| Plastic film mulching can improve the yield of crops,and has a great contribution to global food security.However,plastic film mulching would cause a large amount of macro-and micro-plastic residues,which not only pollute rural ecological environment,but also do harm to agricultural production and sustainable development.The replacement by biodegradable plastic mulch is expected to solve the plastic pollution problem from traditional mulched film.However,the amount and distribution of macro-and micro-plastic in soil after long-term mulching with traditional plastic film are still unclear.In addition,it is not clear whether biodegradable mulching produces microplastics.To this end,the following two experiments are carried on:Experiment I was done in 2019 from the 32-year experimental platform of plastic film mulching and fertilization（established in 1987）at Shenyang Agricultural University,China.Four treatments were selected including:film mulching with nitrogen application(135 kg N ha-1 yr-1),film mulching without nitrogen application,no film mulching with nitrogen application(135 kg N ha-1 yr-1),no film mulching without nitrogen application.The weight and quantity（number of pieces）of macroplastics（>5 mm）in 0～20 cm soil layer,and the quantity of microplastics in 0～100 cm soil profile were measured.Experiment II was based on the one-year mulching of biodegradable plastic film site established in 2018 in Haicheng,Liaoning Province（only mulched in 2018,no more thereafter）.Biodegradable plastic films included two colors as transparent and black,and were both composed of a mixture of PBAT and PLA.Soils under different mulches were collected in spring and autumn 2019,2020 and 2021 to analyze the amounts of macro-and micro-plastics derived from biodegradable plastic mulches.The results are as follows:（1）Not all microplastics in soil after 32 years of plastic mulching are produced by plastic mulching.The types of microplastics included film,fiber and granular were found in soil covered with traditional plastic film for 32 years.Only film microplastic is derived from the mulched film,as its chemical composition is polyethylene.Plastic mulches contributed 33%～56%to the total microplastics in 0～100 cm depth.（2）Microplastics in soil can migrate down to deep soil.The total number of microplastics in the 0～10 cm topsoil ranged from 7183 to 10586 particles/kg soil,with an average of 8885particles/kg soil.In the deep subsoil（80～100 cm）the abundance of micro-plastic ranged as2268～3529 particles/kg,with an average of 2899 particles/kg.（3）Nitrogen fertilizer application can increase the macro-and micro-plastic residues produced by traditional plastic mulching.After 32-year traditional film mulching,the abundance of macroplastic in the fertilized plots（6796 pieces/m2）was as 10 times as that in the non-fertilized plots（653 pieces/m2）,and film microplastics in the fertilized plots（3.7×106particles/m2）was as about 1.7 folder as that in the non-fertilized plots（2.2×106 particles/m2）.（4）Biodegradable mulch also produces microplastics,but their persistence may be shorter.In the third year after biodegradable film mulching,macroplastic disappeared.However,biodegradable mulching film produced microplastics,which reached the highest value（350～567 particles/kg）in spring 2021（3 years after mulching）,and then dropped to 175～250particles/kg in autumn 2021（3.5 years after mulching）.That means microplastics from biodegradable mulch would only last for a few years in the soil.（5）H2O-Zn Cl2 density flotation is an ideal method for the extraction of biodegradable plastic film microplastics.The present method of non-biodegradable microplastic extraction is not suitable for biodegradable film derived microplastics,as the usually used oxidizer（H2O2 or Na OH）to remove soil organic matter,can also oxidize biodegradable film derived microplastic.Our study provides a new method to extract film derived microplastic,which use the density method to remove soil organic matter（because the density of biodegradable film is higher than particulate soil organic matter）,rather than use any common oxidants.The final result of microplastic counting is stable（coefficient of variation is less than 30%）,and has a good recovery,thus is a stable and reliable method for biodegradable film derived microplastic.This study shows that the long-term use of plastic film not only causes serious pollution to the surface soil,but also migrates microplastics into the soil at a depth of 1 m.When plastic moves deeper into the soil,it makes removal and repair more difficult.Biodegradable mulching film can also produce microplastics in soil.According to the current results,the retention time of biodegradable film derived microplastic is like relatively short（maybe several years）in soil,which can be further popularized and applied in the future. |
