Microplastics pollution in soil increases dramatically with long-term application of organic composts in a wheat–maize rotation

The extensive application of organic fertilizers for agricultural use has been considered as a main source of microplastics (MPs) in soils. However, how long-term organic fertilizer applications affect MPs accumulation in soils is poorly understood. Therefore, MPs were characterized in chicken manure (CM), sludge (SC), and domestic waste (DW) composts applied as organic fertilizers, and effects of long-term compost applications on soil MPs accumulation and vertical distribution were explored in a 13-year wheat–maize cropping rotation field experiment. Microplastics concentrations in CM, SC, and DW composts were 14,720 ± 2,468, 8600 ± 1,428, and 11,640 ± 3565 items kg−1, respectively, and after 13 years of addition, MPs concentrations in soils were 2733 ± 160, 2289 ± 270, and 2462 ± 247 items kg−1, respectively. Microplastics concentrations were significantly higher in soils with composts than in those without. Accumulated MPs in soils derived from long-term compost applications ranged from 3.63 × 109 to 4.99 × 109 items ha−1, which were amounts 3 to 4 times higher than those input by composts, indicating MPs derived from composts can be gradually decomposed into smaller pieces after incorporation into soils under long-term application. The percentage of MPs >1 mm in composts (55.4%) was higher than that in soils with composts (40.7%), and abundance and size decreased with increased soil depth. Shapes and polymer composition were similar between composts and soils with composts. Most MPs were different colored fibers and fragments of polypropylene and polyethylene. To conclude, long-term application of organic composts increases MPs pollution risk in agricultural soils, which needs to be evaluated further.