[Microplastic Occurrence Levels， Distribution Characteristics， and Risk Assessment in Different Tillage Layers of...

As an emerging pollutant， microplastics （MPs） pose significant threats to agricultural ecosystems globally. This study investigates the occurrence， spatial distribution， migration mechanisms， and ecological risks of MPs in arid irrigated farmlands， focusing on the Urad Irrigation area-a critical agricultural zone in the Hetao Irrigation District of Inner Mongolia， China. Given intensive plastic mulching （annual application： ~45 kg·hm^-2）， deep tillage practices （up to 40 cm）， and saline-alkaline soil conditions， this region offers a unique model for understanding MPs behavior in water-scarce environments. A total of 69 stratified soil samples （0-10， 10-20， and 20-30 cm） were collected from 23 monitoring sites along irrigation-drainage networks， combined with Fourier transform infrared spectroscopy （FTIR）， Mantel test， and other multi-index risk assessment methods， to reveal the distribution characteristics， migration mechanism， and ecological risk of soil MPs in the irrigation area. The results showed that： ① The abundance of MPs showed significant spatial heterogeneity （2.42×10⁴-1.28×10⁵ n·kg^-1）， which was at a high level in China， and the horizontal distribution was driven by irrigation retreat， which was in the eastern > western upper layer， the eastern > western middle layer， and the northern/southern western lower layer > western lower layer. The vertical distribution showed surface enrichment （8.92×10⁴ n·kg^-1 in the upper layer > 6.18×10⁴ n·kg^-1 in the middle layer > 5.43×10⁴ n·kg^-1 in the lower layer）， but 21.7% of the sites had lower layer > middle layer due to agricultural tillage （depth up to 40 cm） anomalous enrichment of the middle layer. ② The polymer composition was mainly polyethylene （PE， 37%， proportion， the same below）， polystyrene （PS， 27%）， and polypropylene （PP， 26%）， and morphological analysis showed that PE was mainly film/fibrous， and PS was mostly block， which was consistent with the characteristics of local agricultural mulch film. At the same time， a variety of plastic additives （such as benzyltriethylammonium chloride） were detected， suggesting the risk of MPs aging. ③ Mantel test identified soluble salts （r=-0.43） and pH （r=-0.41） as inhibitors of MPs mobility， promoting aggregation via ionic strength； specific surface area （r=0.44） and organic matter （r=0.29） enhanced MPs retention； middle-layer moisture （r=0.42） facilitated vertical transport via irrigation leaching； and coarse-textured soils （D₅₀ > 50 μm） in deeper layers reduced MPs retention （r=-0.44）. ④ PHI indicated low-to-moderate risk （I-II grade）， inversely correlated with MPs abundance due to localized PS enrichment （high toxicity coefficient=30）； RI reached "strong risk" levels （RI=106） at drainage hubs （e.g.， T17）， reflecting hydrological "pollution traps；" and I_geo demonstrated significantly higher accumulation in middle/lower layers （moderate-to-heavy） than in surface soils （light-to-moderate）， with spatial gradients （east > west） mirroring irrigation flow paths. This study establishes a tripartite driving model （"hydraulic-salinity-tillage"） unique to arid irrigated agroecosystems in which hydraulic processes （irrigation return flow） govern horizontal MPs distribution， soluble salts inhibit MPs mobility via aggregation， and deep tillage （≥30 cm） induces subsoil MPs enrichment. The results highlight the urgent need to optimize mulch film recovery （<30% currently） and regulate drainage management to mitigate MPs export to downstream sinks （e.g.， Wuliangsuhai Lake）. The risk assessment framework underscores the necessity of polymer-specific toxicity evaluation beyond abundance metrics. This work provides a scientific foundation for MPs pollution control in global arid irrigation zones， supporting sustainable agriculture in the Yellow River Basin.