Spatial and temporal variance of microplastics in agricultural soils

Microplastics (MPs) as polymeric environmental pollutants have been detected worldwide within arable soils. As agricultural practice poses direct input pathways for plastics to soils and carry the risk of endangering important soil functions, soils microplastic community has developed a strong focus on the investigation on MPs within arable land. However, data on large scale spatial as well as temporal MP occurrence in agricultural soils is still lacking. One possible way to access both, spatial and temporal variance of MPs in agricultural soils is the analysis of reserve samples from permanent soil observation (PSO) monitoring programmes. Based on the example of the PSO programme within the federal state of Hesse (Germany) we have analysed microplastics (300–5000 µm) from topsoil samples of 16 PSO locations sampled in 2003/2004 and 2014/2019. The PSO locations include 12 croplands, three grasslands and one forest soil as a control site across the federal state and its major soilscapes with typical soil formations. We used a MP extraction protocol based on density separation, Nile Red staining and fluorescent analysis coupled with a polymer analysis via ATR-FTIR. Own spiking experiments for common polymers yielded recovery rates of 100% for large MPs (>1000 µm) and 92% for MPs above our lower detection limit of 300 µm. We found MP concentrations of 34.80±23.37 p kg-1 with increasing concentrations by decreasing particle sizes. Overall, MPs occurred dominantly as films, followed by fragments and filaments made of PEs, synthetic rubbers or EPDMs, followed by common thermoplastic polymers. With regard to land use and soil management practice, MP concentrations were significant higher in grassland soils than cropland soils, while particle sizes and polymeric composition stay consistent. Surprisingly, we found the highest MP concentration of 65.22 p kg-1 within mineral forest topsoils of a remote low mountain area. A clear temporal trend, like the MP concentration increase over time, was not detectable. Moreover, we found high temporal variances depending on PSO locations. However, MP concentrations are significant higher at PSO locations in short distances to urban areas, indicating the role of land use intensity and potential littering as MP sources. Our findings enable a discussion on large scale plastic input pathways and their temporal variance. Furthermore, different questions about spatial contaminant patterns and the long-term behaviour of plastics in soils have been raised, that should be discussed within our contribution.