1 Introduction

Plastics are widely used due to their superior mechanical properties, while the ensuing plastic pollution has created enormous challenges to ecosystems. In particular, the demand for single-use plastic products has skyrocketed with the outbreak of the COVID-19 pandemic (Hu et al. 2022). Global plastic waste production is estimated to be 1.6 million tons per day, and approximately 3.4 billion single-use facemasks/face shields are discarded every day (Benson et al. 2021). Microplastics (MPs; < 5 mm in size) have ubiquitously found in the atmospheric environment (Liao et al. 2021; Wang et al. 2020b), aquatic environment (Elgarahy et al. 2021; Koelmans et al. 2019), and terrestrial environment (de Souza Machado et al. 2018; Wang et al. 2019), even in organisms (Lu et al. 2019; Zhang et al. 2020), which has been regarded emerging environmental contaminant. So far, studies on MPs have mainly focused on aquatic environments, while their occurrence in terrestrial ecosystems has been studied to a much lesser extent.

Agricultural soils are considered as a crucial sink of MPs through the inputs from mulching films, sewage irrigation, atmospheric deposition, and compost, negatively affecting agroecosystems and plant productivity (Ng et al. 2018). A growing body of evidence has shown that MPs in soil have detrimental impacts on soil biota, as well as alter the soil properties and biogeochemical cycles (de Souza Machado et al. 2019; Guo et al. 2020; Khalid et al. 2020; Wang et al. 2021). For example, Jiang et al. (2019) found that MPs cause oxidative damage and genotoxicity in Vicia faba, meanwhile, Yu et al. (2020b) discovered that MPs induce oxidative stress and intestinal injury in nematodes. Moreover, MPs are easily transferred into groundwater systems, creating a hazard by chemical additives releases (Ren et al. 2021). Apart from their small size, complex nature, and potential toxicity, MPs can act as vectors of dangerous pollutants and pathogens, posing indeterminate ecotoxicological risks through bioaccumulation and biomagnification in the food chain (Bradney et al. 2019; Yu et al. 2020a). The increasing MP pollution in soil may threaten terrestrial food systems and human health. It has been estimated that humans might ingest 80 g of MPs through edible plants daily (Ebere et al. 2019), and MPs have been detected in human food (Jin et al. 2021), human placenta (Ragusa et al. 2021), and human blood (Leslie et al. 2022), showing the potential risk to human health. Additionally, as hotspots of antibiotic resistance genes and potential pathogens, soil “plastisphere” exacerbates the antibiotic resistance issues in soil (Liu et al. 2021; Zhu et al. 2021). Therefore, research on MP pollution in agricultural soil is of great significance.

Co-occurring MPs and heavy metals in soil pose an alarming threat to soil biodiversity and agroecosystem (Roy et al. 2022). Heavy metals, the nonbiodegradable inorganic pollutants, have long-term negative impacts on human and ecosystem health. Corresponding studies have highlighted that the simultaneous exposure to MPs and heavy metals in soils joint affects soil properties, microbial diversity and functions (Feng et al. 2022), root symbiosis, and plant performance (Wang et al. 2020a), and their combined toxicity is tremendously dangerous than individual toxicity (Khalid et al. 2021; Yan et al. 2020). Moreover, the weathering of MPs can significantly promote their heavy metal adsorption capacity (Chen et al. 2022). A significant amount of plastic waste eventually ends up in landfills or incineration plants. As the MP reservoirs, mismanaged landfills can transfer MPs to their surroundings via an air-borne pathway or the leakage of leachate (He et al. 2019; Rillig 2012). More MPs were detected in the atmospheric fallout collected from landfills (Thinh et al. 2020) and in the groundwater samples around a landfill (Natesan et al. 2021). Additionally, heavy metal pollution was more prone to occur in the soil around waste incineration plants and the operation of waste incineration plants may facilitate the accumulation of heavy metals in soil (Ma et al. 2018). It is more likely that the combined pollution of MP and other pollutants occurs in the soil surrounding the solid waste treatment center. However, to date, the occurrence of MPs in agricultural soils around solid waste treatment centers has not been explored. This study systematically investigated the occurrence, spatial distribution, and characteristics of MPs in agricultural soil near a solid waste treatment center in southeastern China to provide valuable information for estimating and monitoring MPs’ presence in the complex agroecosystem.

