Introduction

Plastics have a range of unique properties and have numerous applications; they can be used at an extensive range of temperatures, they are corrosion resistant, very strong, and though. Furthermore, their low cost, diversity, and utility make them suitable for various applications (Table 1) (Andrady and Neal 2009; PlasticsEurope 2021). Nowadays, we are in the plastic age; the current global usage of plastic is enormous and has been increasing in recent years, reaching 368 million tons in 2019 (Thompson et al. 2009; PlasticsEurope 2021). Plastics represent 10% of waste generated around the world, while some plastic wastes are recycled, and the majority end up in the environment like landfills and agriculture fields (Barnes et al. 2009; Wang et al. 2019). According to Horton et al. (2017), between 473,000 and 910,000 tons of plastic waste are released and retained annually in continental environments of the European Union. These quantities correspond to between 4 and 23 times the estimated amount that is released in the oceans.

Table 1 Types of plastics commonly used worldwide (Rocha-Santos and Duarte 2017; PlasticsEurope 2021). *No information
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Plastic pollution is considered to be a major factor responsible for the global decline in biodiversity. This is a threat to the soil system’s functioning and has been documented in ecosystems worldwide (Barnes et al. 2009; Qi et al. 2020). Therefore, the abundance and persistence of plastics and microplastics (MPs) is a severe environmental risk (Scheurer and Bigalke 2018; Steffen et al. 2015).

The term “MPs” was coined by Thomson et al. in 2004, to refer to microscopic-sized plastics and have often been defined as particles between 5 mm and 100 nm in diameter. MPs are classified according to their origin; in this way, we can distinguish primary and secondary MPs. Primary MPs are those intentionally manufactured microscopic and can be found in personal care products like toothpaste, cosmetics, and cleaning products. On the other hand, secondary MPs are originated from the fragmentation of larger plastic products, such as plastic mulch films and household garbage (Duis and Coors 2016; Qi et al. 2020; Rocha-Santos and Duarte 2017; Wang et al. 2019). MPs contain mixtures of chemical additives, fillers, residual monomers, catalysts, and non-intentionally added substances (NIAS). Also, they act as a vector for pathogens and absorb contaminants such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), hexachlorocyclohexane (HCH), pharmaceuticals, pesticides, perfluoroalkyl substances (PFAS), and heavy metals. Furthermore, they accumulate in the food web by direct uptake from the soil or by consumption of contaminated soil biota (Besseling et al. 2017; Fendall and Sewell 2009; Hodson et al. 2017; Huerta Lwanga et al. 2017; Rochman et al. 2013; Wang et al. 2019).

MP pollution is listed as one of the top environmental problems by the United Nations Environment Programme (UNEP) and have gained attention due to their adverse effects on the soil, soil biota, and ecosystems in general. These effects are produced due to their small size and ubiquity (Rocha-Santos and Duarte 2017; Scheurer and Bigalke 2018; UNEP 2014). There are numerous sources of MP entry to soils (Fig. 1) and have been detected in industrial areas, agricultural soils, greenhouses, home gardens, coastal soils, and alluvial plains with a wide range of concentrations, which are well summarized in the study of Xu et al. (2020).

Fig. 1
figure 1

Main sources of dispersion and entry of MPs to soils. A Use and disposal of household products that have primary and secondary MPs. B Wastewater treatment plants produce sewage sludge which contains MPs, due to their high nutrient load they are used to fertilize agricultural and forestry fields. C Household garbage is disposed of in landfills, where plastics are fragmented forming MPs. D MPs in watercourses reach the adjacent soils by irrigation. E Soil erosion by wind disperses the MPs in the environment. F Forest’s foliage traps MPs present in the air, with the fall of leaves or the rain they are deposited on soils. G Plastics fragmentation of greenhouses generates MPs. H Plastic fragmentation of mulch film generates MPs. I Car tires release MPs due to physical wear of the tires, as well as the wear of the brakes. J Organic amendments such as compost, made with household or municipal waste will contribute MPs to the soils. K Industrial zones are highly contaminated with MPs. L Air masses transport MPs, which pollute soils by atmospheric deposition

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It has been estimated that up to 430,000 and 300,000 tons of MPs enter each year to agricultural land in Europe and North America, respectively (Nizzetto et al. 2016). Moreover, China has reported between 50 and 260 kg ha-1 of plastic in farmland soils after 30 years of extensive use of agricultural plastic films (Liu et al. 2014). In Australia, concentrations as high as 7% of MPs have been reported in highly contaminated topsoils (Fuller and Gautam 2016).

