Microplastic accumulation in agricultural soils: Source apportionment and impact on soil microbial community structure

Agricultural soils have become significant reservoirs for microplastic contamination, yet the sources and ecological consequences remain poorly characterized. This research investigated microplastic accumulation patterns in intensively cultivated agricultural soils, identified primary contamination sources through polymer fingerprinting, and assessed impacts on soil microbial community structure. Soil samples were collected across a depth gradient from surface to 30 cm in fields with documented histories of plastic mulch usage, irrigation with treated wastewater, and organic amendment application. Microplastic extraction employed density flotation with zinc chloride solution, followed by visual identification and Fourier-transform infrared spectroscopy for polymer characterization. Agricultural soils contained microplastic concentrations averaging 847.3 particles/kg in surface layers, declining exponentially with depth to 98.3 particles/kg at 25-30 cm. Source apportionment attributed 36.8% of microplastics to plastic mulch fragmentation, 22.1% to irrigation water, 18.4% to organic amendments, and 10.5% to atmospheric deposition. Polyethylene and polypropylene dominated the polymer composition at 67.3% combined. Microbial community analysis by 16S rRNA gene sequencing revealed significant diversity reductions in highly contaminated soils, with Shannon index declining from 4.82 to 3.42 across the contamination gradient. These findings establish agricultural practices as major microplastic sources while documenting adverse effects on soil biological functioning.