Impact of plastic mulching on microplastic contamination in mountainous agricultural soils

Microplastics (MPs) have extensively contaminated both aquatic and terrestrial ecosystems. However, research on their distribution and impacts in soil remains limited, particularly in remote agriculture landscapes. This study reports on the influence of plastic mulch on MP contamination in remote mountainous agricultural soils of Kakani, Nepal. Soil samples were collected from two depths (0–15 cm and 15–30 cm) across plastic-mulched farms, non-mulched farms, and adjacent forests. MPs were extracted through density separation and digestion, quantified under a stereomicroscope, and characterized using Fourier Transform Infrared (FTIR) Spectroscopy. The spike recovery experiments had a recovery rate of 70% (n=10). Results showed a significantly higher MP accumulation in plastic-mulched soils (average=577 particles/kg), followed by non-mulched soils (average=393 particles/kg) and forest soils (80 particles/kg) (p < 0.05). MPs were predominantly small (100 – 500 µm) and fragment-shaped, with notable vertical movement into deeper soil layers. The MP counts in surface soil (0-15cm) were significantly higher than in deep soils samples (15-30 cm) in all three land use types (p < 0.05). The presence of MPs in non-mulched and forest soils suggests multiple contamination sources, including atmospheric deposition and agricultural inputs. No significant correlation was found between MP accumulation and soil organic matter or pH, highlighting the complexity of MP-soil interactions. These findings emphasize the role of agricultural practices in MP contamination and underscore the urgent need for further research on the long-term ecological and agronomic impacts of MPs in soil environment.