Microplastic Accumulation in Agricultural Soils with Different Mulching Histories in Xinjiang, China

Microplastics (MPs) in soils have been widely studied, yet very little is known about their distribution in agricultural soils and the impact of mulching history. In this study, soil samples were taken across 3 soil layers of 60 sites with varying years of mulching history (<5 years, 5–10 years, 10–20 years and >20 years) in agricultural film-mulched cotton fields of Xinjiang, China. Microplastics were obtained from the soils using oil separation combined with density separation. Stereomicroscopy and Fourier transform infrared spectroscopy (FTIR) were used for identification. The average microplastic abundance of the sites with different years of mulching history are 538, 1484, 5812 and 9708 pieces/kg, respectively. The microplastics with sizes 1000–5000 and 200–500 μm are dominant in soils with less than 10 years and over 10 years of continuous mulching history, respectively. The results show that the abundance of microplastics increases and the size of microplastics decreases gradually as the number of years of mulching history increases. In addition, the best polynomial fitting curves were found between microplastic abundance (y) and mulching years (x) in different soil layers, and the relationship in the topsoil layer can be fitted as the following equation: y = 20.6x2 = 41.39x + 198.65 (p < 0.01, R2 = 0.62). The results indicate that residual agricultural mulching film is the dominating source of microplastics in cotton fields. This study provides rationale for further research on microplastics prediction in agricultural film-mulched fields.