Study On Soil Microplastics Migration In Mulching Farmlands In Bashang Region,Hebei Province

Microplastics(<0.5 mm)(MPs)are extremely prone to fragmentation,migration,and accumulation in the environment due to their light,small,and easily degradable properties,resulting in MPs pollution spreading throughout geographical environments such as the atmosphere,hydrosphere,pedosphere,and even the biosphere.Some external forces,such as wind and water,are the main driving force for the long-distance and transzonal migration of MPs.The farmland covered with plastic film is widely distributed in agricultural and pastoral ecotone in northern China.The adjacent distribution characteristics of farmland and grassland are obvious,and plastic waste from other agricultural sources superimposed on plastic film debris has caused serious MPs pollution in the surface soil.In spring,when it is dry and windy,strong wind erosion events are very likely to occur in the northern agricultural and pastoral ecotone.Under the effect of wind,MPs in the soil are easy to disperse in the wind,which leads to MPs pollution in the atmosphere and water and soil resources in the downwind area.However,the migration characteristics and laws of MPs driven by wind erosion are still unclear.In this study,we selected Kangbao County of Hebei bashang region as the study area,and collected MPs contaminated surface soil samples from 23 typical mulched farmland sample sites and 8 "upwind mulched farmland-downwind grassland" sample strips on the basis of field survey.The MPs characteristics in dry soil aggregates were revealed and the MPs characteristics of different transport distances to downwind grass were analyzed.The main research results are as follows:(1)It was found that part of the MPs in the topsoil were bound to the dry soil aggregate,defined as the "bound state",and part of the aggregates were dispersed in the soil,defined as the "free state".About 35% of the MPs are bound to the soil aggregate in the "bound state" and about 65% of the MPs are dispersed in the soil in the "free state".If the dry soil aggregate ability is less stable,some of the MPs in the loose soil will change from the "bound" state to the "free" state,and will then be more easily released and transported by the wind.(2)For the MPs characteristics of dry soil aggregate in the surface soil of the mulched farmland,the fiber(non-fiber)was found in seven different dry aggregate size(<0.106,0.66-0.25,0.25-0.5,0.5-0.85,0.85-2,2-5,5-20 mm)were 78.17%(21.83%),77.13%(22.87%),84.97%(15.03%),84.57%(15.43%),respectively.87.95%(12.05%),79.64%(20.36%),91.15%(8.85%).The abundance of fiber and non-fiber decreased with the increase of soil aggregate size.Fibers and non-fibers abundances decreased exponentially with increasing particle size of soil aggregates.(3)The abundance of fibers approximately less than 2.5 mm in size and non-fibers less than 0.5 mm in size linearly decreased exponentially with increasing dry aggregate size(Mann-Kendall test,P<0.05).In this study,the Logistic model best depicted the MP size distribution.This indicated that the MP abundances generally increased slowly with small sizes(less than about 0.1 mm)and then increased dramatically with median sizes(from about 0.1 to 2 mm).This suggested that the distribution of MPs in dry soil aggregate is related to their size and shape.(4)The average abundance of MPs presented a decreasing trend with increasing downwind distance “upwind mulch farmland and downwind grassland” transect(Mann-Kendall test,P<0.01).The abundances of fibers approximately greater than 0.5mm in size and non-fibers greater than 1 mm in size decreased exponentially with increasing downwind distance(Mann-Kendall test,P<0.05).(5)Research found that the MPs size distributions at different distances showed similar sigmoid trends described by Logistic models.The MPs fiber size was concentrated from 0.5 to 2 mm,while most of the MPs non-fiber sizes were less than1 mm.Interestingly,MPs fiber size fraction in the farmland between 0.5 mm and 2mm was higher than that in the grassland.In contrast,MPs non-fibers’ size(<1 mm)fraction in the farmland was less than that in the grassland.The size of transported MPs at the short-distance was larger than that of long-distance dispersal.(6)MPs accumulate in downwind grasslands and have a potential impact on the ecological environment.MPs transported to the plant surface are easily eaten by animals.MPs deposited into the soil will potentially affect the soil microenvironment,when wind erosion occurs,MPs enriched to the downwind grassland will rise again and transport to the downwind area,becoming a secondary source of pollution.