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Plastic mulched agricultural fields in Xinjiang are regarded as potential "hotspots" of microplastic (MP) contamination in China, whereas the abundance of MPs in this region is still unclear. As a carbonaceous material, current conventional methods for measuring soil organic carbon (SOC) generally do not separate the MPs from soils, which probably overestimated the soil carbon (C) sequestration. In this study, 77 agricultural soil samples under plastic film mulching were collected in Xinjiang. Afterward, the average abundance of agricultural MPs and the contribution of microplastic-carbon (MP-C) to the SOC pool were evaluated. The abundance of MPs was 12,589 pieces kg−1 soil (ranging from 4198 to 47,420 pieces kg−1 soil), and small-sized (<0.5 mm) plastic particles accounted for 93.3% of the total MPs. Interestingly, the soil salt content was positively related to the proportion of 0.1–0.5 mm MP but negatively correlated with the proportion of 0.02–0.1 mm MP, indicating that soil salinization probably controlled the degradation process of plastic residues. The average content of MP-C in the 0–20 cm layer was 25.33 kg ha−1 (ranging from 1.60 to 192.57 kg ha−1), which had a contribution of 1.59‰ (ranging from 0.05 to 14.24‰) to the SOC pool. Accordingly, we roughly estimated that the MP-C storage (0–20 cm layer) was approximately 88.66 Gg in the plastic film mulching fields of Xinjiang. Although MP is undeniably organic C, this environmental pollution cannot be regarded as "true" soil C storage, which induces the overestimation of soil C sequestration in agricultural fields. Therefore, our results highlighted that MP-C should be subtracted when estimating SOC sequestration in plastic film mulching fields of Xinjiang. [Display omitted] • We identified microplastics (MPs) in agricultural fields of Xinjiang. • The average abundance of MPs in the plastic film mulching fields was 12,589 pieces kg−1. • Small MPs (<0.5 mm) accounted for 93.3% of the total MPs. • The contribution of MP-carbon to soil organic carbon was approximately 1.59‰. [ABSTRACT FROM AUTHOR]