2 Materials and methods

2.1 Studied area and sampling methods

A solid waste treatment center which is located in Xiamen, southeast China (N 24°32′–24°50′, E 118°10′–118°27′) was chosen as a studied area. It is the only multi-functional large-scale solid waste comprehensive treatment base in this city that integrates domestic waste classification and treatment, fermentation and fertilizer production, sanitary landfill, incineration and power generation, animal harmlessness, waste recycling, and water reuse, and it is responsible for the disposal of domestic garbage, medical garbage, and industrial hazardous waste (Fig. S1). Equipped with treatment plants such as landfills and waste incineration plants, the solid waste treatment center was first operated in 2008 and now handles more than 3700 tons of domestic waste and more than 200 tons of kitchen waste per day. The studied area has a typical subtropical oceanic climate and the rainfall increases due to the influence of typhoons from July to September each year, with an average annual rainfall of 1143.5 mm and relative humidity of 77%.

Since easterly wind prevails throughout the year and the eastern part is a mountainous area, the studied area was classified into upwind, center, and downwind areas. Taking the solid waste incineration plant as the center, the 500 × 500 m grid method was used to collect 57 agricultural soil samples with a depth of 0–20 cm at a total area of 36 km2 along with a 3.0-km extension from north to south and east to west (Fig. 1). There were 31 sampling points in the upwind area (S1–S31), 8 sampling points in the center area (S32–S39), and 18 sampling points in the downwind area (S40–S57). Five sub-samples were collected from each sample point using the five-point sampling method and a pre-cleaned stainless shovel in October 2020. After removing litter, grassroots, insect shells, gravel, and other impurities, the five sub-samples were mixed and homogenized into a composite sample on site. Finally, about 500 g of the composite sample was packed in a clean glass bottle, then transported to the laboratory. In the laboratory, the soil samples were wrapped in aluminum foil and dried to constant weight in a drying oven at 30 °C.

Fig. 1
figure 1

Geographic location of the studied area. Since there are mountains in the eastern study area, the sampling area is selected on the left side of the study area

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2.2 Extraction of MPs

MPs in the soil samples were separated and extracted using the density separation method used by Thompson et al. (2004) but with slight adjustments. For each sample, 50 g of dried soil was placed in a glass beaker and mixed with saturated NaCl solution (1.2 g/cm3, dissolved in Milli-Q water), and the mixture was stirred for 10 min to mix thoroughly and left for 24 h. After removing the supernatant, the saturated NaCl solution was added to the beaker with soil again, and this step was repeated three times to fully extract MPs. The collected supernatant was passed through a 2-mm sieve and then filtered with a vacuum filtration device with a glass-fiber filter membrane (Sartorius, diameter 50 mm, pore size 0.45 μm). The material on the filter membrane was rinsed slowly with saturated NaCl solution to a clean glass Erlenmeyer flask and floated again to avoid clogging of filter membrane for soil particles are not completely separated from the NaCl solution. Following density separation, H2O2 (30%, v/v, XIHUA) was added to the conical flask containing the supernatant at a ratio of 10:1, then transferred to a 70 °C water bath and heated until the organic matter was completely digested. The digested solutions were cooled down and filtered through a 0.45-μm glass-fiber filter membrane. The MP-containing filter membrane was moved to a clean glass petri dish, dried, sealed, and preserved for further analysis.

2.3 MPs’ identification

The color, shape, and size of the suspicious MPs on the membrane were visually identified under a microscope (Olympus SZ61, Japan), and the quantity was recorded (Fig. 2). There were three shapes observed: fragments (flaky with obvious outline), fibers (slender strip), and particles (stereo particles), and six colors: white (transparent included), blue, red (pink included), black, yellow, and green. The chemical components of the representative MPs and suspect particles in the different membranes were determined using a Raman microscope (Renishaw in Via, UK, spectral range: 100–3200 cm−1, incident laser: 785 nm). Raman micro-spectroscopy analysis was repeated three times in three different runs under the same conditions to guarantee the accuracy of the results (Fig. S2) (Sobhani et al. 2019; Dong et al. 2020).

