Accumulation And Migration Of Microplastics And Its Influencing Factors In Coastal Saline-Alkali Soils Amended With Sewage Sludge

Sewage sludge can be used as an efficient soil amendment to promote the transformation from coastal saline-alkali soil to agricultural soil,which is highly significant to alleviate the shortage of arable land in China and guarantee the national food security.However,the environmental safety in the process of land use of sewage sludge has always been the focus of attention.A large number of studies have focused on heavy metal pollutions during the agricultural use of sewage sludge,while sewage sludge contains a large number of microplastics（MPs）less than 5 mm,and the study on the accumulation and migration of MPs in the process of applying sludge to improve saline-alkali soil is an important support for the safe utilization of sewage sludge.In this study,a field plot experiment was conducted to investigate the accumulation,migration characteristics and influencing factors of MPs in coastal saline-alkali soil after one-time sewage sludge application（SSA）at the rates of 0,25,50,100 and 200t ha^-1.The results showed that:（1）Sewage sludge input can lead to MP accumulation in the Tiaozini coastal saline-alkali soil,and the MP abundance increased with sewage sludge application rates.The MP abundance in 0-20 cm soil was significantly higher than that in 20-40 cm soil.In sludge-treated salinealkali soil,fragments and fibers were the two most common forms of MPs.MPs in 0-20 cm soil were mainly composed of fragments,while those in 20-40 cm soil were mainly composed of fibers,and the fragment proportion in 0-20 cm soil was significantly higher than that in 20-40 cm soil.More than 80%MPs were<200 μm.50-200 μm MPs were predominant in 0-20 cm soil,while<50 μm MPs were predominant in 20-40 cm soil.With increasing sewage sludge application rates,<50 μm MPs in 0-40 cm soil first decreased and then increased.Black was the predominant MP color type.PP and PA were the most common MP composition types in 020 cm and 20-40 cm soil,respectively.MP accumulation rate（0-40 cm）and migration rate（2040 cm）decreased gradually with increasing sewage sludge application rates.（2）Sewage sludge input can lead to MP accumulation in the Bencha coastal saline-alkali soil,and the MP abundance increased with sewage sludge application rates.The MP abundance in 0-20 cm soil was significantly higher than that in 20-40 cm soil.In sludge-treated salinealkali soil,fragments and fibers were the two most common forms of MPs.MPs in 0-40 cm soil were mainly composed of fragments in May,while in October,MPs in 0-20 cm and 20-40 cm soil were mainly composed of fibers and fragments,respectively.In October,the fragment proportion in 0-20 cm soil was significantly lower than that in 20-40 cm soil,while the fiber proportion in 0-20 cm soil was significantly higher than that in 20-40 cm soil.More than 80%MPs were<200 μm.50-200 μm MPs were predominant in 0-20 cm soil,while<50 μm MPs were predominant in 20-40 cm soil.With increasing sewage sludge application rates,<50 μm MPs in 0-40 cm soil first decreased and then increased.Black was the predominant MP color type.PP and PE were the most two common MP composition types.PP was the most common MP composition type in May and PA was the most common MP composition type in October.MP accumulation rate（0-40 cm）and migration rate（20-40 cm）decreased gradually with increasing sewage sludge application rates.（3）Correlation analysis showed that MP accumulation rate was significantly positively correlated with BD,pH and EC,and significantly negatively correlated with SOM and SSA.MP migration rate was significantly positively correlated with pH and negatively correlated with SSA.Furthermore,in 0-20 cm soil,fragment proportion was significantly negatively correlated with BD,pH and EC,and significantly positively correlated with SOM and SSA.Fiber proportion was significantly positively correlated with BD,pH and EC,and significantly negatively correlated with SOM and SSA.Film proportion was significantly negatively correlated with BD and pH,and significantly positively correlated with SSA.（4）Stepwise regression analysis showed that MP accumulation rate in soil was affected not only by SSA,but also by EC,BD,pH and SOM,while MP migration rate was only affected by pH and EC.Furthermore,in 0-20 cm soil,fiber proportion was affected by EC,BD and SOM.Fragment proportion was affected by BD and pH,and film proportion was affected by SSA and EC.