Abundance and morphology of microplastics in an agricultural soil following long-term repeated application of pig manure

Highlights: • More microplastics occurred in soil with long-term manure addition than unamended. • An estimated 3.5 million particles ha⁻¹ a⁻¹ accumulated from manure application. • Soil and manure had similar types and composition of microplastics. • The microplastics in the manured soil had complicated weathered surfaces. Microplastics occur widely in the terrestrial environment and they currently occur in organic fertilizers applied to agricultural land. However, there is little information available on the accumulation of microplastics in soils fertilized over the long term. Here, we investigate the characteristics of microplastics in both pig manure and soil following long-term manure application in an attempt to assess their accumulation and the potential risk to agricultural soils of repeated application of pig manure. Microplastics were separated from soil and pig manure samples using a sequential flow separation and flotation method. The abundances of microplastics were 16.4 ± 2.7 and 43.8 ± 16.2 particles kg⁻¹ in control plots (CK, no manure applied) and plots amended annually with pig manure for 22 years (PM), respectively. The microplastics (especially fragments) were significantly enriched in PM-amended soil compared with the control plots. The average annual abundance of microplastics was 1250 ± 640 particles kg⁻¹ in manure. Interestingly, the type and polymer composition of microplastics were very similar in the soil and manure. Differences in color and particle size indicate that microplastics sourced from pig manure may be gradually weathered and degraded after incorporation into the soil. The average accumulation rate of microplastics in the agricultural soil with long-term application of pig manure was estimated to be 3.50 ± 1.71 million particles ha⁻¹ a⁻¹. The microplastics in the manured soil displayed complicated weathered surfaces. The presence of carbonyl groups suggests that the weathered microplastics in soil may have the potential to adsorb contaminants.