Global soil microplastic assessment in different land-use systems is largely determined by the method of analysis

Although microplastics (1 μm - 5 mm, MPs) are increasingly recognised as a novel entity of pollutants, we still lack a basic understanding of their prevalence in different terrestrial environments, such as managed soils. Here, we aimed at elucidating the global MP pollution of managed soils, with specific focus on continental differences, input pathways, and land use types as impact factors, whilst considering the effect of the respective analytical methods on the reported results. After evaluation of 305 sites from 51 studies, we found that the analytical method mostly determined the reported MP load, as analysis of both small and large MPs, use of high density separation solution, and of post-density separation soil organic matter removal yielded significantly higher MP loads. The global means of MP loads benchmarks 2,081 ± 6323 MP items kg-1 soil (global median of 476 (0 - 72,200) MP items kg-1), with 75% of studies located in Asia. Highest mean numbers of MP items were found for Asia and The Americas (2,408 ± 7,194 and 2,138 ±  2,142 MP items kg-1 respectively), with the former being significantly higher than the mean MP concentration of Europe (1,153 ± 1,721 MP items kg-1, p < 0.05). Maximum MP numbers were found for soils under plastic mulching (2,576 ± 8,568 MP items kg-1), followed by greenhouses and polytunnels (1,980 ± 1,200 MP items kg-1), and sludge amendments (1,845 ± 1,925 MP items kg-1). There was no evidence of elevated MP loads in horticultural fields relative to other agricultural management practices (agronomy). Yet, quantitative comparisons were biased by the methodology selected for MP analyses, as looking at the effect size methodology had the largest effect on MP loads. Hence, we conclude that based on the current database, comparisons across studies and input pathways as well as land-use systems are hampered by methodological inconsistencies.