Vertical distribution and multi-source pathways of microplastics in agricultural soils: A study of typical irrigation areas in the upper Yellow River basin

Agricultural production inputs, irrigation water and atmospheric deposition are the primary sources of microplastics (MPs) in farmland soils. Irrigation exacerbates the vertical migration of MPs from surface soils to deeper aquifer, posing significant ecological and healthy risks to groundwater. However, most studies focused upon surface soils, with limited effort on the spatial distribution and influencing factors of MPs. This study compared forest and grassland soils in the upper Yellow River region, analyzed the vertical distribution of MPs across six soil layers (0-100 cm) under border and drip irrigation patterns. Additionally, the specific composition of MPs in atmospheric source in 1-year settling period, irrigation water, fertilizers, and seed coatings was investigated. The results showed that MPs were detected in all 0-100 cm soil layers, with significantly higher abundance in farmland soils (5688.8 ± 609.38 items/kg) than in forest (2387.05 ± 152.47 items/kg) and grassland (669.64 ± 131.83 items/kg) soils. The MPs abundance in farmland increased with prolonged plastic film mulching. Overall, MPs abundance decreased with soil depth, but accumulation occurred at 20-30 cm due to tillage and irrigation. The MPs abundance in the border-irrigated farmland (5688.79 ± 1497.11 items/kg) was twice that in the drip-irrigated farmland (2744.44 ± 996.93 items/kg), indicating that border irrigation intensifies surface soil contamination and accelerates MPs migration to the deeper layers. The primary MPs components in farmland soils were PVC, PU, CPE, ACR, PE, EVA, FKM, PET and PTFE. PVC and PU mainly originated from fertilizers, seed coatings, irrigation water, and atmospheric deposition, while CPE, PI, PLA, and PSF were likely derived from the irrigation water and atmospheric deposition. In conclusion, the soil MPs mainly originated from atmospheric source, irrigation water and agricultural inputs. The MPs distribution was influenced by tillage intensity and irrigation practices. These findings provided a better understanding of the MPs distribution patterns and influencing factors, thereby aiding in development of effective pollution control strategies.