Determination of MPs in soils

MP pollution has been documented in various environments, and their determination is highly challenging. Therefore, is essential to choose correct methodologies in the stages of sampling, processing, detection, and quantification of MPs (Fig. 2) (Möller et al. 2020; Zhang 2007). The soil is a heterogeneous matrix comprised of minerals with a range of particle sizes, distributions, and organic matter at varying stages of decomposition. Also, the distribution and quantity of MPs can vary considerably. Therefore, the first stage in determining MPs in soils is the sampling, which must be representative and always avoid adding MPs from the sampling or transport materials (IAEA 2004; Möller et al. 2020; Yang et al. 2021).

Fig. 2
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Methodologies used in the main stages of MP determination in soils

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Sample processing

To date, there is no consensus methodology for soil processing; the analytical methods for MPs research vary among research groups. First, the sample must be dry, and the purpose is to analyze a known quantity of mass to normalize by MP abundance (g, mg, or particles) per kilogram of dry soil (Möller et al. 2020; Yang et al. 2021). Then, the soil must be sieved (5 mm), in order to separate stones, roots, or other more prominent elements. Is recommended to disrupt the soil aggregates and pass them through the sieve to recover MPs from the soil aggregate fractions (Möller et al. 2020; Yang et al. 2021; Zhang and Liu 2018).

After sieving, the MPs must be isolated from the soil matrix, and several methods exist (Fig. 2). Density fractionation methods are widely used to extract MPs from complex matrices such as soil and compost. This technique uses solutions with a similar density to plastics, and plastic particles have a lower density than sediments and soil (typically 2.65–2.7 g cm-3) (Li et al. 2020a; Rocha-Santos and Duarte 2017). The best results have been using saturated NaI solution (density 1.8 g cm-3), an expensive reagent. However, it can reach a density that allows the vast majority of MPs to be separated without damaging them (Scheurer and Bigalke 2018; Yang et al. 2021; Zhang and Liu 2018)

Oil-based extraction techniques take advantage of most plastics’ lipophilic properties. This methodology consists of mixing the sample with water and some oil, then MPs are separated from the matrix by shaking. Due to its oleophilic surface, MPs remain in the oil, from where they can be filtered and extracted (Crichton et al. 2017; Mani et al. 2019; Möller et al. 2020; Scopetani et al. 2020). The oil-based extracting technique has several advantages; spiked polymers are not chemically altered during treatment, and it requires minimal reagents and essential laboratory equipment. However, filters and MPs need to be carefully rinsed with hexane to remove oil traces and the interaction is not strong enough to extract fluorinated plastics like polytetrafluoroethene (PTFE) from solid samples (Crichton et al. 2017; Mani et al. 2019; Möller et al. 2020; Scopetani et al. 2020).

Fuller and Gautam (2016) developed an extraction method based on pressurized fluid extraction (PFE); this technique uses solvents at subcritical temperature and pressure conditions as an alternative. This method is fully automatized and fast, as it does not require sample purification. However, it is a destructive method, it does not allow to extract MPs larger than 30 μm, and only enable mass-quantitative analysis, not providing information on number, size, and shape of the polymer particles (Bläsing and Amelung 2018; Fuller and Gautam 2016; Möller et al. 2020).

To guarantee a reliable identification and quantification of MPs in soil, they must be purified from any biogenic material present (alive and non-living). Soil organic matter (SOM) should be removed because it interferes with some MP identification techniques such as Fourier transform infrared (FTIR) and Raman spectroscopy (Bläsing and Amelung, 2018). Various methodologies have been developed (Hurley et al. 2018; Scheurer and Bigalke 2018; Yang et al. 2021) (Fig. 2). Hurley et al. (2018) obtained the best results with Fenton’s reagent, which is an oxidation reagent that uses H2O2 in the presence of a catalyst (Fe2+). This method is performed at room temperature, it is low cost, fast, and effectively destroys highly chlorinated aromatic or inorganic compounds, typically recalcitrant in H2O2. Recently, Mbachu et al. (2021) developed a simple protocol for soil samples based on the application of cellulase, hemicellulase, lipase, and protease enzymes that digest the natural components of lignocellulosic biomass. This method proved to be effective reducing approximately 90% of organic matter. However, the authors used plant materials to simulate organic matter; nevertheless, it will be relevant to study this method in different soil samples.

