Vertical profile of low-density microplastics in vegetable cultivation soils in Nuwara Eliya, Sri Lanka

In recent years, growing global attention has focused on microplastic (MP) pollution and its environmental impacts. Various agronomic practices, such as plastic mulching, the use of plastic implements for transporting fertilizers and seedlings, and irrigation with MP-contaminated water, are potential sources of MP accumulation in agricultural soils.  This study addresses the knowledge gap on plastic pollution in Sri Lankan agricultural soils by examining the occurrence, abundance, and vertical distribution of low-density MPs in selected vegetable fields in Nuwara Eliya. The sites, located in urban and semi-urban settings and under continuous cultivation for over a decade, provide valuable insights into long-term MP accumulation and transport within soil profiles. Thirty-six soil samples were collected from three depths (0–10, 10–30, and 30–50 cm) across two farmer-managed vegetable plots. Microplastics (density < 1.00 g/mL) were extracted through density separation following optimized chemical digestion in 100 mL of acidified water containing 5 mL of 30% H₂O₂ at 60–70 °C. MPs were categorized by shape and color under a stereomicroscope, and polymer types were confirmed by ATR-FTIR analysis. Statistical analysis using Minitab 18.0 compared MP concentrations between sites and depths.  Results showed significant site-specific variation (p = 0.007), with the urban site (Site 2) exhibiting higher MP levels (340.0 ± 158.6 particles/kg) than the semi-urban site (246.7 ± 162.8 particles/kg), likely due to proximity to residential areas. MP abundance declined with depth, while particles smaller than 2 mm tended to migrate beyond the plough layer, accounting for 31% and 40% of total MPs at depths > 10 cm in Sites 1 and 2, respectively. Fragments dominated across all depths, though fibers penetrated deeper.  The findings indicate widespread MP contamination in Nuwara Eliya’s vegetable soils, influenced by agricultural inputs and urban activities. Vertical migration of MPs may increase the risk of groundwater contamination, underscoring the need for monitoring and regulatory thresholds to manage microplastic pollution.