Rapid quantification of fluorescent micro- and nanoplastics (≤2 μm) in soil

Micro- and nanoplastics (MNP) pose an emerging threat to soil ecosystems, with particular concern for small MNP (≤10 µm). Although small fluorescent MNP are widely used to track MNP distribution and transport in aquatic environments and organisms, analyses of small fluorescent MNP in soil remain qualitative. Here, we present the first direct quantification approach for MNP ≤2 μm in soil using fluorescence microscopy, with recoveries calculated from fluorescence area ratios between MNP in filtered soil suspensions and pure water. Direct detection yielded rapid and reliable quantification with recovery rates (49.1–53.1%) comparable to magnetic separation (60.4–61.7%) and about 7 times higher than conventional separation (6.8–7.5%). We established a robust calibration line (R² = 0.988) converting fluorescence area ratios to MP concentrations across a wide range (0.01–1000 mg/kg) in loamy sand, with systematic underestimation corrected through linear calibration. Under more realistic conditions (MP mixed with larger amounts of soil and incubated for 15 d), quantification yielded an average symmetric mean absolute percentage error ranging from 15% to 39%. The calibration curve was applicable across diverse soil types (R²= 0.979-0.998) and MNP sizes down to 500 nm (R²=0.961). This direct detection method provides a highly efficient, robust tool to advance mechanistic understanding of MNP behaviour and environmental hazards in soils.