The industrial production of plastics exploded in the mid-20th century. Since then, plastics have directly influenced the global economy, and have been mass-produced in a large number of industrial sectors due to their low cost, and great durability and versatility for varied applications [1]. Before and during the COVID crisis, plastic production increased worldwide from 359 t in 2018 to 369 t in 2020, with the highest production in Asia (49%) and specifically in China (32%). The European demand of plastics, mainly for packaging (ca. 40%), represented a total of 49.1 × 10⁶ t in 2020, with Germany (23.3%) and Italy (14.1%) at the head, among the different European countries [2].

Despite their excellent properties and cost effectiveness, conventional plastics produced from non-renewable resources have a major drawback: they are extremely resistant to natural degradation processes. In other words, they persist in the environment for long periods of time, with half-lives between 0.035 and 9000 years [3]. At present, a great effort is being made to reduce the impact of plastics in the environment through the establishment of different measures such as more stringent regulations on their use and disposal in landfills, the improvement of their reusability and recyclability, or their total or partial replacement by novel materials with similar properties and less impact (for instance, bioplastics and/or biodegradable and compostable polymers) ^[4][5][4,5]. Nevertheless, plastic debris is frequently pointed out as the protagonist of serious pollution episodes of natural ecosystems. Of special concern are plastic particles smaller than 5 mm, commonly referred to as microplastics (MPs), and even smaller (less than 0.1 µm or than 1 µm, depending on the authors), defined as nanoplastics (NPs) ^[6][7][6,7], which have emerged in recent years as main contributors of pollution since they are dispersed in aerial, terrestrial and aquatic environments. Hence, these emerging pollutants may potentially accumulate in the trophic chain and cause detrimental effects on plants, animals and human beings [7].

Monitoring studies have shown worldwide the presence of MPs in aquatic sources, with polypropylene (PP) and polyethylene (PE) as the most frequently found ^[8][9][8,9]. Already in the 1970s, Carpenter and Smith [10] alerted about the risks associated with the increasing production of plastics and their uncontrolled disposal. Although the concentration of MPs in the aquatic environments is continuously increasing and raising public concern, MPs are usually more abundant in soil than in water. In fact, their content in terrestrial ecosystems has been estimated to be between 4 and 23 times higher than in the oceans [11]. Furthermore, the presence of MPs in freshwater environments has been reported to be a direct consequence of land transference or runoff ^[12][13][12,13]. However, when Xu et al. [14] studied a large watershed estuarine area of Bohai Sea, they found double as much MP in the soil (40–980/kg) than in the adjacent sediment (6.7–320/kg). The authors were unable to establish relationships between the two compartments, which suggests low transfer between them, acting the soil as a sink for MPs. In addition, MP vertical and horizontal transport will depend on the type and density of the vegetation covering its surface, on the topography and on the rainfall amount [15]. However, the zones heavily polluted with MPs are mainly placed in densely populated areas and their abundance diminishes with altitude [16].

2. Sources of Plastics in Agricultural Soils

Monitoring studies in the last decades has shown worldwide the presence of MPs in aquatic sources, both in marine and freshwater environments ^[17][18][23,24], even in remote high mountains lakes ^[19][25]. Although previously a few works had considered the presence of plastics in terrestrial environments, it was not until 2018 when several articles ^[20][21][26,27] and reviews ^[6][22][23][6,28,29] alerted about the presence of plastics in agricultural soils, and since then, the related scientific literature has grown exponentially. However, one of the main challenges for establishing the actual level of MP pollution in soil is the development of robust and reliable measurement methods. This, together with the renewed general interest for MPs, has led to the development of novel analytical technologies ^[24][25][26][30,31,32] aimed to simplify and standardize the existing laborious protocols. According to recent review reports, landfills, beaches, urban areas and agricultural lands are the soil ecosystem areas most largely affected by MPs ^[22][27][28,33]. In general, MPs reach soils by either direct addition of plastics manufactured from varied industrial sectors (for instance personal care and hygiene products, paints, adhesives, pharmaceuticals or synthetic agrochemicals), or indirectly after embrittlement and subsequent fragmentation of plastic parts from varied sources such as landfills, household items, or agricultural practices ^[6][11][28][6,11,34]. Among all the entry