Effects of the coexistence of polystyrene microplastics and imidacloprid on nitrogen and phosphorus transformation in soil

Microplastics have received increasing attention in soil ecosystems, and their potential impacts on soil properties have raised concerns. Pesticides are the most prevalent pollutants in soil, but their combined effects with microplastics on the soil environment have not been elucidated. In this study, polystyrene microplastics (PS MPs) and imidacloprid (IMI) were added to the soil to investigate their combined effects on soil physicochemical characteristics, nitrogen and phosphorus contents, related transformation activities, and the composition of nitrogen- and phosphorus-transforming microorganisms. The results revealed that the coexistence of PS MPs and IMI led to a significantly higher soil pH level and lower water-stable aggregate (WSA) content. Additionally, it increased the relative abundance of nitrogen- and phosphorus-transforming microorganisms, including ammonia-oxidizing archaea and bacteria, nitrite-oxidizing bacteria, heterotrophic denitrifying bacteria, phosphate-solubilizing bacteria. PS MPs increased the soil potential denitrification rate by 14.53% owing to a significantly higher pH level. However, this promotion disappeared when they combined with IMI. The coexistence of PS MPs and IMI caused a significant decrease in WSA content, thereby improving soil aeration and increasing the relative abundance of phosphate-solubilizing bacteria, which led to a 14.54% and 44.79% increase in soil phosphatase activity and Olsen-P content, respectively. Variance partitioning analysis revealed that the coexistence of PS MPs and IMI mainly influenced nitrogen and phosphorus transformations by altering soil pH and WSA content. These results reveal the combined effects of PS MPs and IMI on soil nitrogen and phosphorus transformations and elucidate soil environmental risks associated with microplastics and pesticides.