Automated identification and quantification of invisible microplastics in agricultural soils

Microplastics in agricultural soils have become the research hotspot in recent years, however, the quantitative methods based on the traditional visual inspection may have a high false detection rate. Here we combined the laser direct infrared (LDIR) and Fourier-transform infrared (FTIR) methods to investigate the microplastics in farmland with long-term agricultural activities. The results showed that the total abundance of microplastics reached 1.98 ± 0.41 × 10⁵, 1.57 ± 0.28 × 10⁵, 1.78 ± 0.27 × 10⁵, and 3.20 ± 0.41 × 10⁵ particles/kg soil in cotton fields with film mulching of 5, 10, 20, and >30 years, respectively. LDIR results indicated that microplastics ranging from 10 to 500 μm accounted for 96.5-99.9 % of the total microplastic amounts in the soils. Additionally, a total of 26 polymer types of microplastics were detected, among which polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyamide (PA), and polytetrafluoroethylene (PTFE) were dominantly observed. For the microplastics detected by FTIR (500 μm-5 mm), PE polymer was majorly observed (88.0-98.9 %). Most microplastics were films (88.2 %), while fibers and pellets were also found. The reclaimed water from sewage treatment plants, the drip irrigation utilities, and the residual plastic film are the potential sources of microplastics in the farmland soils. By using the automated quantitative and identifiable approaches, this study suggested that the commonly used visual counting method may underestimate the microplastic contamination in agricultural soils.