Sample Collection, Preparation, and Identification of Microplastics in Soil: Issues and Recommendations

Abstract The omnipresence of microplastics (MPs) in agricultural soils, driven by practices such as compost and fertilizer addition, sewage sludge application, plastic mulching, and wastewater irrigation, is the main input of MPs to soil and poses significant risks to soil health and food safety. The paper evaluates methodologies from 39 global studies to identify gaps and inconsistencies in MP sampling, preparation, and identification protocols. Key findings reveal diverse sampling strategies, with composite sampling across depths (0–30 cm) and grid/quadrant approaches recommended to account for soil heterogeneity. Pre-treatment methods, including organic matter removal via 30% H₂O₂ or enzymatic digestion, and density separation using NaCl or NaI, vary widely, impacting MP recovery rates. Identification techniques predominantly combine microscopy with spectroscopic methods (FTIR, Raman), though challenges persist in false positives, polymer degradation, and nanoparticle detection. Methodological disparities in reporting units (e.g., MP/kg vs. mg/kg) hinder data comparability, complicating risk assessments and policy formulation. Quality assurance measures, such as procedural blanks, contamination controls, and spiked recoveries, are inconsistently applied, underscoring the need for standardised protocols. Recommendations emphasise harmonised sampling designs, optimised extraction workflows, and integration of advanced techniques like laser direct infrared spectroscopy for accurate quantification. Establishing global standards is imperative to generate reliable datasets, inform regulatory frameworks, and mitigate agricultural plastic pollution effectively.