Polyester microplastic fibers rearrange soil physical quality indicators without compromising hydraulic function in six Mediterranean soils: Insights from S-theory indicators

Microplastics contamination in agricultural soils represents an emerging threat to soil health and ecosystem functioning. This study investigated the effects of polyester (PES) microplastic (MP) fibers on soil physical quality (SPQ) using indicators derived from soil water retention curve (SWRC) inflection point, based on Dexter's S-theory. Six soils with different textures were contaminated with PES fibers at concentrations of 0.25 %, 0.5 %, and 1 % (w/w) and incubated for about six months. Four SWRC models (van Genuchten with Mualem constraint, VGM; van Genuchten with Burdine constraint, VGB; van Genuchten unconstrained, VGN; and Kosugi, KSG) were fitted to experimental water retention data. The VGN and VGM models provided the best fitting accuracy across soil types. Overall, MP contamination altered key SPQ indicators determining: a decrement of the pressure head at inflection point (h*) and of effective porosity (Φ*), indicating larger modal pore diameters; an increment of the slope at inflection point (S*), suggesting enhanced soil aggregation. Effects were soil type and concentration dependent, with changes primarily occurring at 1 % MP concentration. Soils with moderate clay content (clay < 30 %) showed improved S* values, while clay rich soils showed minimal response. Capacitive indicators (air capacity and plant available water capacity) remained largely unaffected, suggesting preserved total porosity despite internal pore structure modifications. These findings demonstrate that PES MP fiber contamination can alter soil pore architecture and aggregation without substantially impacting bulk hydraulic properties, highlighting the complexity of MP-soil interactions and the value of inflection point indicators for detecting subtle changes in soil physical quality.