Residues and accumulation characteristics of plastic fragments and microplastics in farmland soil of Guanzhong Plain, Shaanxi

The application and extension of agricultural film-mulching technology has greatly promoted the development of agriculture in arid and semi-arid areas. However, soil pollution caused by agricultural film residues and their fragmentation have become a great challenge to the development of green agriculture and sustenance of ecological security. In this study, the topsoil (0−30 cm) of farmland from the Guanzhong Plain in Shaanxi Province planted with different crops was taken through field investigation, and samples were analyzed using advanced plastic fragments and microplastic extraction and microscopic scanning identification method to study the characteristics of plastic fragments and microplastics and accumulation in two case study areas: vegetable cultivation area in Caijiapo (S1) and greenhouse planting area in Yangling (S2). The results showed that plastic fragments were detected in all the soil samples, and the residue color was red, black, white, and blue in the vegetable planting area. In the greenhouse planting area, the residues from the culture cups were mainly black. The number of plastic fragments decreased significantly with increasing soil depth (P<0.05) and increasing area. Specifically, in the vegetable planting area, 37.8% of the fragments were in the 0.25−2 cm² group, followed by those in the 2−10 cm² and 10−25 cm² groups, and the lowest fragment group was >25 cm². In the greenhouse planting area, 67.5% of the debris was in the 0.25−2 cm² group, followed by that in the 2–10 cm² and 10–25 cm² groups, and large plastic residue (>25 cm²) was almost not detected. In the different soil layers, the content of plastic residues decreased significantly with increasing soil depth (P<0.05). Specifically, the content of residues in the 0−10 cm soil of S1 and S2 was 26.2 μg·g⁻¹ and 34.9 μg·g⁻¹, respectively, and the residual content in 20−30 cm soil layer was the lowest; it was 2.48 μg∙g⁻¹ in vegetable plot and 4.79 μg∙g⁻¹ in greenhouse nursery. However, the detection rate of microplastics in the vegetable growing area was 23.9%, with a maximum of 1.8×10³ particles·kg⁻¹ in the 0–10 cm soil layer of S1-2. In the greenhouse planting area, the detection rate of microplastics was 39.5%, with a maximum of 500 particles·kg⁻¹ in the 0–10 cm soil layer of S2-2. In conclusion, the long-term application of agricultural plastic film and plastic products in agriculture leads to an increase in plastic residues in farmland soil. Significant differences in plastic residue and microplastic accumulation were found in different soil layers, and they may be harmful to soil properties and crop growth, reducing soil fertility, declining crop yield, and causing other adverse consequences. Therefore, further studies are needed to understand the fragmentation process of plastic residues and the cumulative and migration risk of microplastics in soil to provide a survey-based dataset for comprehensive assessment of plastic pollution in the Guanzhong Plain.