Previous research has focused mainly on agricultural or urban soils, where high MP emissions from direct sources, such as plasticulture, fertilization, and littering, are expected9<\/a><\/sup>. Forests, as a key type of terrestrial ecosystem and covering 31% of the global land surface10<\/a><\/sup>, remain as a blind spot in our understanding of terrestrial MP occurrences11<\/a><\/sup>. Previous studies have demonstrated the presence of MPs in forest soils in the Republic of Korea with mean concentrations of 160 particles per kg soil dry weight (p kg\u20131<\/sup>)12<\/a><\/sup>, primary and secondary rainforests in China with mean concentrations of 630 p kg\u20131<\/sup>13<\/a><\/sup>, neotropical rainforest and pine plantations in Mexico with mean concentrations of 1500 p kg\u20131<\/sup>14<\/a><\/sup>, and alluvial forests in Serbia with mass concentrations ranging of 0\u20136\u2009g\u2009kg\u20131<\/sup>\u200615<\/a><\/sup>. However, these studies are limited because they did not measure small sizes of MPs (<40\u2009\u00b5m), have no comprehensive polymer identification, and mostly focus on mineral forest soil and negating organic soil horizons formed from leaf litter. In contrast to, for example, agricultural soils, forest mineral soils are covered by one or more organic soil horizons formed by biogeochemical turnover processes of leaf litter. Up to three horizons form, consisting of minimally decomposed (Oi), intermediate (Oe), and strongly (Oh) decomposed material on top of mineral soil horizons (e.g., A and B horizons)11<\/a><\/sup>.<\/p>\n\n\n\n In the absence of direct MP sources, atmospheric deposition is thought to act as the major source of MPs to forest ecosystems. Forest filter effects and the trapping of atmospheric particulate matter are already known for trace metals16<\/a><\/sup>and organic compounds (e.g., DDT, PFAS, PAHs)17<\/a>,18<\/a><\/sup>. For MPs, mean MP throughfall depositions of 365 p m\u20132<\/sup> d\u20131<\/sup>and a majority of MP particle sizes of <50\u2009\u00b5m were found in remote forests in the Pyrenees (France)19<\/a><\/sup>, whereas other studies found mean MP throughfall depositions of 331\u2013512 p m\u20132<\/sup> d\u20131<\/sup> during short-term20<\/a><\/sup> and 147 p m\u20132<\/sup> d\u20131<\/sup> (during long-term sampling in near-urban forests (Germany)21<\/a><\/sup>, both with the majority of MPs occurred in sizes <64\u2009\u00b5m20<\/a><\/sup> or <30\u2009\u00b5m21<\/a><\/sup>. Similarly, MP deposition on leaves was already demonstrated under laboratory conditions, with rates of capture reaching 0.87 p cm\u20132<\/sup> during a two-week exposure to airborne MPs22<\/a><\/sup>. However, all previous research on atmospheric deposition of MP in forest systems did not differ between throughfall deposition (including resuspension of previously deposited MPs) and the direct atmospheric deposition (accounting only for the entry of MPs to the forest systems)19<\/a>,20<\/a>,21<\/a><\/sup>.<\/p>\n\n\n\n Combining previous studies on MPs in temperate or tropical forests soils with the outcome of atmospheric deposition studies, it is clear that soil-related studies show higher MP size detection limits; consequently, the majority of atmospheric deposited MPs cannot be detected12<\/a>,13<\/a>,14<\/a><\/sup>. Additionally, the removal of forest litter during sampling12<\/a>,14<\/a>,15<\/a><\/sup>leads to a missing link between atmospheric deposition and MP occurrence in forest soils. The potential importance of forest soil litter was highlighted by the verification that atmospheric MPs can become trapped on leaf surfaces in tree canopies23<\/a>,24<\/a><\/sup>. Concentrations of 0.14\u201325 p cm\u20132<\/sup> for MPs >10\u2009\u00b5m in size were found after washing leaves with water25<\/a><\/sup>, whereas 57 p m\u20132<\/sup> d\u20131<\/sup> was found on near-urban oak leaves (Japan) after extraction using a 10% potassium hydroxide solution26<\/a><\/sup>.