Microplastic fate in a chronosequence of biosolid-amended agricultural soil in Southern Ontario, Canada

Municipally sourced biosolids are commonly used as cost-effective fertilizers, diverting material from landfills and contributing to the circular economy. However, biosolids contain high concentrations of microplastics (MPs), which are emerging contaminants of concern due to their ubiquity in the environment. Despite this, there is a lack of environmentally relevant field studies. In 2022, composite topsoil samples (0–15 cm depth) were collected from seven agricultural fields in Southern Ontario, Canada, representing a chronosequence of biosolid applications ranging from 1 to 9 years since amendment and a control (untreated) field. MP particles down to 20 μm in size were extracted by density separation, enumerated, characterized by stereomicroscope and polymers identified using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Here, we report on the characteristics, abundance and polymer type of MP particles in the study area to assess their fate in biosolid-amended soils. The average MP concentration among fields was 6.87 ± 1.47 MP g⁻¹ (3.43 ± 0.74 mg MP kg⁻¹). Additionally, the MP soil pool increased with repeated applications of biosolids. The dewatered biosolid plastic content of 8816 ± 1809 MP g⁻¹ dry weight (11.6 ± 17.5 g MP kg⁻¹ dry weight) was used to estimate a mean MP loading of 94.5 ± 10.9 kg ha⁻¹ to each field per application, suggesting that 7% of the MP soil pool persisted over time. Quantifying the MP pool in biosolid-amended agricultural soil will inform evidence-based plastic policy changes in our global effort to understand and reduce plastic pollution.