Impact of Plastic Mulching on Microplastic Contamination in Mountainous Agricultural Soils

Abstract

Microplastics (MPs) have extensively contaminated both aquatic and terrestrial ecosystems, yet their distribution and impacts in soil—both a source and a sink for MPs—remain poorly understood, particularly in remote agricultural landscapes. This study investigates the influence of plastic mulch on MP contamination in the mountainous agricultural soils of Kakani, Nepal. Soil samples were collected from plastic-mulched farms, non-mulched farms, and adjacent forests at two depths (0–15 cm and 15–30 cm). MPs were extracted using density separation and digestion, quantified under a stereomicroscope, and characterized through Fourier Transform Infrared (FTIR) Spectroscopy. Spike recovery experiments yielded a 70% recovery rate (n = 10), confirming the reliability of the extraction method. 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 concentration in topsoil (0–15 cm) was significantly higher than in subsoil 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. However, 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.