routes, municipal solid waste, wastewater treatment plants (WWTPs) and, even, atmospheric deposition after wind and rain events have been identified as major contributors to soil pollution by MPs ^{[29][30][31][32][33]}[35,36,37,38,39]. Focusing on agricultural soils, the sources of pollution by MPs have been recently reviewed and consider direct (plastic mulching, plastic pipes, plastics used to cover plants and, in general, plastic materials used in agriculture) or indirect inputs (sewage sludge, compost, irrigation, etc) ^[34][35][36][40,41,42]. In agricultural soils, Huerta Lwanga et al. ^[36][42] found >3000 MP/kg soil, increasing to >5000 MP/kg soil for some agricultural practices, such as sewage sludge amendment or plastic mulching (mostly PE). Wang et al. ^[35][41] reported higher values (>42,000 MP/kg) in Chinese agricultural soil, while very high values were found in suburban soil (4.1 × 10⁵ MP/kg) ^[34][40]. From the above results, mulching (mulch films and greenhouse materials) has been identified as a major contributor of MP entry in agricultural soils ^[27][37][33,43]. Meng et al. ^[37][43] also point to high-intensity machine tillage as responsible for higher fragmentation of macro-plastics and more severe MP pollution. In areas with extensive mulching, the combined effect of light exposure and mechanical forces like tillage favors the formation of MPs ^[38][44]. Indirect plastic input routes are irrigation (polluted water, wastewater) and use of amendments rich in organic matter, such as urban sewage sludge or compost, which are agricultural practices that are very usually employed in OC-poor soils or in arid/semiarid environments ^[39][40][41][45,46,47]. The WWTPs are usually very efficient in removing MPs, being able to induce a decrease of >90% and up to >98% in some cases ^{[42][43][44][45]}[48,49,50,51]. Apart from the specific wastewater treatment, the efficiency of MP removal depends greatly on the nature, shape and size of MPs, with fibers generally being less retained. Despite the high removal efficiency and due to the large volumes processed, the small fractions of MPs released from WWTPs result in significant amounts of MPs entering the environment when agricultural soils are irrigated with wastewater. The second main indirect way of MP entry to the soil is the use of organic amendments, both fresh and after biological treatment. Tan et al. ^[46][52] reported that, among different organic materials including food waste, livestock manure and sludge, the latter has greater capability to accumulate MPs followed by food wastes and manure. The major fraction of MPs included PE, PP and polyethylene terephtalate (PET), both as fibers and films. The presence of cracks, holes and other defects in MPs after biological treatment (for example composting) indicated that MPs could be further fragmented during the process, which implies greater ecological risks. To avoid this, the authors recommended the minimization of MPs input into the waste before the treatments. Regarding this, Yang et al. ^[47][53] demonstrated that the removal of plastic packages reduced the amount of MPs and the variability of polymer types in a digestate produced through anaerobic digestion of biogenic wastes. Due to their effective removal from wastewaters in WWTPs, MPs concentrate in sewage sludge, passing to the agricultural lands when it is applied as soil amendment ^{[48][49][50][51][52]}[54,55,56,57,58]. It has been estimated that application of sludges can provide up to 430,000 t of MPs to the agricultural lands in Europe ^[53][59]. The type of treatment in the WWTP affects the ability to reduce MPs in the effluents. Bayo et al. ^[54][60] found that fibers were less efficiently retained in WWTPs than particulate MPs (56.2 vs. 90.0%) suggesting that some purification treatments clearly discriminate between MP forms. Similarly, in a study in China, the dominance in effluents of fibers (76.7–90.0%) and small particle sizes (<2.0 mm, 62.5–81.5%) suggested that they escaped easily from WWTPs ^[55][61]. Recently, van den Berg et al. ^[51][57] studied the accumulation of MPs in 16 agricultural fields located in the east of Spain with 0–8 sewage sludge applications at a rate of 20–22 t ha⁻¹ per application. Soils without addition of sewage sludge had an average load of 930 ± 740 and 1100 ± 570 for low- (<1 g cm⁻³) and high-density (>1 g cm⁻³) MPs, respectively. After soil amendment, these amounts increased to 2130 ± 950 for light density plastics and to 3060 ± 1680 for high-density plastics. On average, each consecutive application of sewage sludge enhanced the concentration of MPs by 280 MPs kg⁻¹ for low-density plastics and 430 MPs kg⁻¹ for high-density plastics. In other words, consecutive application of sewage sludges led to the accumulation of MPs in soils. Rough estimates point to an annual deposit on European farmlands of 63,000–430,000 t of MPs through sewage sludge application ^[56][62]. This situation creates a pathway for the entrance of MPs in the soil environment with unknown consequences. For instance, although