The association of microplastics with water-stable aggregates formed under controlled conditions

Water-stable aggregates (WSAs) are a key indicator of soil structural stability and influence water erosion potential. Microplastics (MPs) have been shown to impact WSA formation and their association with WSAs may strongly influence their transport in soils. However, the effect of MPs on WSA formation and their simultaneous association with WSAs has not yet been addressed. This study assessed how MP polymer composition [polylactic acid (PLA) and polyethylene terephthalate (PET)], size (<63 µm and >250 µm) and concentration (0.2, 0.5, and 1 wt%) as well as soil texture (loam and loamy sand) affect WSA formation and MPs-WSA association over a two-week laboratory incubation. The formation of WSAs was decreased most (i.e., 10-13%) by small PLA fragments at low to medium concentrations. Other treatments showed weak or no effects. Across all incubations, MPs were predominantly associated with WSAs (>65%), whereas large PLA fragments at low and small PLA fragments at high spiking concentrations showed comparatively less association. Collectively, MP polymer type, size, and concentration in soil impacted WSA formation and their association with WSAs, implying that the mutual interactions of MP presence and aggregation processes in soils have the potential to affect aggregate stability and particularly MP transport.