems,polyethylene microplastics,which are currently used more frequently in agricultural production,were selected as the subject of this paper.The effects of microplastics on soil physical properties,microbial community structure,soil nutrients and soil enzyme activities were investigated under four concentrations(5.00 g·kg^-1,10.00 g·kg^-1,20.00 g·kg^-1 and 30.00 g·kg^-1)with six particle sizes（5μm,180μm,800μm,1 mm,3 mm and 5 mm）by soil exposure experiments.Furthermore,a 360-day dynamic change study of soil nutrients and soil enzyme activities was conducted.The main results are as follows,（1）Short-term effects of polyethylene microplastics on the physical properties of the soil were analyzed.Polyethylene microplastics treatment increased soil bulk density and non-capillary porosity,and decreased soil maximam water holding capacity,capillary moisture capacity,field capacity,capillary porosity and total porosity.The effect of microplastic particle size on soil bulk density was consistent in the range of millimeter-scale microplastics and micron-scale microplastics when concentration of microplastics was the same.After entering the soil,microplastics can occupy capillary pores to weaken the capillary effect of soil which in turn reduces soil water holding capacity and accelerates soil cracking.（2）Short-term effects of polyethylene microplastics on soil nutrients.After soil aging was completed,the contents of total nitrogen,total phosphorus,available phosphorus and available potassium decreased by0.73%～8.35%,2.21%～10.13%,3.83%～29.71%and 1.16%～17.99%,respectively,compared with the control group.Soil total potassium content increased by 0.44%～6.87%.（3）Short-term effects of polyethylene microplastics on soil enzyme activity.After soil aging,microplastic exposure decreased soil catalase,sucrase and urease activities by 0.90%～18.24%,0.45%～6.55%and 10.03%～31.56%,respectively.Compared with the control group,soil alkaline phosphatase activity increased by 23.03%～54.66%.（4）Short-term effects of microplastics on soil microbial community structure were analyzed.Polyethylene microplastics significantly increased soil microbial richness,but had no significant effect on soil microbial diversity.Under low concentration microplastic exposure conditions(5.00 g·kg^-1 and 10.00g·kg^-1),soil microbial richness decreased with increasing soil microplastic particle size.However,there was no significant effect of microplastic particle size on soil microbial richness under high concentration microplastic exposure conditions.The results of microbial community structure analysis showed that microplastic exposure significantly reduced the abundance of Gemmatimonadetes by 3.46%～15.03%,while the abundance of Bacteroides and Firmicutes were significantly increased by 12.32%～46.87%and1.82%～225.1%,respectively.（5）Effects of polyethylene microplastics on dynamic changes of soil nutrients were analyzed.Microplastic exposure generally decreased soil total nitrogen,total phosphorus,available phosphorus,total potassium and organic matter by 0.80%～27.60%,0.02%～22.20%,1.18%～17.77%,0.49%～12.46%and0.77%～19.28%,respectively.However,the content of soil available nitrogen and available potassium increased by 0.83%～16.71%and 1.16%～21.69%,respectively.At the same microplastic concentration,the contents of soil organic matter and available phosphorus in micron-scale microplastic treatment group were higher than those in millimeter-scale microplastic treatment group.The contents of total nitrogen,available nitrogen,total phosphorus and total potassium in micron-scale microplastic treatment group were lower than those in millimeter-scale plastic addition treatment group.（6）Effects of polyethylene microplastics on dynamic changes of soil enzyme activities were analyzed.Microplastics promoted soil alkaline phosphatase activity and soil catalase activity with the promotion rates of 0.58%～14.29%and 1.71%～29.02%,respectively.However,soil urease activity was inhibited in the range of 0.12%～16.15%.Low concentration of microplastics exposure(5.00 g·kg^-1)inhibited soil sucrase activity in the range of 0.04%～9.19%.But high concentrations of microplastics exposure(10.00 g·kg^-1,20.00 g·kg^-1and 30.00 g·kg^-1)promoted the activity of sucrase,and the promotion rate ranged from 1.99%to 21.83%.