Microplastics In Different Types Of Agricultural Land Soils:Distribution,Sources,and Control Measures

In recent years,soil microplastic pollution has received widespread global attention.Due to their small size and large quantity,microplastics（MPs）are highly susceptible to fragmentation,migration and accumulation in the environment and have the ability to carry other contaminants.Once MPs enter agricultural soils,the agroecosystem may be seriously threatened,with implications for food security and human health.Previous studies had shown that the development of modern agricultural systems and intensive human activities have resulted in widespread soil microplastic contamination of agricultural land.It has been confirmed by domestic and international studies that the distribution of MPs in different types of agricultural soils varies according to a number of factors.At present,such studies are almost always limited to field surveys and tests in small areas,and data analysis and systematic assessment of soil microplastic pollution characteristics on a national scale are still lacking.In addition,few studies had been reported on the specific sources of MPs in different types of agricultural soils.This had affected the establishment and implementation of concrete and feasible countermeasures for MPs prevention and control.Considering the large differences in the agronomic practices and human activities of different agricultural land types soils,this paper classified agricultural land into five categories:vegetable land,garden land,grain field,forest land and grassland according to the different types of crop cultivation.A large scale database of MPs pollution distribution was established nationwide,and a Meta-analysis method model was used to analyze the MPs pollution in various types of agricultural land soils in China.We also explored the distribution,pollution characteristics and sources of MPs in the soil of each agricultural land by investigating the actual soil microplastic residues in typical areas.Based on the above study,two-dimensional indicators for MPs pollution source analysis and control priority evaluation were established to clarify the specific control measures and control priorities of MPs in various types of agricultural land soils,with a view to providing important data support and practical basis for the prevention and control of MPs pollution in agricultural land soils in China.The main contents and conclusions of this study were as follows:（1）Significant differences existed in the distribution of MPs in soils from different agricultural lands across the country.Meta-analysis determined that the general pattern of relationship between agricultural land use practices and MPs abundance was:vegetable field>orchard>cropland>grassland（woodland data showed high heterogeneity and were not discussed in this paper）.Meanwhile,the subgroup analysis examined the effects of agricultural production practices,anthropogenic factors and geospatial factors.Results indicated that agricultural mulching,PM_2.5concentration,carbon emissions,altitude and latitude might all contribute to the enrichment of MPs in various types of soils.Agricultural film mulching significantly increased soil microplastic abundance,most significantly in orchards;changes in carbon emissions and PM_2.5concentrations were consistent with trends in MPs abundance;altitude and latitude influenced the soil distribution of MPs,particularly at mid-altitude（100-2000 m）and high latitude（40-50°）,but there were no significant east-west differences.（2）Significant differences were found in the distribution and contamination characteristics of MPs in different agricultural land types in the typical region.All the agricultural land types in the study area had MPs in their soils,with the mean abundance of MPs in vegetable field（6555.19±573.54 items/kg）>cropland（4657.70±656.47 items/kg）>forest land（4186.88±1930.21 items/kg）>orchards（3048.72±543.82 items/kg）.The abundance of soil MPs was significantly higher in vegetable plots than in orchards（p<0.01）and croplands（p<0.05）.Variations in contamination characteristics were reflected in the fact that MPs residues in the vegetable plots were the result of multiple channels and high frequency of exogenous inputs adding to each other;in the orchards and grain fields MPs were mainly introduced by a large number of agricultural necessities;in the woodlands MPs were more homogeneous in character and attributed to the relatively low level of human activities and agricultural practices in the woodlands.（3）Control priority of MPs in different types of agricultural soils and control measures.Based on the survey and testing results,a two-dimensional index of"abundance and MPs diversity"was established to classify MPs pollution in agricultural land in Gansu Province into four levels.The results showed that vegetable fields are the next priority target for control,which requires both policy and technical support,and should be controlled in terms of both breadth and depth.The high MPs diversity of grain fields may pose a threat to food and arable land security,and should be controlled as early as possible.It is necessary to standardize modern agricultural operations and monitor water quality in irrigated areas on a regular basis.The orchard MPs were kept at a low level,so the main task should be to optimize the daily production activities of local fruit farmers and guide them to develop good planting habits.At present,the main focus should be on optimising the daily production activities of local fruit farmers and guiding the development of good planting habits.The pollution of forest land is not obvious,and the control of soil MPs is mainly preventive.Finally,from the overall goal of national agricultural soil microplastics control,and in view of the complex reality of soil microplastics in China,four measures were proposed to strengthen the monitoring and assessment of key enterprises at source,promote the implementation of prevention and control collaborative treatment programs,innovate MPs pollution control technology,and improve the legal and regulatory mechanism of MPs pollution,in order to provide countermeasure ideas for achieving effective prevention and control of MPs pollution in agricultural soil in China.