Tracking biodegradable microplastics: Mobility in agricultural soils under different environmental conditions

Biodegradable mulching films have been developed as an alternative to conventional agricultural plastics; however, concerns persist regarding the microplastics (MPs) generated during their degradation. This study investigates the mobility and fate of polybutylene adipate terephthalate (PBAT) fragments derived from biodegradable mulching films in surface runoff and infiltration under two scenarios: bare and cultivated soil. Six experimental units with soil amended with PBAT fragments (0.5 wt% in the upper 4 cm of soil) were used. Barley (Hordeum vulgare) was cultivated in three of the units. Irrigation and simulated rainfalls were applied, and natural rainfall was monitored. Runoff and infiltration water were collected over a 5-month period, and soil samples were analysed after harvesting. Results show that most biodegradable MPs remained in the topsoil. Cultivation enhanced MP retention, with 70% of particles found in the 0-5 cm layer and 29% in the 5-10 cm layer, compared with 50% and 7%, respectively, for bare soils. This suggests that vegetation reduces MP losses via runoff while promoting their movement towards deeper soil layers. Indeed, MP mobilization by runoff was very low in bare soils (1.6%), and cultivation further limited surface transport (0.02%). Less than 0.001% of MPs infiltrated through the unsaturated zone. The type of water-input event strongly influenced MP mobilization: high-intensity rainfall increased runoff-driven transport in bare soils, while the volume of infiltrating water conditions increased the penetration of MPs to deeper soil layers. Overall, this study provides critical insights into the behavior of biodegradable plastics in soil ecosystems.