Fig. 2
figure 2

Typical image of MPs under the microscope: fragment (a–c), fiber (d, e), and particle (f)

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2.4 SEM–EDS analysis

After being identified by Raman analysis, the MP was coated with a thin film of platinum and placed under a scanning electron microscope (SEM, Hitachi S4800, Japan) to observe the surface morphology, and an energy spectrometer (X-Act, Oxford) was used to obtain the metal elements attached to the MPs’ surface. The visualization was repeated three times in different positions to ensure accuracy (Wang et al. 2017).

2.5 Physico-chemical parameters of soils

The dried soil samples were ground through a 0.15-mm sieve. Soil pH and electrical conductivity (EC) were measured using suspended samples with soil/water ratios of 1:2.5 and 1:5, respectively. Total organic carbon (TOC) in the soil is determined by microwave digestion. The data of physico-chemical parameters of soils are listed in Table S1.

2.6 Quality control

To prevent plastic contamination during the experiment, only glass utensils were used and rinsed with Milli-Q water and continuously covered with aluminum foil. Throughout the trial, the researchers donned white cotton lab coats and nitrile gloves. Ten blank control experiments were conducted to determine the MP pollution induced by Milli-Q water, reagents, and experimental processes, but no MPs were detected, showing that the impact of the experimental environment can be ignored.

2.7 Statistical analysis

The statistical analysis was performed by IBM SPSS 22.0, and one-way ANOVA was used to compare the significant differences in the abundance of MPs in different studied areas. All figures and tables were drawn using ArcGIS 10.8, Excel 2019, and Origin 2021.

3 Results and discussion

3.1 Abundance and distribution of MPs

MPs were widely observed in the 57 soil samples, and there were significant differences in their spatial distribution. The overall abundance of MPs varied from 280 to 2360 items/kg, with an average of 820 items/kg. The MP concentration in the upwind, center, and downwind region ranged from 400 to 1480 items/kg, 280 to 1240 items/kg, and 340 to 2360 items/kg, respectively (Fig. 3a). The highest value occurred at site S48, located in the village area in the downwind area. Abundant MPs were not found in the samples near the solid waste center but found in those closer to the villages or roads in the downwind area. Unexpectedly, no statistical differences were found in the abundance in the downwind, center, and upwind areas, while the mean abundance of MPs in the downwind area was higher than that in other areas (Fig. 3b). These different distribution observations might be explained by the irregular planting patterns in the sampling area. Since the study area is not a large-scale vegetable planting area, and there are irregular planting patterns, the villagers tend to apply mulch film technology to some areas with fragile vegetables to increase their production. A higher abundance of MPs in the downwind area implied that the solid waste treatment center might have a small contribution to MP pollution in the surrounding agricultural soils. Loppi et al. (2021) found that the impact of MPs on air emissions from landfills was spatially limited by detecting the deposition of MPs on lichens in the vicinity of landfills. The solid waste treatment center’s limited effect in this study might be attributed to its strong management. For example, modern processing techniques are employed, the operating area and time are substantially reduced, and the fog cannon cars continuously spray special medicine for disinfection and deodorization during the working period. All these measures may have contributed to lessening the pollution of the solid waste center to the surrounding soil.