Identification and quantification of MPs

There are several methods to determine MPs in soil samples. From simple visual sorting (MPs are identified by color, shape, or surface texture), to more complex techniques where MPs are determined by their chemical composition (Li et al. 2020a; Zhang et al. 2018). All these techniques are summarized with their advantages and limitations in Table 2. Nowadays, FTIR spectroscopy techniques are the most popular to identify and quantify MPs and are a promising tool for automated MP analysis. This technique provides information regarding MP abundance, shape, size, and precise identification of polymer types by recording the spectral chemical fingerprint of samples and comparing them with spectral databases. It has been used to detect MPs down to 5–10 μm (Chen et al. 2020a; Li et al. 2020a; Möller et al. 2020; Yang et al. 2021). Another common technique is Raman spectroscopy; this technique identifies substances with aromatic bonds, where FTIR has weaker intensity. Therefore, the combination of FTIR and Raman spectroscopy would be optimal for complete and reliable chemical characterization of MPs (Chen et al. 2020a; Käppler et al. 2016).

Table 2 Most common techniques used to MPs determination in soils
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Another promising tool for MP determination in soil samples is visible-near-infrared spectroscopy (vis-NIR). This technique, which allows to be implemented in portable devices, measures the reflectance spectrum of a sample, that can be used to the identify its chemical composition (Corradini et al. 2019b).

Hyperspectral imaging (HSI) has also been used in soils by Shan et al. in 2018. In this technique, the spectrum, which can be obtained in the vis-NIR or middle infrared (MIR) region, is recorded in each pixel of an image, giving spatial context to chemical information (Möller et al. 2020).

On the other hand, Watteau et al. (2018) used pyrolysis-gas chromatography/mass spectrometry (Pyr-GC–MS); to determine MPs in soil amended with municipal solid waste composts. This technique decomposes the sample in an inert gas at high temperature, then separates using gas chromatography, and finally analyzes by mass spectrometry the composition of the MPs (Junhao et al. 2021).

Accumulation of MPs in agricultural environment

Soils are essential components of terrestrial ecosystems and have intense pressure due to MP contamination. Rillig in 2012 was the first to expose this problem; he documented that there is a large accumulation of MPs in the environment mainly due to factors such as its durability and the existing technological limitation to discard or recycle the plastic produced (Barnes et al. 2009). MP’s presence in THE soil trigger changes in physical and chemical parameters, which can alter the degradation of organic matter (Liu et al. 2017). Qi et al. (2020) incubated soil with low-density polyethylene (LDPE) MPs for 4 months. They observed an electroconductivity increase, which is relevant because along with pH, and it affects the mobility of nutrients and heavy metal absorption by plants (Marschner and Rengel 2012; Zeng et al. 2011). It has also been shown that MPs can adsorb heavy metals on their surface. Soil incubation experiments with high-density polyethylene (HDPE) MPs demonstrated adsorption of zinc and an increased soil desorption capacity of cadmium (Cd). This suggests that MPs can increase the percentage of exchangeable Cd (Hodson et al. 2017; Wang et al. 2020a).

MP’s presence also produces alterations of soil physical parameters; in agricultural soils, it has been shown that 72% of the MPs were associated with soil aggregates (Zhang and Liu 2018). de Souza-Machado et al. (2018) and de Souza-Machado et al. (2019) demonstrated that polyamide (PA), polyester (PES) fibers, and PA microspheres decrease the water-stable aggregates, unlike the HDPE, polyethylene terephthalate (PET), polypropylene (PP), and polystyrene (PS) fragments that did not show statistically significant results. This indicates that the shape of the microplastic, especially the microfibers, is an essential factor influencing the soil aggregates and would decrease the soil’s structural stability (Zhang and Liu 2018). In addition, PA, PS, and HDPE increased the evapotranspiration in the soil. Evapotranspiration is relevant for numerous processes like microbial activity, precipitation, and the associated latent heat flux that helps to control surface temperatures (de Souza Machado et al. 2019; Jung et al. 2010).

Plastics are often less dense than many minerals present in soil; therefore, there is an bulk density parameter decreased by the addition of HDPE, PET, PP, and PS MPs at a concentration of 2% w/w (de Souza-Machado et al. 2018, 2019). However, experiments with lower concentrations of PS microfibers (0.3%) did not alter soil bulk density significantly (Zhang et al. 2019c). Due to MP pollution, a decrease in bulk density alters the soil pore structure, which may reduce penetration resistance for plant roots, enhance soil aeration, and influence water transport, increasing evaporation rate. In addition, physical and chemical parameters affect soil water dynamics, decomposition of organic matter, and biogeochemical cycles (de Souza Machado et al. 2019).

Regarding agricultural soil’s contamination, the most important MP entry-ways are sludge from sewage treatment and the use of plastic covers. There are also other ways of contamination, such as the use of organic amendments, compost, irrigation, flooding, fragmentation of plastic waste, and atmospheric deposition (Bläsing and Amelung 2018; Ng et al. 2018; Nizzetto et al. 2016; Xu et al. 2020).

