Spatiotemporal evolution of small microplastics in agricultural soils from long-term pig manure application

Long-term application of organic fertilizers serves as a nutrient source in agriculture, yet the contamination of these materials with small microplastics (sMPs, 20-500 microm) remains poorly understood. Research on the accumulation and morphological transformation of sMPs in soils under extended fertilization regimes is currently scarce. This study employed Laser Direct Infrared (LDIR) Spectroscopy to quantify and characterize sMPs in soils subjected to four fertilization regimes: no fertilizer (CK), pig manure (M), nitrogen-phosphorus-potassium (NPK) fertilizer, and a combination of NPK and pig manure (MNPK). Temporal and spatial dynamics of sMPs were assessed across treatments with prolonged organic input. A progressive increase in both the abundance and type of sMPs was detected in pig manure, reaching 21,376 ± 1008 items kg^-1 in 2023-an increase of 180 % compared to 1979.The initial soil sMPs concentrations in 1979 were approximately 3000 items kg^-1; after 44 years, levels in M and MNPK treatments reached 7183 ± 568 items kg^-1 and 5557 ± 329 items kg^-1, respectively. Soils receiving pig manure consistently exhibited higher sMPs concentrations than untreated controls. The relatively elevated levels of sMPs suggest in-situ degradation of larger MPs. Except in the CK treatment, sMPs abundance increased with soil depth. Across all fertilization types, particles within the 30-100 microm range comprised over 46 % of total sMPs, indicating a consistent size distribution. The polymer types and composition in pig manure-amended soils mirrored those identified in the manure itself. These results demonstrate that long-term pig manure application markedly elevates soil sMPs concentrations, increasing the potential for sMPs contamination in agricultural systems.