Fig. 3
figure 3

Abundance of MPs at each sampling point (a), and boxplots for abundance analysis of MPs, no significant differences were detected (b)

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Compared with the gathering research on MPs in agricultural soil (Table 1), the MPs’ pollution level in this study is moderate. The abundance of MPs in different research regions varies due to diverse factors such as pollution sources, soil properties, planting patterns, extraction, and quantification methods. The concentration of MPs in farmland soil around an e-waste dismantling zone in China reached up to 22,800 items/kg, with e-waste as the primary source (Chai et al. 2020). The cultivated soil of the Yunnan Plateau, where the planting industry was the basis, contained 900–40,800 items/kg of MPs (Huang et al. 2021). Higher MP concentrations were found in the mulching farmlands than in non-mulching farmlands (Zhou et al. 2020a). Owing to the limitation of the density of the flotation solution, high-density MPs failed to float, which may have resulted in an underestimated MPs abundance in this study. Moreover, the studied area is far away from the industrial area, so industrial sources could not be considered, implying agricultural activities might be the primary cause of MP contamination.

Table 1 Characteristics of MPs in agricultural soil in different regions
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3.2 Size, color, shape, and composition characteristics of MPs

The most frequent plastic particle size was less than 0.05 mm, accounting for 65.8% of the total samples, followed by the particle size of 0.05–1 mm (20.2%) and 1–2 mm (14.0%) (Fig. 4a). Similar to previous research (Wang et al. 2022), the particle size distribution showed a tendency that the smaller the particle size, the larger the proportion, attributing to the formation of more small particles by aging (ultraviolet radiation, mechanical plowing, and biodegradation). The < 1 mm MPs were detected in greater abundance in the downwind area than in the center and upwind areas, which can be ascribed to the fact that smaller and lighter MPs may be carried by the wind (Rezaei et al. 2019), and to some extent, it was revealed that the solid waste center may contribute to the MPs in downwind area via atmospheric deposition. Studies have demonstrated that the smaller the particle size of MP, the greater pollutant adsorption capacity and the easier it is for it to be swallowed by soil creatures or migrate to deeper layers of soil, causing harm to soil animals and the agricultural ecosystem (Boots et al. 2019; Lahive et al. 2019). Given the higher risk of small-particle MPs, more research is urgently required on their distribution and toxicity.

Fig. 4
figure 4

Physicochemical characteristics of MPs in different study areas. a Size characteristics. b Color characteristics. c Shape characteristics. d Component composition

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The color distribution of MPs was significantly different: white (58.20%), blue (12.66%), black (9.47%), red (8.50%), yellow (7.52%), and green (3.65%) (Fig. 4b), which is related with the characteristics of local plastic pollution. White plastic products such as plastic film are widely used in cultivated land. Moreover, the colored MPs will weather and turn white, releasing harmful additives and intermediates (oligomers and dissolved organic matter (DOM)) to cause ecological risk (Zha et al. 2021). In this study, fragments accounted for 58.4% of all samples were the dominant type of MPs, followed by fibers (31.86%) and particles (9.74%) (Fig. 4c). Weithmann et al. (2018) discovered that all compost samples with different substrates were rich in MPs, of which fragments and particles accounted for a large proportion. A higher proportion of fibers was detected in the downwind area, which is frequently found in irrigation water and atmospheric deposition. Additionally, different shapes of MPs have distinct mobility, the particles are more likely to migrate to deep soil, whereas the fragments and the fibers may hinder the migration of MPs in the soil (Zhou et al. 2020b).

The components of MPs were identified as polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET). The most common polymer types identified were PE (52.86%) and PP (27.14%), while PS (11.43%) and PET (7.14%) were the second most abundant in all samplings (Fig. 4d). PE and PP are widely utilized in manufacturing and daily life as raw materials for a variety of packaging, containers, plastic mulch, pipes, and other plastic products. In this study, the high abundance of PE and PP was attributed to the extensive use of plastic film and compost. Huang et al. (2020) confirmed that plastic mulching was the main source of MPs in agricultural soils through the research on plastic residues on agricultural land in China in the past 5 years. MPs were widely detected in rural domestic waste compost (Gui et al. 2021) and livestock manure (Huerta Lwanga et al. 2017; Yang et al. 2021). Moreover, PS is widely used in the manufacture of transparent films, daily decoration, and packaging, particularly in cosmetics products. Likewise, field surveys of this study discovered that numerous plastic mulch waste was removed improperly, plastic garbage such as plastic bags or bottles used to contain fertilizers and pesticides were scattered everywhere, and composting was applied. In addition, wastewater irrigation was observed in the analyzed zone. PET is mainly applied in the textile industry, and PET-MPs are transferred to the environment through domestic sewage. In recent years, certain evidence indicates that abundant MPs have been detected in terrestrial water bodies such as rivers, lakes, and ponds and can be disseminated in agricultural soils as a result of irrigation. For example, PET-MPs (30.2%) were widely identified in agricultural land irrigated with wastewater in Baoding, China, (Du et al. 2020). Based on these results, it can be concluded that agricultural activities contributed significantly to MP pollution in this study. Additionally, MPs can enter soils by road dust, surface runoff, and atmospheric deposition which may explain why MPs are greater in downwind locations. Therefore, further research works on the contributions of atmospheric MPs need to be carried out subsequently.

