Microplastic background levels in German soils: The influence of site-specific characteristics and land-use practices

Microplastics (< 5 mm) are ubiquitous environmental pollutants. The extensive accumulation of microplastics in marine and terrestrial ecosystems has become a critical global issue, driven by their ecotoxicological impacts and persistence. The knowledge about its occurrence, especially in the agricultural ecosystem, remains limited, which makes environmental assessments and the development of mitigation strategies difficult.

This study aims at assessing the background levels of microplastics in German soils to elucidate the influence of site characteristics and land-use practices on their spatial distribution. To this end, soil was sampled from 400 cropland and 200 pasture sites in Germany. The samples were characterized for soil type, soil organic carbon (SOC), and pH. Polyethylene (PE), polypropylene (PP), and polystyrene contents are quantified using solvent-based pyrolysis GC/MS. An oxidative digestion with hydrogen peroxide (H₂O₂) was used to reduce SOC in the samples. Microplastics was density-separated from the soil matrix using a saturated NaCl solution.

To validate the method, a recovery test was performed, as the samples contained varying SOC (0.7–39.0%). The recovery experiments were conducted on the following soil types: a clay soil (2.4% SOC), a sandy loam (1.7% SOC), a sandy silt (12.5% SOC), and a quartz sand. Recoveries varied with respect to polymer and soil type. Higher polymer spikes (25 µg/g) yielded higher recoveries (12.5 – 88.0 %) than lower concentrations (5 µg/g, 0 – 48.9 %). The highest recovery rate was 88% for PS with quartz sand. Among the polymers tested, PE showed the highest recovery, whereas PP exhibited the lowest. Recovery experiments with different soil types are crucial in microplastic research to ensure accurate quantification, as varying soil properties can significantly affect the efficiency and reliability of the extraction process.
First results from 52 out of the 600 soil samples indicate that concentrations are overall low and near the analytical detection limits (0,5 – 3 mg kg⁻¹). Average PE levels were 3.4 ± 0.3 mg kg⁻¹ in arable soils and 10 ± 20 mg kg⁻¹ in grasslands. PP concentrations were comparable in both land-use types, ranging from 2.0 to 2.1 mg kg⁻¹. PS levels in both land-use types were below the limit of detection. The values determined must be considered in relation to their SOC content, which may interfere with polymer quantification. Differences in microplastic types could also be attributed to the different site factors depending on land use and potentially driven by atmospheric deposition or littering.

These findings are preliminary and based on a subset of the sampled sites. Data collection and analysis are ongoing and will be extended to the full set of the 600 sites. The data generated in this project will help to develop a geoinformatics-based assessment framework for classifying microplastic pollution in the German agricultural landscape and to derive recommendations for mitigating microplastic inputs.