Hernández-Arenas et al. ^[57][63] found that sewage sludge containing MPs fostered the growth of tomato plants, it also delayed and reduced fruit production, but it was not completely clear whether other factors were simultaneously occurring. It has also been reported that the application of MP-containing sewage sludge widened MPs contamination to nearby land with no history of direct sewage application ^[58][64]. On the other hand, neither soil erosion nor surface runoff have been identified as relevant mechanisms of MPs transport/export processes, pointing to the fact that agricultural soils can be considered as long-term accumulators of MPs in arid and semi-arid regions ^[59][60][65,66]. Compost represents another major carrier of MPs into agroecosystems ^[61][62][67,68]. Irrespective of the origin of the materials to be composted, fragmentation processes of plastics into MPs accelerate under composting conditions, thus posing a threat of agricultural soil pollution when composts are used as organic amendments. Composting of the organic fraction of municipal solid wastes has been recently reported to produce a total of 10–30 plastic particles g⁻¹ dry compost, from which MPs accounted for 5–20 particles ^[63][69]. These authors conducted a comprehensive analysis focusing on the shapes and types of the MPs present in the compost, and highlighted the dominance of fibers. Moreover, they found that PE, polystyrene (PS), polyester, PP, polyvinyl chloride (PVC) and acrylic polymers (in order of abundance) represented 94% of the MPs items. In a prior study, van Schothorst et al. ^[64][70] urged for the need to establish threshold levels for MPs in composts in order to effectively regulate the total influx of plastic pollution to agricultural soils. They demonstrated that a compost produced from municipal organic waste had major potential to accumulate MPs (PE and PP in the size range 0.03–2 mm) than that obtained after the composting of gardening waste. When used as organic fertilizers in two Dutch farmlands with an annual compost application of 10 t ha⁻¹, the composts led to MPs accumulation in soil samples, averaging 888 ± 500 MPs kg⁻¹ soil. After the composting of biowaste and sewage sludge, Scopetani et al. ^[65][71] detected a concentration of MPs in compost of 6.6 ± 1.5 pieces kg⁻¹. They estimated that 4–23 × 10⁷ MP pieces ha⁻¹ year⁻¹ could reach agricultural soils fertilized with such compost by following common recommendations for compost application. The addition of composted household organic waste has been also reported to release MPs on agricultural soils. In this sense, Gui et al. ^[66][72] concluded that the composting of rural domestic waste was a significant source of MPs in soils, since MPs amount in compost related to both the quantity and type of plastic before the production of the compost. They found 2400 ± 358 MPs items kg⁻¹ dry compost, with polyester, PP and PE in the form of fibers and films as the most common MP types. 

3. Effect of MPs on Soil Health and Functioning

As indicated before, soil is the main sink of MPs in the environment [11]. The presence of MPs in the soil profile may alter some relevant soil properties related to the behavior of pesticides in soil.  MPs adsorb on soil by surface sorption, electrostatic or hydrophobic interactions, and this sorption increases with soil organic carbon (OC) and Fe/Al oxide contents but decreases with clay content and increasing values of pH and ionic strength ^[67][68][69][73,74,75]. MPs will be mainly concentrated in top soil layers, because infiltration capacity is very low ^[59][60][65,66], or because plants’ root growth will accumulate them in the top soil ^[70][76]. The presence of plastics in soil has been reported to affect some relevant physicochemical soil properties, although contradictory results about the trends observed are often found. In general, the studies analyzed agree that MPs, especially fibers, reduce the soil bulk density and modify the size distribution and stability of soil aggregates altering soil pore structure and water transport ^[71][77]. Therefore, MPs may potentially play a crucial role in the leaching of pollutants to deeper soil layers. These effects in soils have been proven in alterations in the performance of some crops, such as in the water-use efficiency of maize ^[72][78] or in the blockage of root growth of garden cress ^[73][79]. De Souza Machado et al. ^[74][75][80,81] found that a wide variety of plastics differing in length and diameter reduced, in general, soil bulk density because plastics are usually less dense than soil. Moreover, some MPs affected the soil water holding capacity or induced a decrease in the water-stable aggregates. In line with this, the presence of large-sized PE in soil (2–10 mm) has been reported to significantly accelerate the water evaporation process from soil, forming surface cracks, because they destroy the integrity of the soil surface structure and reduce the tensile strength of the soil surface at their interface ^[76][82]. However, Zhang et al. ^[77][83] found no