Distribution And Accumulation Of Soil Microplastics In Typical Small Watersheds In Hehuang Valley And Its Influencing Factors

Microplastics generally refer to various plastic particles with a diameter of less than 5mm.In recent years,as a emerging pollutant,it has become a widely studied hot spot.Due to its characteristics of small size,persistence,easy migration,and pollution enrichment,microplastics are easily migrated and accumulated in various environmental media,causing environmental impacts.Its migration and accumulation characteristics are also affected by spatial environments.The Hehuang Valley region is an important agricultural area in Qinghai Province.With the development of the economy,the land use in the area has changed greatly.However,little is known about the distribution and migration and accumulation of microplastics in the typical small watersheds with different proportions of land use.Therefore,this study selected four dam-controlled small watersheds（Haizigou,Dongsanxiang,Li’erpu,and Hongyazi）within Huazhong,Huzhu,and Minhe counties of the Hehuang Valley basin with different proportions of land use as research objects.The spatial distribution and morphology of microplastics in different land use types of soil and sedimentary soil were studied using density separation,hydrogen peroxide digestion,electron microscopy observation,and spectral analysis,in order to clarify the distribution characteristics of microplastics in farmland,grassland,forest,and barren land in the watershed,the influence of the characteristics of microplastic distribution in deposited soils,and the relationship between soil basic physicochemical indicators and microplastics abundance and morphological composition.The main results of the study are as follows:（1）There are differences in soil physical and chemical indicators among typical small watersheds with different land use in the Hehuang Valley area:the average grassland water content（12.25%）,p H value（8.52）,and organic carbon content(9.07 g·kg^-1)are relatively high,while the average conductivity value of bare land is high(12.43 ms·cm^-1),and the average content of available phosphorus(2.24 mg·kg^-1),nitrate nitrogen(108.71 mg·kg^-1),and ammonia nitrogen(37.36 mg·kg^-1)in cultivated land soil is relatively high.As the depth of soil layer increases from 0-20 cm,20-40 cm,to 40-60 cm,soil p H increases,while the contents of organic carbon,available phosphorus,and nitrate nitrogen decrease.In sedimentary soils in different layers of dammed land,the basic physical and chemical properties of the soil show a fluctuating trend.Due to the different characteristics of the watershed,there are also differences between soils of the same type.The electrical conductivity of Hongyazi small watershed soil is higher than that of other small watersheds,and the p H,organic carbon,total phosphorus,nitrate nitrogen,and ammonia nitrogen contents are relatively small compared to other small watersheds.In dammed soil of different small watersheds,the p H,conductivity,and nitrate nitrogen content are similar to those in different land use types within the watershed,but there is no clear regularity in other indicators’changes.（2）The average abundance of soil microplastics in four typical small watersheds is as follows:Dongshan Township(137.0 n·kg^-1)>Haizigou(95.7 n·kg^-1)>Lierbao(76.0 n·kg^-1)>Hongyazi(26.3 n·kg^-1).In Haizigou,Dongshan Township,and Lierbao watersheds,the abundance of different types of microplastics is in the order of cultivated land>grassland>forest land,and the abundance of microplastics is in the order of 0-20 cm>20-40 cm>40-60cm at different depths.The average abundance of microplastics in grassland and bare land in Hongyazi watershed is the smallest,at 27.9 n·kg^-1 and 24.6 n·kg^-1 respectively.The proportion of microplastics less than 0.5 mm in size in cultivated land is higher（59.28%）than that in other land use types,and the proportion of film-like microplastics is in the order of cultivated land>grassland>forest land.Fiber microplastics have the highest proportion（54.19%）in the four soil types,with transparent（45.81%）and black（32.90%）being the main colors.PE is the primary component（65.78%）.（3）The surface abundance of microplastics in sedimentary soils is higher than that in watershed soils.The content of microplastics in different sedimentation layers decreases overall as the depth increases,and the abundance of microplastics in special layers shows a fluctuating distribution.The surface abundance of microplastics in sedimentary soil is influenced by upstream land use,with the highest abundance of microplastics in Lierbao small watershed with residential land use at 519.4 n·kg^-1.The order of surface abundance of microplastics from high to low is Dongshan Township(458.9 n·kg^-1)>Haizigou(311.1 n·kg^-1)>Hongyazi(278.62 n·kg^-1)with the proportion of upstream cultivated land decreasing.Compared to soils in different upstream land use types,transparent,film-like,large-sized,and PP components of microplastics are more abundant in dammed soil,which is affected by the characteristics of the microplastics themselves.（4）There is a certain correlation between the distribution of microplastics and soil basic physical and chemical properties in typical watersheds:organic carbon in soil is significantly positively correlated with microplastic abundance（p<0.05）.In addition,the abundance and number of different characteristic types of microplastics show an overall increasing trend with increasing water content,total phosphorus,and available phosphorus content.The impact of nitrate nitrogen and ammonia nitrogen in soils with different land use on microplastic abundance and morphology is not significant.The correlation between microplastics and soil basic physical and chemical properties in dammed sedimentary soil is different from that in upstream soil.The abundance,size,shape,color,and nitrate nitrogen and ammonia nitrogen are positively correlated.There is no obvious regularity with changes in organic carbon,total phosphorus,and available phosphorus content.The abundance of microplastics in Hongyazi small watershed is negatively correlated with conductivity.This study clarifies the distribution characteristics of microplastics under different land use types in small watersheds and the cumulative characteristics of microplastics in dammed sedimentary soil at different times,and reveals the relationship between soil basic physical and chemical indicators and microplastic abundance and morphology.The research provides basic data and theoretical basis for understanding the distribution characteristics of microplastics in soils with different land use types and studying the migration and accumulation mechanisms of microplastics in small watersheds.