Characteristics And Ecological Effects Of Soil Microplastic In Typical Agricultural Region With Plastic Film Mulching In China

Microplastics have often been defined as particles smaller than 5 mm in size,which are now recognized as an important environmental pollutant,being almost ubiquitous in the atmosphere,water,soil,and other environmental media.Due to their small particle size and very slow biodegradation rate they can be easily absorbed by organisms and subsequently transported through food webs.Microplastics are emerging pollutants that have been extensively detected in aquatic ecosystems,especially oceans.However,there is a knowledge gap regarding microplastic pollution in agricultural soils and terrestrial ecosystem.In this paper,according to the main problems of microplastics in agricultural environment,we focused on the following aspects:(i)determine and optimize the economic and effective methods for separating and extracting microplastics from soil;(ii)extract microplastics from soils of typical agricultural regions(Beijing,Shandong and Xinjiang)with plastic film mulching in China,and analysis the distribution characteristics and influencing factors of microplastics in agricultural soils;(iii)study the effects of microplastics on soil microbial community structure and plasticizer by indoor culture experiment and ¹⁴C isotope tracer technique;(iv)explore the function of soil microplastics in terrestrial environment.The main results are as follows:(1)In this study,the suited conditions of different suspension agents,digestion solution,digestion temperature and digestion time was optimized by response surface analysis;and ultimately three economic and efficient soil microplastics separation methods were determined.Three methods are respectively Rapid and economical separation method:Considering the time cost and economic benefit,the most rapid and economical separation method is to use SC1(1.18 g cm^-3 Na Cl solution)suspend the floating matter and rinse it with deionized water,then digestion using 10%Na OH solution in a water bath at 50?for 6 h;High precision separation method:The most efficient separation method is to suspend with SC3(1.53 g cm^-3 Zn Cl₂ solution),wash with SC2(1.27 g cm^-3 Zn Cl₂ solution),and digest with 30%H₂O₂ solution for 10 h in a water bath at 61?;Comprehensive balanced separation method:Overall consideration of the time cost,economic benefit and environmental risks,the optimal experimental conditions are using SC1(1.18 g cm^-3 Na Cl solution)to suspend,deionized water to rinse,and then digestion with DS1(30%H₂O₂ solution)in a water bath at 50?for 15 h.(2)The abundance,morphological characteristics and composition of microplastics in soils were various in typical agricultural areas in China(Beijing-suburban areas,Shandong-intensive farming areas,Xinjiang-long-term planting areas).Overall,the abundance of microplastics in Shandong was48298±22170 N kg^-1,significantly higher than that in Xinjiang(34343±12584 N kg^-1)and Beijing(18211±5050 N kg^-1)under greenhouse cultivation mode.In conventional mulched soil of Beijing,Shandong and Xinjiang,the abundance of microplastics were 18329±4963 N kg^-1,34878±14685 N kg^-1and 3195±7796 N kg^-1,respectively,and the differences among them were significant(P<0.05).Microplastic particle size in agricultural soil ranged from 50.02?m to 3345.57?m,mainly consisting of films or particles with small particle size.The distribution of microplastics with different particle size decreased with the increase of particle size.PE,PP,PA and UF were the main components of microplastics,accounting for 50.00%,19.72%,13.90%and 13.90%,respectively.Under greenhouse cultivation mode,PP was the main microplastics type,accounting for 36.99%;while PE dominated in conventional soil,accounting for 63.36%.Moreover,PE was the main type of microplastics in the soils of Beijing,Shandong and Xinjiang,accounting for 53.46%,47.09%and 48.19%,respectively.(3)The types and addition amount of microplastic and their interactions significantly affected soil physicochemical properties(p H value,EC,TN,TC,SOC,MBC)and the Alpha diversity of soil bacteria and fungi(P<0.05).The range of soil physicochemical properties were as follows:p H value of 8.38?9.16;EC of 120.60?319.80?s cm^-1;TN of 0.97?1.52 g kg^-1;TC content of 21.81?107.22 g kg^-1;SOC of12.61?97.40 g kg^-1;MBC of 95.04 of 2579.24 mg kg^-1,respectively.In general,the addition of different microplastics(PE,PVC,PBS,PHA,PBAT and PPC)tended to increase the contents of TC,SOC and MBC in soil,and the effect of degradable microplastics(PBS,PHA,PBAT and PPC)on the contents of TC,SOC and MBC in soil was greater than that of PE microplastic.(4)The community structure of soil bacteria and fungi was significantly different under different microplastics types and addition amount.The OTUs sequences of soil bacteria were classified into 14phyla,27 classes,44 orders,48 families and 48 genera.The OTUs sequences of soil fungi were classified into 5 phyla,7 classes,12 orders,16 families and 15 genera.The microbial community in soil amended with degradable microplastics(PBS,PHA,PBAT and PPC)was relatively concentrated,which was significantly different from that with non-degradable microplastics(PE and PVC).Compared with PBS,PHA and PPC,three biodegradable microplastics,the alpha diversity index of soil bacteria and fungi was significantly higher when PBAT and non-biodegradable microplastics(PE and PVC)were added;At phylum level,microplastics reduced the abundance and alpha diversity of dominant phylum.Soil TN,p H and MBC were vital factors affecting the alpha diversity of soil microbial community under different microplastics addition conditions.(5)The addition of PE microplastics inhibited the conversion of DEHP to CO₂ in soil,but promoted the conversion of DBP to CO₂ in soil.Under DEHP addition,¹⁴C-DEHP not used by microorganisms was more than ¹⁴C-MBC with PE and PVC addition.In addition to DBP,¹⁴C-DEHP under PVC treatment was more than ¹⁴C-DEHP under PE treatment,while microbial utilization was more.Compared with DEHP,DBP with small molecular weight degraded relatively quickly and was basically stable after 14 days.The degradation of DEHP and DBP in soil is closely related to soil type,microplastic type,plasticizer addition amount and their interaction.Plasticizer type(DBP and DEHP),Soil C,PE and PAEs concentration were important factors affecting the transformation of exogenous DEHP and DBP in soil,accounting for 27.80%of the total variables(P<0.05),27.80%(P<0.05),7.00%(P<0.05),4.80%(P<0.05)and 3.20%(P<0.05).