The Effect Of Microplastics On Soil Physical Properties And Organic Carbon

Plastics are man-made high molecular polymers,which are widely used in modern society due to their durability and lightness.However,a large amount of plastic waste,especially microplastics,has caused serious harm to terrestrial ecosystems.The current research speculates that the impact of microplastics on the soil ecosystem is closely related to the changes in soil physical properties caused by it.However,the current research results on the effects of microplastics on the physical properties of soil are scattered and contradictory.So it is difficult to systematically and correctly recognize the effects of microplastics on the physical properties of soil.Based on this,this study intends to explore the effects of different types of microplastics on the physical properties of soil,and attempt to reveal the mechanism of microplastics on physical quality and its influencing factors.At the same time,this study also explored whether heat treatment and adding natural organic matter can reduce the impact of microplastics on soil properties.This study studied two kinds of microplastics through cultivation experiments,namely polyester microfiber（PMF,0%,0.1%,0.3% and 1% soil dry weight）and low-density polyethylene microfilm（LDPE,0%,0.1%）,0.3% soil dry weight),and natural organic matter（Populus nigra leaf litter（OM,0%,1%,3% soil dry weight））that impact on the physical structure of the soil,soil water holding capacity and natural organic matter accumulation.At the same time,it was explored whether heat treatment and adding natural organic matter can reduce the impact of microplastics on soil properties.The main findings are as follows:（1）This study explored the effect of microplastics on the physical structure of soil,trying to reveal the effect of microplastics on the stability and pores of the soil structure.The results show that,compared with no microplastics（CK）,the addition of PMF and LDPE significantly reduces the MWD of the soil.But they have no significant effect on the number of <30μm pores and >60μm pores in the soil.After the soil heat treatment（HPMF）with the addition of polyester microfibers,HPMF has no effect on the water-stable large aggregates and MWD of the soil.Compared with 0%HPMF treatment,1.0%HPMF treatment significantly increased the number of soil storage pores,while reducing the number of soil >60μm pores.Adding OM to the soil can reduce the impact of LDPE treatment on soil MWD,but it cannot change the impact of PMF treatment on soil MWD.After the addition of OM,HPMF treatment had no significant difference in the number of soil MWD,<30μm pores and >60μm pores.This indicates that the addition of PMF and LDPE may reduce the stability of soil structure.Heat treatment changes the effect of PMF on the physical structure of the soil.The addition of OM cannot change the influence of PMF on soil structure.but it reduces the effects of LDPE and heating PMF.（2）By exploring the effects of different types of microplastics on the soil water characteristic curve（SWCC）under low suction and soil water constant,trying to reveal the mechanism of microplastics affecting soil water holding capacity.Model analysis shows that the van Genuchten model has the best fit for SWCC in the low suction section.Compared with CK,the addition of PMF significantly reduces the macropore space（MPS）,hysteresis（H）and pore distribution index（I）in the soil.But it significantly increases the water entry value（ψw）in the soil.But it has no effect on the air entry value（ψa）,field water holding capacity,available water,hygroscopic coefficient and wilting coefficient of the soil.The addition of LDPE has no effect on SWCC parameters under low suction and soil water constant.After heat treatment,HPMF treatment had no significant effect on soil MPS,I,ψa,field capacity,hygroscopic coefficient and wilting coefficient.However,compared with 0%HPMF treatment,1.0%HPMF treatment significantly increased soil H,field capacity and available water,and significantly reduced ψw.After adding OM to the soil,the influence of PMF and HPMF on the soil water characteristic curve under low suction and soil water constant is changed.This shows that: different types of microplastics have different effects on soil water content.Filament PMF increases soil water holding capacity,but flake LDPE has no effect on soil water holding capacity.Although the heat treatment has melted the polyester microfibers,its effect on the water holding capacity of the soil still exists.The addition of OM reduces the effect of microplastics on soil water holding capacity.（3）This experiment explores the influence of different types of microplastics on the accumulation of natural organic carbon in the soil,and attempts to clarify the effect of microplastics on the decomposition of natural organic matter in the soil.The results showed that the accumulation of soil organic carbon decreased with the increase of PMF and HPMF concentration,and the addition of LDPE had no significant effect on the accumulation of soil natural organic carbon.This shows that PMF promotes the decomposition of natural organic matter,while LDPE has no effect on it.After heat treatment,PMF still affects the accumulation of natural organic carbon in the soil.This study shows that the addition of polyester microfibers and low-density polyethylene reduces the stability of soil structure,but they have no significant effect on the characteristics of soil pore distribution.Different types of microplastics have different effects on soil water holding capacity.The addition of low-density polyethylene has no significant effect on the water holding capacity of the soil,but the addition of polyester microfibers increases the water holding capacity of the soil.After heat treatment,the polyester microfiber changes the physical properties of the soil and the water holding capacity of the soil.Adding natural organic matter and heat treatment can partially reduce the influence of microplastics on soil physical properties.Regardless of heat treatment or not,the addition of polyester microfibers promotes the decomposition of natural organic matter in the soil.