3.3 Surface morphology of MPs

Different types of MPs randomly selected presented complex surface morphologies via SEM. The MPs have weathered as a result of long-term exposure to the soil environment and show aging features like rough and porous surfaces. On the surface of the fragments, there are numerous fissures and uneven rips (Fig. 5a). The fibers are badly worn, with no original shape and many linear convex plastic residues generated by ripping; filiform fibers, in particular, have been eroding more severely (Fig. 5b,c). The particles are heavily irregularly damaged, with some undigested strips clinging to the surface, showing a complicated structure (Fig. 5d). Long-term UV light, frequent physical abrasion, and microbes all played an important role in the weathering of MPs in the soil environment, altering their surface shape and characteristics (Duan et al. 2021).

Fig. 5
figure 5

SEM images of different shapes of MPs at different magnifications. a Fragments. b Fibers. c Filamentous fibers. d Particles

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3.4 Elements attached to the surface of MPs

SEM–EDS analysis of the surface element distribution of the MP samples was carried out for 9 metal(loid)s (Cd, Hg, Cr, Fe, Mn, As, Zn, Cu, Ni) (Fig. 6). The EDS image demonstrated that more diverse metal(loid) elements can be detected on the surface of MPs with aging characteristics such as substance attachments, depressions, protrusions, and cracks or broken edges. The increased specific surface area, oxygen-containing functional groups, and surface electronegativity of the aged MP result in greater sorption to metals (Duan et al. 2021). The presence of metal elements on MPs can be linked to inherent load and external environment adsorption. Metals were added to plastic throughout the production process as stabilizers, pigments, or catalysts to improve its performance and esthetics. Laboratory simulation experiments and environmental monitoring data have indicated that MPs can adsorb metals from the external environment (Godoy et al. 2019; Sighicelli et al. 2018). In this study, there are different metal elements observed on different surface sites of individual MP, suggesting that part of the metals attached to the surface of the MPs originated from the external environment. In other words, the MPs can act as carriers of metals to facilitate the accumulation of heavy metals in soil and aggravate soil complex pollution. Zhou et al. (2019) confirmed that the MPs in soil were vital transport carriers of metals, and heavy metal content in MPs was significantly impacted by soil content levels. Given the mobility and capacity of MPs to absorb and accumulate metals, soil MP contamination deserves more attention and research.

Fig. 6
figure 6

SEM–EDS images of MPs in different study areas: a upwind area, b center area, and c downwind area

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4 Conclusion

This study described the occurrence, distribution, and characteristics of MPs in agricultural soil around a solid waste treatment center in southeast China. The < 1-mm MPs were prevailing, while PE and PP fragments were commonly detected. MPs generally exhibited complicated weathered surfaces that had the potential to absorb heavy metals. The presence and distribution of MPs in soil samples have been considerably influenced by agricultural operations. The solid waste treatment center has a small contribution to the propagation of MPs showing that well-management could prevent the spread of MPs. Nevertheless, higher MP levels found in the downwind area, especially small-sized ones, revealed certain potential risks. Further research works are urgently required to reveal the distribution of the atmospheric MPs in areas along different wind directions.