Sewage sludge

Agricultural soils are one of the main reservoirs of MPs, and the application of sludge from water treatment plants corresponds to the highest entry of MPs (Nizzetto et al. 2016). Sewage sludge is widely used as fertilizer because its richness in organic and inorganic plant nutrients is economically advantageous to increase yields in agricultural applications (Bläsing and Amelung 2018; Nizzetto et al. 2016). Nizzetto et al. (2016) estimated that through direct application of sewage sludge, between 125 and 850 tons MPs per million inhabitants are added annually to European agricultural soils. In addition, MPs accumulate in soils with successive sludge applications over time, thus increasing their concentration. Moreover, fibers have been found in agricultural soils where sewage sludge was applied 15 years ago, and these fibers were still maintaining their original properties (Corradini et al. 2019a; Zhang and Liu 2018; Zubris and Richards 2005).

Comparing MP concentrations of sewage sludge from different countries, Chile has an average of 34,000 MP particles kg-1 (Corradini et al. 2019a), Spain has an average of 50,000 MP particles kg-1 (Van den Berg et al. 2020), and Canada has up to 11,469 MPs kg−1 (Crossman et al. 2020). MP polymer differs too. For example, Ren et al. (2020) found that 41% of MP particles in sewage sludge from Yangling in China were PVC. In contrast, Crossman et al. (2020) reported mainly PS (44%) in sewage sludge from Ontario, Canada. The differences in concentration and resin are because the regions have different dietary habits, human activities, industrial manufactures, and different wastewater treatment processes (Zhou et al. 2020b).

Fragmentation plastic covers or mulch film

Plastic films are covering around 128,652 km2 of agricultural land worldwide. The 80% of the mulched surface is found in China with estimated applications of around 700,000 t year-1, where the growth rate is approximately 25% per year (Espi et al. 2006; Zhang et al. 2019a). Plastic mulch films are widely used in intensive production systems because they contribute to modify soil temperatures, improve the water content reducing evapotranspiration, increase rooting, control weeds, and significantly increase the productivity of crops. However, plastic polymers efficiently accumulate other harmful pollutants from the surrounding environment during its use, including several persistent, bioaccumulative, and toxic substances like PCBs, dioxins, DDTs, and PAHs (Nizzetto et al. 2016). For example, Ramos et al. (2015) evidenced a concentration of deltamethrin in mulch film (584–2284 μg pesticide g-1 plastic) higher than the concentration in soil (13–32 μg pesticide g-1 soil). Furthermore, there was a recalcintrant effect on the degradation of deltamethrin adsorbed in PE film. This aspect could be very concerning because PE and PP MPs have been found in agricultural soil where plastic mulch was applied for at least 20 years and with unknow consequences for soil biota and/or biodiversity (Piehl et al. 2018).

Recent studies in soils with plastic covers and mulch films have shown up to 18,760 MPs per kilogram of soil. Moreover, soils with mulch film have more than twice MPs compared to non-mulch since the remaining plastic decomposes into smaller pieces under the action of various physical, chemical, and biological factors (Zhang and Liu 2018; Huang et al. 2020; Zhou et al. 2020a).

Atmospheric deposition

The third most important entryway of MPs to soils is through atmospheric deposition. Atmospheric deposition is understood as the flux of substances from the atmosphere onto the earth’s surface. Due to their small size and relatively low density (compared to other natural sediments), MPs are easily transported by air masses. Moreover, MPs can be transported to remote locations as has happened to MPs found in the Alps, the Pyrenees, and even the Arctic (Allen et al. 2019; Bergmann et al. 2019; Dris et al. 2016; Evangeliou et al. 2020; Klein and Fisher 2019). Currently, studies of the presence of MPs in atmospheric deposition have focused mainly on urban centers due to the possible impact on human health (Liu et al. 2019). In monitoring carried out throughout the year in Creteil (France), the atmospheric deposition of MPs ranged from 2 to 355 particles m-2 per day, indicating a high annual variability (Dris et al. 2016). In the case of the Hamburg metropolitan area (Germany), an average abundance of 275 particles m-2 per day has been reported, similar to the Chinese city of Dongguan, where up to 313 MPs particles m-2 per day were found (Cai et al. 2017; Klein and Fisher 2019).

An essential source of MPs into the atmosphere is road traffic. Cars release MPs due to physical wear of tires, as well as the wear of brakes (Kole et al. 2017). Dowarah et al. in 2020 studied the abundance of MPs in road dust in 16 sites in India, finding 227 particles per 100 g of dust, where most were fibers (92%). An alarming fact about this situation is there is a correlation between changes in the dominant wind direction and the number of MPs measured during the same period (Klein and Fisher 2019). It has also been shown that the wind can erode the soil, such as uncovered agricultural soil, and drag MPs that can be re-suspended to the atmospheric load and be transported to remote sites (Rezaei et al. 2019).

