Influence of microplastics on soil aggregate formation: Insights into biological binding agents

Microplastics (MPs) are widespread in agricultural soils and can disrupt soil structure by altering aggregate formation and stability. However, the mechanisms by which MPs influence aggregate formation through aggregate binding agents remain unclear. This study investigated the effects of polyethylene MPs (PE-MPs, 0.40 % w/w) in different shapes (granule, fiber, and film) and aging states (pristine and UV-aged) on aggregate formation and mechanisms in purple soils with two levels of soil organic matter (SOM, 1 % and 3 %). Granule-shaped PE-MPs promoted aggregate formation, whereas fiber- and film-shaped PE-MPs inhibited it. EPS content, positively correlated with aggregate formation, was significantly influenced by PE-MP shape and SOM content. Fiber- and film-shaped PE-MPs significantly reduced EPS content. PE-MPs also significantly altered microbial composition, reducing the abundance of EPS-associated taxa such as Candidatus_Solibacter and Mortierella. Structural equation modeling revealed that PE-MPs can directly affect aggregate formation, and can also indirectly affect aggregate formation by interfering with EPS synthesis and microbial diversity by changing soil microbial biomass and activity. This complex process is mainly regulated by SOM content. These findings elucidated the multi-pathway mechanisms through which PE-MPs impact soil aggregate formation and provides a basis for understanding their broader effects on soil structure.