Association of microplastics with water-stable aggregates formed under laboratory conditions

Water-stable soil aggregates (WSAs) are a key to the structural stability of soils. The presence of microplastics (MPs) has been found to affect WSA formation. Additionally, the association of MPs with WSAs may strongly alter the transport behavior of MPs in soils. However, the association of MPs with newly formed WSAs has not yet been investigated as a function of MP and soil properties. In this study, we assessed how polymer composition [polyethylene terephthalate (PET), polylactic acid (PLA)], particle size (small: nominally <63 µm; large: nominally >250 µm), MPs concentration in soil (0.2, 0.5, 1 wt.%) and soil texture (loam, loamy sand) impact WSA formation and MPs-WSA association over a two-week incubation under controlled laboratory conditions. Small PLA fragments reduced WSA formation more strongly and tended to be less associated with WSAs than small PET fragments, potentially due to the slightly greater hydrophobicity of PET. Across all incubations, coarse PLA fragments at 0.2 wt.% showed the largest share of unassociated fragments with approximately 35% of all MPs introduced into the system. The effect of small PLA fragments on WSA formation was concentration-dependent, with reduced aggregation at low and intermediate concentrations but near-control levels at high concentration, despite a higher fraction of unassociated MPs. These non-monotonic effects suggest that MPs affect WSA formation through opposing mechanisms and monotonic concentration–response assumptions may be inappropriate for intra- or extrapolating MP effects on WSA formation. Altering soil texture from loam to loamy sand did not impact the share of unassociated small PET fragments. Collectively, MPs polymer composition, size, and concentration in soil impacted WSA formation and their association with WSAs under the experimental circumstances, showing potential for reduced MPs transport in soils and altered soil structural stability.