Impact of MPs on soil fauna

Soil fauna is the total population of endopedonic (living inside the soil) and amphihabitant animals (living for a time in the soil and then outside) (Bunnenberg and Taeschner 2000). Studies show that MPs and soil contaminated with MPs negatively affect soil fauna, and the magnitude of its impact depends on several factors such as the species, concentration, size, and polymers present (Huerta Lwanga et al. 2016; Pflugmacher et al. 2020). Additionally, MPs can indirectly affect soil fauna by changing the soil’s physicochemical parameters (Kim et al. 2020). Decomposition of SOM is performed in 90% by microorganisms such as bacteria and fungi. Also, decomposition is facilitated by ants, termites, earthworms, and others, which create channels, pores, aggregates, and mounds that influence the gases and water transport (Brussaard 1997; García-Palacios et al. 2013).

According to the body width, soil fauna is classified into three categories. The macrofauna (fauna of size >2 mm in diameter) are recognized as litter transformers by converting organic matter into organic structures (fecal pellets) (Xu et al. 2020). Selonen et al. (2020) studied the effect of soil contaminated with 0.02 to 1.5% w/w of PS microfibers on Porcellio scaber and observed that contaminated soil decreases feeding activity and allocates energy resources from proteins and lipids to carbohydrates, suggesting a potential depletion in energy reserves. Prendergast-Miller et al. (2019) also evidenced the effects of PS fibers in Lombricus terrestris. Treatments of 1% w/w showed a 1.5-fold lower cast production and a change in stress biomarker genes responses (24.3-fold increase metallothionein expression and a 9.9-fold decline in heat shock protein-70 expression). On the other hand, the significantly higher concentration of LDPE MPs (<150 um, 28% w/w) increased mortality and decreased the growth rate of L. terrestris (Huerta Lwanga et al. 2016). Otherwise, Song et al. (2019) demonstrated that PET microfibers can be ingested and depurated throughout the digestive system of terrestrial snails Achatina fulica. This behavior caused effects like villi damage, decreased food intake, excretion rate, glutathione peroxidase content, and total antioxidant capacity (T-AOC).

In the case of mesofauna (fauna with a size between 100 μm and 2 mm in diameter), studies have focused on the species Enchytraeus crypticus and Folsomia candida. Pflugmacher et al. (2020) showed that an increase in the concentration of HDPE MPs of 0 to 8% w/w in soil resulted in an increased E. crypticus mortality from 2 to 14%, respectively. Furthermore, when enchytraeids are exposed to soils with different concentrations of MPs, they preferred an environment with lower MP dose or an MP-free environment. MP particles used in this study (4 mm) were too large to be consumed by the oligochaete. It probably changed the soil structure, which resulted in unfavorable conditions for the Enchytraeids (Pflugmacher et al. 2020). Similar behaviors were evidenced in F. candida; springtails exhibited avoidance behaviors at 0.5 and 1% of PE MPs (w/w), and the avoidance rate was 59 and 69%, respectively. Other effects in springtails (1% MPs in soil w/w) were a decrease in the reproduction rate (70.2%) and an increase in mortality (26%) compared to the control group (Ju et al. 2019).

Lin et al. (2020) studied a high-dose of MP addition (15 g m−2), finding a decrease of abundance of oribatid mites, dipteran larvae, lepidopteran larvae, and hymenoptera ants. However, Barreto et al. (2020) found no effects on the abundance and species richness of the groups Oribatida, Prostigmata, Astigmata, Mesostigmata, and Collembola in a loamy sand soil with addition of PE and PP MPs (0.4% w/w). These different results can be explain by the use of different MPs and concentrations in both studies.

Regarding microfauna (fauna of size <200 μm in diameter), in vitro experiments using Caenorhabditis elegans nematode, show that effects depend on MPs’ concentration, size, polymer content, and additives. MPs triggered a decrease in offspring and survival rates and also produce more oxidative stress, intestinal damage, and shorter defecation intervals than the control group (Lei et al. 2018; Schöpfer et al. 2020). Recently Kim et al. (2020) studied the effect of soils contaminated with PS nanospheres or microspheres on C. elegans, finding that offspring number significantly decreased at concentrations of 10 mg kg-1 of soil, and nematodes were more sensitive to MPs (530 nm) than nanoplastics (42 nm). Moreover, a principal component analysis showed that soil composition and properties like bulk density, cation exchange capacity, clay, and sand content significantly affect the toxicity induced by these 530-nm-sized PS particles (Kim et al. 2020).

