Environmental Erosion Research Journal

Agricultural and rangeland soils represent major reservoirs of microplastic accumulation, yet the mechanisms governing their transport under water erosion remain insufficiently understood. This study investigated the effects of rainfall-induced erosion on the transport and enrichment ratio of microplastics in rangelands located in the northern part of the Nahrmian watershed. Two erosion plots (2 × 22 m) were established under vegetated and bare-soil treatments. Polyethylene microplastics in two size classes, 125–63 μm (fine) and 500–250 μm (coarse), were mixed into the upper 10 cm of soil. Surface soil samples were collected before and after three rainfall events, yielding 24 soil samples along with sediment samples from plot outlets. After sample preparation, including drying, coarse-particle separation, sieving, washing, and removal of organic matter using hydrogen peroxide, changes in microplastic concentration and enrichment ratio were assessed using a stereomicroscope. Results showed that particle size is the primary factor controlling microplastic fate. Fine particles exhibited the greatest decrease in surface soil, with average losses of 14% in the bare-soil treatment and 13% in the vegetated treatment. Coarse particles showed lower reductions: 5% in the vegetated plot and 4% in the bare plot. Enrichment ratios indicated greater accumulation of coarse particles in exported sediments, reflecting their stronger tendency for horizontal transport by runoff, whereas fine particles were more prone to vertical infiltration and retention in soil. Regression analysis further showed a significant decline in enrichment ratio with increasing erosion and sediment yield. These findings highlight the immediate threat posed by highly erodible coarse microplastics to surface-water quality, alongside the long-term risks associated with fine particles accumulating in rangeland soils and potentially infiltrating groundwater. Vegetation cover also modulated particle export through selective erosion.

Keywords: Microplastics, Soil erosion, Runoff, Experimental plot, Vegetation cover

     

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