Long-Term Application of Organic Compost is the Primary Contributor to Microplastic Pollution of Soil in a Wheat–Maize Rotation

Microplastics (MPs) are posing new threats to soil ecosystems. Organic fertilizers are considered as an emerging contributor to MPs accumulation in agricultural soils. However, few studies have focused on the MPs fate in soils under long-term organic fertilizer application. Based on an 11-year field test with wheat-maize cropping rotation, this study investigated the characteristics of MPs in pig manure and cow manure composts, and examined the impact of long-term compost application on soil MPs accumulation, surface morphology, and distribution. The MPs contents in pig manure and cow manure composts were 3547 and 4520 items kg, respectively. Microplastics abundances in soils under long-term use of these two composts were 144 to 287 and 140 to 316 items kg, respectively, which increased significantly with increasing compost application amount and was substantially higher than that in soils without compost. Accumulated soil MPs sourced from long-term compost application were 1.73 × 10 to 7.22 × 10 items ha, accounting for 43.0 %-75.9 % of the total, and the contribution value doubled as the compost application rate doubled. The proportion of MPs <1 mm in composts (31.0 %) was lower when compared with that in compost-amended soils (43.8 %), and size and abundance reduced with increasing soil depth. Microplastics shapes and polymer types in composts and compost-amended soils were similar and mostly included fragments of polyethylene and polypropylene and fibers of polyethylene terephthalate. Microplastics in compost-amended soils showed complicated weathered surface morphologies, and soil mineral colloids were attached. These results demonstrate that compost-derived MPs in soils can be gradually weathered and degraded into smaller particles under long-term compost application. These findings provide key insights into the pollution level of soil MPs with organic fertilizer application and serve as a scientific basis for developing MPs mitigation measures in agricultural soils.