Gut microbiota present in soil fauna (springtail F. candida and oligochaete E. crypticus) has also been studied. Insects exposed to soils with MPs had a different structure of gut microbial community than insects in soils without MPs; gut microbes play a vital role in host reproduction, nutrient supply, and immunity (Ju et al. 2019; Zhu et al. 2018). Exposure to HDPE MPs increased the relative abundance of Bradyrhizobiaceae, Ensifer, and Stenotrophomonas, all associated with N fixation (Ju et al. 2019). It is estimated that biological fixation contributes globally with 180 million metric tons of ammonia per year, and these fixation processes are performed by a great variety of bacteria that have nitrogenases (Tilak et al. 2005).

Effect of MPs on the soil microbiota and nitrogen cycle

MPs are a novel surface and serve as a substrate for microorganism colonization; this ecosystem which in marine environments was called “Plastisphere” (Zettler et al. 2013) is also present in soils. Recently, next-generation sequencing (NGS) analysis of MP surface from soils evidenced different microbial communities, with lower richness and evenness in MPs compared to microbial community of soil (Huang et al. 2019; Yi et al. 2020). Also, there was differences in the microbiome on PET and LDPE, suggesting that chemical properties of MPs play an important role directing the evolution of the soil microbiome (Huang et al. 2019; Ng et al. 2021; Wang et al. 2021).

MPs in soil serve as a “special microbial accumulator” as well, enriching the bacterial groups involved in their own biodegradation (Zhang et al. 2019b). An example of this colonization is the phylum Actinobacteria, which is the most sensitive to MP addition, because it decreases in the soil, but is enriched on the surfaces of PE (Huang et al. 2019; Yi et al. 2020; Wang et al. 2020b). Actinomycetes produce extracellular polymers such as dextran, glycogen, levan, and N-acetylglucosamine-rich slime polysaccharides, facilitating their attachment to plastic surfaces for subsequent microbial action (Amobonye et al. 2020).

N dynamics in the biosphere include biological processes such as, N fixation, mineralization, nitrification, denitrification, and anaerobic oxidation of ammonium. Its incorporation is essential for soil fertility and, therefore, for plant productivity. Microbial communities play a significant role in these processes, and when soil microbial ecology is disturbed, biological processes such as nutrient cycling will be affected (Cerón and Aristizábal 2012; Rong et al. 2021). Several studies have reported that MP addition to the soil could have an impact on the N cycle at different levels; altering the microbiota and the abundance of genes, and therefore, the enzymes that catalyze the different stages of the N cycle (Fei et al. 2020; Huang et al. 2019; Qi et al. 2020; Rong et al. 2021; Wang et al. 2020b).

There is consensus that phyla Acidobacteria, Bacteroidetes, Gemmatimonadetes, and Proteobacteria are significantly more abundant in soils with the addition of PE and PP, and the composition of microbial communities plays a fundamental role in SOM decomposition (Fei et al. 2020; Huang et al. 2019; Rong et al. 2021; Yi et al. 2020; Wang et al. 2020b). In Proteobacteria, there are the families Burkholderiaceae (which is documented as a N-fixing bacteria), Pseudomonaceae (with the ability to promote both nitrification and denitrification), and Xanthobacteraceae (with fixing-N capacity). These three families increased their abundance in loamy and sandy soils with the addition of LDPE (1 and 5% w/w), PVC (1 and 5% w/w), and PP (2% w/w) (Fei et al. 2020; Yi et al. 2020; Wiegel 2006). Furthermore, in the study of Qian et al. (2018), the MP addition produced an increase in nitrite-oxidizing bacteria belonging to the Phylum Nitrospirae, which also participate in soil nitrification in agricultural ecosystems. On the other hand, the phylum Acidobacteria decreased with the addition of LDPE MPs at both 1% and 5%. Some members of this group have been reported as nitrate reducers (Kielak et al. 2016; Qian et al. 2018). Regarding silty loam soil, exposure of LDPE MPs of 150–200 μm (2 and 7% w/w) affected the soil bacterial diversity and structure. It triggered a shift in the abundance of some bacterial genera involved in soil N-cycling processing. Bacterial diversity significantly increased with 2% of LDPE MPs amendment at day 7 and significantly decreased in soils with 7% w/w of LDPE MP amendment at day 60 (Rong et al. 2021). Besides, a high concentration of LDPE MPs (7% w/w) altered the structure of nitrogen-cycling bacterial community. This increased the proportions of Mycobacterium, Gordonia, and Rhodococcus, but decreased the proportion of Azoarcus compared to control (Rong et al. 2021). In general, MPs alter microbial communities of the soil and these changes mainly depend on the polymer’s shape, quantity, and composition (de Souza Machado et al. 2018; Xu et al. 2020).

