Sources of Microplastics in the Soil and Use of Laser Direct Infrared Imaging for Microplastic Detection

Abstract Agricultural soils have emerged as critical sinks for microplastics (MPs), with contamination driven mainly by plastic mulch films, sewage sludge, wastewater irrigation, organic fertilizers, and atmospheric deposition. These MPs, ranging from 1 μm to 5 mm, perturb the soil ecosystems by adsorbing heavy metals, organic pollutants, and antibiotic resistance genes while disrupting soil structure, microbial communities, and plant health. Despite their pervasive presence, conventional detection methods—such as FTIR and Raman spectroscopy—remain labour-intensive, making any large-scale assessments challenging. This study underscores the transformative potential of laser direct infrared (LDIR) imaging, a high-throughput technique leveraging quantum cascade lasers and rapid spectral scanning in the 975–1800 cm −1 range to characterize MPs by polymer type, size, and morphology with minimal pretreatment. Global data highlight alarming MP accumulation in sludge-amended soils, retaining up to 3.50 MP g −1 after five years, while mulched farmlands in China exhibit 1.08 MP g −1 after 24 years. Untreated wastewater irrigation further introduces 8–15,620 MP m −3 into soils, exacerbating contamination. LDIR’s efficiency, demonstrated by 80–100% recovery rates for 200–500 μm particles and analysis times of ~3 hours per sample, addresses critical gaps in current protocols. Its integration with machine learning and expanding spectral libraries promises enhanced precision for sub-10 μm particles, enabling scalable monitoring. By advancing detection accuracy and standardisation, LDIR emerges as a pivotal tool for elucidating MP dynamics, guiding policy, and mitigating risks to soil health, biodiversity, and food security in the face of this escalating environmental crisis.