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Purpose: Within the emerging field of soil related microplastic (MP) research, little is known about spatio-temporal variations of MP concentrations in soils. To overcome this current knowledge gap, we suggest the exploitation of long-term soil monitoring programmes (LTSM) and its archived soil samples, to gain first data on spatio-temporal variations within soil MP contamination.

Methods: We analysed 32 archived topsoil samples of 13 cropland and 3 control sites, including grassland and forest sites, which are part of the LTSM programme of the German federal state of Hesse. Analysed samples cover a time span of 10–16 years. MP (>300 µm) have been extracted via sodium chloride (NaCl, 1.2 g cm⁻³) density separation, Nile Red staining and fluorescent optical identification followed by ATR-FTIR analysis of individual MP.

Results: We found MP contamination (mean 34.66±24.08 p kg⁻¹) from mainly PE and synthetic rubber polymers. Furthermore, we were able to detect spatial MP contamination hotspots, with unexpected high MP concentrations in control grassland and forest LTSM sites. In case of temporal trends, we cannot conclude a general increase of MP concentrations over time. However, as we found MP in older and newer samples, MP contamination is likely to be ubiquitous in topsoils and has already been present in the environment for several decades.

Conclusion: Our findings underline the role of cropland topsoils as MP reservoirs, but raise questions on the MP storage in grassland and forest soils. Applying MP analytical tools to archived LTSM samples enables first insights within non-targeted spatial source analysis and temporal trends of soils MP pollution. If enhanced analytical methods will be applied to LTSM samples, it will be possible to gain more detailed insights within spatio-temporal MP dynamics in soils.