Other ubiquitous microorganisms in which the influence of MPs has also been studied are arbuscular mycorrhizal fungi (AMF). Studies showed that MPs alter symbiosis with roots. MPs of PES and PP increased the root colonization ∼8 and ∼1.4 times, respectively, but PET reduced root colonization ∼50% (De Souza Machado et al. 2019). In addition, next-generation sequencing (NGS) analysis evidenced that depending on the type and dose, MPs also alter the structure and diversity of the AMF community (Wang et al. 2020a). MPs in soil have the potential to alter the role of AMF in the nitrogen cycle, like improving soil structure and nitrogen retention (the global AM fungal N pool may be at least 70% of that in the root pool) (Hodge and Fitter 2010). This role is connected to key ecosystem services important for soil and, eventually, human health (Leifheit et al. 2021). In conclusion, the addition of MPs affects different stages of the nitrogen cycle, as seen in Fig. 3.

Fig. 3
figure 3

Factors affected by MPs that alter the soil nitrogen cycle. A Bacterial communities, genes, and enzymes. B Soil physiochemical parameters (soil aggregate fractions, evapotranspiration, soil bulk density, electroconductivity). C Soil fauna (macrofauna, mesofauna, and microfauna). D Arbuscular mycorrhizal fungi

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Effect of MPs on genes related to the nitrogen cycle

The alteration of bacterial communities due to MP addition changes the abundance of bacterial genes related to the nitrogen cycle. To date, there are few studies on the effect on these genes. Qian et al. in 2018 studied the use of plastic film and its effect on soil communities involved in the nitrogen cycle. They found that the abundance of the nifH gene increased by approximately 48%; this gene is used as a marker in the nitrogen fixation stage. On the other hand, the abundance of amoA (marker gene related to the nitrification stage) decreased by 9.8%. Regarding the nosZ and nirS genes, these genes increased 80 and 83%, respectively, but the abundance of nirK decreased 37% (Qian et al. 2018). The abundance of the nirS and nosZ genes was positively correlated with the activity of nitrate reductase. However, it showed no correlation with nirK gene abundances, indicating that the marker genes of the denitrification stage nirS-type and nosZ-type contribute more to nitrate reduction and are more active. This suggests that functional communities involved in denitrification respond differently to soils covered with plastic (Iqbal et al. 2020; Qian et al. 2018).

Nitrogen fixation

The study of the MP effects on the abundance of nitrogen cycle marker genes is a recent topic in soils as in other ecosystems as sediments and freshwater systems. However, to date, it has been shown that soils with LDPE MPs at 0.5% (w/w) did not produce significant effects on the abundance of marker genes of the nitrogen fixation stage, such as the nifD, nifH, and nifK genes (Feng et al. 2022). But high doses of LDPE (7% w/w) promoted the abundance of nifH gene (Rong et al. 2021). This results can be associated with the increase of certain genera related to nitrogen fixation, such as the genus Burkholderiaceae that significantly increased after MP addition (LDPE 1% and 5% w/w and PVC 5% w/w) (Fei et al. 2020). Furthermore, mass balance calculation of total nitrogen at the beginning and at the end of a microcosm experient with freshwater suggested a possible N input caused by biological nitrogen fixation produced by biofilms on PP MPs (Chen et al. 2020b).

Nitrification

Regarding the nitrification stage, studies with soil are based on the abundance of amoA gene, which codes for the ammonia monooxygenase enzyme that oxidizes ammonia (NH3+) to hydroxylamine (NH2OH) (Seeley et al. 2020). Rong et al. (2021) showed that addition of LDPE MPs (2% w/w) promoted the bacterial amoA gene abundance on day 15, but not the following days. These results showed a positive correlation with nitrifying bacteria Nitrosopira (r = 0.662, p = 0.007). Moreover, the addition of high-dosage LDPE MPs (7% w/w) promoted the bacterial and archaeal amoA genes abundance on day 60. However, the addition of LDPE MPs (2% and 7% w/w) also produced a decrease in the amoA gene abundance of archaeas on day 15. This suggests that LDPE MPs can occasionally inhibit the abundance of ammonia-oxidizing archaea (AOA)-amoA gene (Rong et al. 2021). The amoA gene abundance has also been studied in other environments such as sediments and freshwater. In sediment studies with MPs, it has been shown that abundance of ammonia-oxidizing bacteria (AOB)-amoA gene increased from day 7 to day 16, suggesting enhanced nitrification potential with time (Seeley et al. 2020). Furthermore in freshwater systems, the addition of MPs with biofilms further increased nitrification ability in the system (Chen et al. 2020b).