<\/p>\n\n\n\n Following the suggestion of a forest trapping function for MPs, the question arises to which extent canopy-trapped MPs and atmospheric MPs reach forest soils via throughfall deposition, as well as whether forest soils act as a sink for atmospheric MPs? Therefore, this study aimed to apply an aligned extraction method on a representative study site in central Germany to answer the following questions:<\/p>\n\n\n\n Our results indicate that direct atmospheric MP deposition and canopy-trapped MPs reaching the soil by throughfall and litter fall are the major sources of MPs to forest soil. Ground-based MP sources, such as littering or outdoor activities, are a possible additional minor source. MPs enter the soil from the surface and are finally accumulated in lower mineral soil by litter turnover processes. The total MP stocks and concentrations in the soils are high, indicating diffuse MP pollution.<\/p>\n\n\n\n MP concentrations within the investigated temperate forest soils ranged between 120\u201313,300 p kg\u20131<\/sup> with a mean of 4440\u2009\u00b1\u20093690 p kg\u20131<\/sup> (mean\u2009\u00b1\u2009SD<\/em>) considering all samples (Supplementary Table S3<\/a>). The highest mean concentrations were found in more decomposed organic forest soil horizons (Oe, Oh) with 5940\u2009\u00b1\u20094800 p kg\u20131<\/sup>, followed by mineral soil horizons (A, B) with 3670\u2009\u00b1\u20092800 p kg\u20131<\/sup> and the uppermost litter horizon (Oi) with 3020\u2009\u00b1\u20091600 p kg\u20131<\/sup>. No statistically significant (p<\/em>\u2009\u2265\u20090.05) differences were observed between MP concentrations in different forest soil horizons. The mean MP concentrations at the different forest sites were not significantly different (p\u2009\u2265\u20090.05) and were 3300\u2009\u00b1\u20092800 p kg\u20131<\/sup> (FS1), 5200\u2009\u00b1\u20094400 p kg\u20131<\/sup> (FS2), 2500\u2009\u00b1\u2009850 p kg\u20131<\/sup> (FS3) and 6800\u2009\u00b1\u20093500 p kg\u20131<\/sup> (FS4).<\/p>\n\n\n\n MP stocks calculated from MP concentration using Eq. 3<\/a> at different sites and in different horizons ranged between 310\u2013870,000 p m\u20132<\/sup> with a stepwise increase from the uppermost litter horizon (Oi) with 1900\u2009\u00b1\u2009900 p m\u20132<\/sup>, to the organic horizons (Oe, Oh) with 14,300\u2009\u00b1\u200912,600 p m\u20132<\/sup>, then to the mineral horizons (A, B) with 230,000\u2009\u00b1\u2009290,000 p m\u20132<\/sup> (Fig. 1a<\/a>, Supplementary Table S3<\/a>). MP stocks in the lowest forest soil horizon (B) were significantly higher (p<\/em>\u2009\u2264\u20090.05) than all other horizons. Within the different forest sites, MP stocks occur without significant differences (p<\/em>\u2009\u2265\u20090.05). The total MP storage in forest soils occurred in sums of 550,000 p m\u20132<\/sup> (FS1), 200,000 p m\u20132<\/sup> (FS2), 240,000 p m\u20132<\/sup> (FS3), and 990,000 p m\u20132<\/sup> (FS4).<\/p>\n\n\n Throughfall deposition fluxes of MPs occurred with an overall mean deposition of 9.1\u2009\u00b1\u20099.4 p m2<\/sup> day\u20131<\/sup> (range 0.6\u201335 p m2<\/sup> day\u20131<\/sup>). MP fluxes in throughfall deposition occurred with a high spatial and temporal variance (Fig. 1b<\/a>), but no significant (p<\/em>\u2009\u2265\u20090.05) mean difference was observed between sampling dates or sites. Comparing MP fluxes over time along the four forest sites, the highest mean concentrations were found at site FS1 with 15\u2009\u00b1\u200915 p m2<\/sup> day\u20131<\/sup> followed by site FS2 (8.0\u2009\u00b1\u20097.8 p m2<\/sup> day\u20131<\/sup>), FS4 (7.4\u2009\u00b1\u20097.8 p m2<\/sup> day\u20131<\/sup>) and FS3 (5.7\u2009\u00b1\u20092.9 p m2<\/sup> day\u20131<\/sup>) (Fig. 1b<\/a>) with overall high variances in MP deposition fluxes.<\/p>\n\n\n\n\n
Results<\/h3>\n\n\n\n
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