Accumulation and migration of microplastics and its influencing factors in coastal saline-alkali soils amended with sewage sludge.

Coastal saline-alkali soil can be transformed to agricultural soil with sewage sludge amendment. However, sewage sludge contains a large number of microplastics (MPs), and the fate of MPs in sludge-treated saline-alkali soil needs to be studied. Therefore, we investigated the accumulation and migration of MPs, and their influencing factors in saline-alkali soil after one-time sewage sludge application (0, 25, 50, 100 and 200 t ha SSA). The results indicated that sewage sludge input contributed to MP accumulation in soil, and the MP abundance in 20-40 cm soil was significantly lower than that in 0-20 cm soil. Fragments and fibers were the most abundant MPs in soil, and the proportions of fragments and 50-200 µm MPs in 20-40 cm soil were lower than those in 0-20 cm soil, while the < 50 µm MP proportion was higher than that in 0-20 cm soil. Correlation analysis showed that MP accumulation rate (0-40 cm) and migration rate (20-40 cm) were negatively correlated with soil organic matter (SOM) content and SSA, but positively correlated with soil pH. Stepwise regression analysis further showed that SOM and SSA were the main factors affecting MP accumulation rate, which explained 47.7% and 46% of its variation, respectively, while pH was the crucial factor affecting the migration rate of MPs, followed by EC and SSA. In conclusion, SSA caused MP accumulation in saline-alkali soil, and SSA primarily affected the MP abundance, while soil OM, pH and EC directly affected MP migration in soil.