Denitrification

Regarding the denitrification stage, the addition of high doses of LDPE MPs to the soil (7% w/w) promoted the abundance of nirK gene on day 90 and nirS genes on days 7 and 15. However, the stimulative effects on nirS gene were temporary and decreases by day 90. The results about nirK gene abundance are positively correlated with the abundances of denitrifying bacterias Pseudomonas, Stenotrophomonas, Brachybacterium, and Achromobacter (r > 0.5, p = 0.5) (Rong et al. 2021). On the other hand, Ren et al. (2020) studied the effect of the MP addition to a fertilized soil on the emission of greenhouse gases, concluding that LDPE MPs (5% w/w) decreased the emission of N2O by changing the abundances of microbes related to N2O emissions. The impact of MPs on the nitrogen cycle has also been studied in sediments, where microcosm experiment with sediment and PVC MPs exhibited a decrease of relative abundance of nirS gene and a low potential rate of denitrification too (Seeley et al. 2020). However, in experiments adding MPs to activate sludge and MPs with biofilms at a freshwater systems, the denitrification has been promoted (Chen et al. 2020b; Li et al. 2020b).

Studies conclude that addition of MPs to the soil produces effects on the nitrogen cycle and additional studies are required to measure the real impact on the different stages of the nitrogen cycle.

Effect of MPs on soil enzymatic activity

Bacterial communities are the main enzyme producers in soils, and MPs alter the bacterial structure and affect the soil enzymatic activity (de Souza Machado et al. 2018; Xu et al. 2020; Zhang and Liu 2018). Urease catalyzes the conversion of urea to ammonium that will be oxidize in the nitrification process. The effects on this enzyme depend mainly on the MPs used, concentration, and experiment extension time. In the study of Yi et al. (2020), a higher urease activity was observed in soils treated with MPs of LDPE and PP at 2% (w/w) on day 14, but this activity decreased by 31% on day 29 compared to the control. However, in a different study, urease activity was stimulated in soil with LDPE MPs during the 90 days of the experiment, although this effect was probably due to the lower concentration of MPs that was used (0.0076% w/w) (Huang et al. 2019). Alterations in the community structure, gene expression, and synthesized enzymes result in variations in the nitrogen content in soil. For example, in the study of Liu et al. (2017), when MPs of PVC at 7 and 28% (w/w) were added, the total dissolved nitrogen (TDN) and dissolved organic nitrogen (DON) content increased significantly after days 7 and 14, respectively. However, between days 7 and 30, the addition of MPs did not produce significant changes in NO3- and NH4+ compared to control. On the other hand, Yan et al. (2020) showed that paddy soil with concentrations of 1% of PVC had a 13% lower NO3- content than soil without MPs. These contradictory results are likely because different concentrations of PVC or soils were used, and therefore, there were different physical, chemical, and biological characteristics. It has also been shown that MPs alter the nitrogen cycle directly too, by enriching the soil with nitrogen, particularly when PA MPs are added because their composition is rich in nitrogen (de Souza Machado et al. 2019).

Conclusión, challenges, and future directions

It is important to know the MP load that the soils contain; for this purpose, a representative sampling and suitable processing of soils must be performed. Then, a combination of FTIR and Raman spectroscopy would be optimal for complete and reliable chemical characterization of MPs. MPs are classified as emerging pollutants and like any pollutant, and it alters the ecosystem it enters. It has been shown that the addition of MPs to soils alters biogeochemical cycles, such as the nitrogen cycle, and does it directly by adding MPs that have nitrogen in their chemical structure. However, these alterations can be indirectly too, i.e., by modifying the microbiota/enzymes that catalyze reactions in the different stages of the nitrogen cycle in the soil, by changing the soil fauna that is responsible for facilitating the decomposition of organic matter, and/or by altering the physicochemical parameters of the soil such as evapotranspiration, electrical conductivity, and/or the proportion of microaggregates.

The global effects of MPs on the nitrogen cycle are still unknown, since the studies to date have been performed under soil plant (leguminous) in laboratory conditions. Also, field experiments to study the changes in the nitrogenous species should be performed for better understanding of the N-biological stages and processes afected. This is necessary since the consumption of plastic is increasing, and with it, the accumulation of MPs, as their natural degradation, is limited. These analyses would show strong evidence that could be used to conduct appropriate agronomic practices and public policies to reduce the consumption and disposal of plastics to mitigate their effects. Additionally, understanding the impact of MPs on the nitrogen cycle is important because this cycle is a key predictor of ecological stability and management in the terrestrial ecosystem.