Soils of croplands especially where plastic film mulch is commonly applied, are normally contaminated by Microplastics (MPs). Microplastics can threaten air quality, food and water health, as well as human health by wind erosion processes. In this research, we investigated MPs collected in four wind erosion events at sampling heights between 0 and 60 cm in typical semiarid farmlands in northern China that employ plastic film mulch. Height distribution and enrichment heights of the MPs were measured. The results revealed that the average amounts of MPs for 0-20 cm, 20-40 cm and 40-60 cm sampling heights were 868.71 ± 249.21, 799.87 ± 271.25, 1102.54 ± 317.44 particles kg-1. The average enrichment ratios of MPs for the different heights were 0.89 ± 0.54, 0.85 ± 0.56, 1.15 ± 0.73. Height distribution of MPs was combined affected by shape (fiber and non-fiber) and size of MPs, wind speed and soil aggregate stability. The amount of fibers approximately <4 mm in size and non-fibers <2 mm in size increased with sampling height, while both shapes of larger MPs than these two sizes was almost independent of the sampling height. Enrichment ratios of microfibers were positively related to wind speed but negatively related to soil aggregate stability at each sampling height. The results revealed that although MPs have similar density to organic matter (OM), the MPs' amount and enrichment were independent of height but the OM content and enrichment ratios increased with height. These results suggested that more attention should be paid to the exploration of the influence of higher sampling heights (>60 cm) on the distribution of MPs in the future and the MPs characteristics for different sampling heights require carefully parameterization in detailed models of atmospheric MPs transport by wind erosion.
Keywords: Enrichment; Microplastics; Saltation layer; Sampling height; Wind erosion.
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