Plastic mulching and crop type govern the vertical distribution and composition of microplastics in a major...

Microplastics (MPs) are emerging pollutants threatening soil ecosystems and food security. Global research on agricultural soil MPs shows distinct geographic clustering, primarily in Asia, Europe, and North America. Against this backdrop, the Hetao Irrigation Area, Asia's largest single-source irrigation system and a key grain-producing region, is facing severe agricultural plastic pollution. However, the distribution and influencing factors of soil MPs remain unclear. In this study, we used an Agilent 8700 Laser Direct Infrared (LDIR) Chemical Imaging System to analyze the vertical distribution, abundance, and characteristics of MPs in these agricultural soils. We observed that MP abundance increased with soil depth. Polypropylene (PP) and polyamide (PA) were the predominant polymers, accounting for 43.76% and 31.58% of the particles, respectively. The PP content specifically increased with increasing depth. While particles primarily ranged from 10 to 500 μm, smaller particles (10-50 μm) were the most prevalent (75.6%-80.4%), although their proportion slightly decreased with soil depth. Mulching duration significantly influenced MP abundance, which peaked in soils mulched for 30 years (31,489 ± 1426 particles/kg) and generally increased before decreasing over longer periods. Abundance was also closely related to crop type, with wheat fields showing the highest levels (26,133 ± 1348 particles/kg), followed by corn, sunflower, and green pepper fields. These findings quantify widespread MP contamination in a critical agricultural region and identify key farming practices influencing accumulation. This highlights the pressing need for additional research on the long-term effects of MPs on soil health and crop productivity.