Similar impact of biodegradable and conventional microplastics on soil hydraulic properties, aggregate stability, and splash erosion

• Impacts of polymer types and concentration explored. • Rainfall simulation and photogrammetry were used for splash erosion. • Effects observed at ≥ 3% (w/w) microplastic concentration. • Reduced hydraulic conductivity but increased water retention. • Splash erosion increased with microplastics abundance. Microplastics, both biodegradable and conventional, are becoming ubiquitous in terrestrial ecosystems, posing potential risks to soil health and ecosystem functioning. Although biodegradable polymers are promoted as environmentally friendly alternatives to persistent plastics such as polyethylene, their effects on critical soil physical properties and processes remain poorly understood, creating a major knowledge gap. We conducted a controlled experimental study by incorporating two types of microplastics, polybutylene adipate terephthalate (PBAT, biodegradable) and low-density polyethylene (LDPE, conventional), into soil at five concentrations (0.1, 0.5, 1.0, 3.0, and 5.0% of w/w). The study used a size mixture to reflect environmentally relevant microplastic distributions: 4000–2000 µm (4.25%), 2000–650 µm (8%), 650–400 µm (8%), 400–250 µm (15%), 250–100 µm (26%), 100–50 µm (21%), and < 50 µm (17.75%). We then employed a mechanistic framework to evaluate the effects of microplastics on key soil physical processes, such as aggregate stability, and their influence on splash erosion dynamics (particle detachment, soil consolidation, and surface roughness), encompassing the effects on soil hydraulic properties (water repellency, hydraulic conductivity, and water retention). Results of the linear regression analysis showed a negative relationship with saturated hydraulic conductivity ( K s ) and a positive relationship with soil water retention (SWR) at matric potential (hPa), water-stable aggregate (WSA), and splash erosion under simulated rainfall, with increasing microplastics concentrations. Furthermore, increasing microplastic concentration did not affect soil water repellency, bulk density, or surface roughness as characterized by photogrammetry. Moreover, a monotonic response of microplastic concentration on soil properties was observed: WSA increased by up to 58% and 53%, splash erosion by 39% and 44%, and SWR by up to 30%, while K s decreased by 44% and 53% for conventional and biodegradable microplastics, respectively, compared to the plastic-free control. PBAT, a biodegradable microplastic, and LDPE, a conventional microplastic, showed comparable effects on soil physical properties, challenging the presumed functional divergence between them. The study also established concentration thresholds above which microplastics begin to alter soil functions in the tested soil type.