obvious changes in the soil density or saturated hydraulic conductivity in clay-like soil in both pot and field trials. The same authors found, only in the pot experiment, an increase in the contents of water stable large macroaggregates. This divergence was explained because of differences in the physical and chemical characteristics of the soils. The results of a meta-analysis ^[78][84] confirmed most of the reported changes in soil properties, such as making the soil more porous or increasing its water retention, but without relevant changes in soil microbial diversity, suggesting that MPs would occupy physical soil space but would not be integrated into the soil biophysical matrix. The presence of MPs has also led to increased ^[79][85], decreased ^[80][86] or not changed ^[81][87] soil pH, depending on MPs type, shape, dosage and exposure time ^[82][88]. MPs increase the dissolved organic carbon (DOC) content ^[82][83][88,89] depending on the degree of MPs degradability. However, since MPs tend to slowly degrade in the environment because plastics are relatively inert [3], DOC from this source could not be generated in a large amount ^[83][89]. MPs will also indirectly affect organic matter and DOC contents due to their influence on soil microbial biomass and activity ^[82][84][88,90], changing the decomposition and transformation of organic materials. However, Steinmetz et al. ^[38][44] alerted that application of plastic mulches would potentially favor soil organic matter degradation because of increased soil temperatures. No significant influence of MPs on the amounts of available phosphate, nitrate, and ammonium, and on crop growth was either observed in a meta-analysis based on 32 works ^[83][89]. In contrast, De Souza Machado et al. ^[75][81] found an enrichment of soil N and N content in onion leaves when polyamide (PA) fibers were added to the soil because this plastic contains N in its molecule that could be released to the environment. In a soil containing six different plastic types, the authors also found a generally positive effect on the growth of onions and their colonization by arbuscular mycorrhizal fungi, which in turn, would contribute to an improvement in nutrient availability and plant nutrient content. The positive effect of MPs on the development of arbuscular mycorrhizal fungi might be attributed to the changes caused in the soil structure, which have a direct impact on the habitat space and conditions for these fungi. Qi et al. ^[85][91] have also reported an increase in the C:N ratio due to the presence in the soil of low-density PE and biodegradable plastic mulch films. MPs have been also shown to affect microbial activity in the soil environment ^[80][86]. For instance, various meta-analyses have recently concluded that MPs would not or only slightly reduce soil microbial diversity but would significantly enhance soil microorganism amounts and activity ^[78][83][84,89]. In contrast, Awet et al. ^[86][92] reported a negative effect of PS on microbial biomass and enzyme activities in soils, with an increase in basal respiration rate and metabolic quotient due to increased cell death. Likewise, Yi et al. ^[87][93] indicated a decrease in urease, dehydrogenase and alkaline phosphatase activities after the addition of three types of MPs (membranous PE, fibrous PP and microsphere PP). The effects of microsphere PP were different from the other two, probably owing to the different sizes and lower concentrations. Fei et al. ^[88][94] added PVC and PE to acid soil (pH 5.5) and found that both MPs inhibited fluorescein diacetate hydrolase activity, stimulated urease and acid phosphatase activities, and declined the richness and diversity of the bacterial communities. The effects were, in general, more severe in the PE-treated soils. On the contrary, Brown et al. ^[89][95] reported no changes in soil bacterial community diversity or in the size and structure of the PLFA-derived soil microbial community, Liu et al. ^[90][96] measured a stimulation of FDAse activity in soil driven by PP, while Zang et al. ^[91][18] found that PE and PVC increased the soil microbial biomass and changed the structure and metabolic status of the microbial community. These changes would significantly influence key pools and fluxes of the soil C cycle with the response being both dose-dependent and MP-specific. The soil properties and functions discussed in this section include those that have been reported as directly modified by the presence of MPs in soils. However, other soil properties can be indirectly affected by changes caused by the aforementioned ones. Apart from soil organic matter, soil electrical conductivity has shown unclear trends (increases, decreases or no modifications) under MP addition ^[80][81][85][86,87,91]. In general, as suggested by Qi et al. ^[81][85][87,91], the effects of plastic debris on soil parameters are highly variable because they are present in different types, sizes and contents, factors that may additionally interact among them in complex and sometimes little understood ways.