Effects of microplastics polluted soil on the growth of Solanum lycopersicum L.

Abstract

This study employed two prevalent plastic products - straws and microfiber as microplastics (MPs) to elucidate their largely unexplored effects on soil’s properties and the growth of the tomato plant (Solanum lycopersicum L.). For this experiment, a completely randomized design (CRD) was adopted where, straw - polypropylene (PP), microfiber - polyester (PES) + polyamide (PA), and their combinations (PP + PES + PA) were mixed with soil using different concentrations – 0% (control), 0.4%, 1%, and 2% (treatments) and kept for 45 days at room temperature. The findings demonstrated that incorporating 2% mixed MPs in soil significantly decreased bulk density and electrical conductivity 7.29% and 67.3%, respectively, while soil pH increased 17.84% in cultures containing 1% microfiber. Maximum water holding capacity (MWHC), soil organic carbon (SOC), and soil organic matter (SOM) showed varied responses based on MPs type and concentration. Specifically, MWHC increased 16.4% with 2% microfiber but declined 13.3% with 0.4% straw. The highest decreased (30.65%) in SOC and SOM were evident in cultures with 1% microfiber whereas increased 9.68% and 8.33% in cultures with 0.4% straw. In terms of the growth traits of S. lycopersicum, substantial reductions in plant height (56.37%), leaf number (54.37%), and girth diameter (56.43%) were observed in 2% straw containing cultures. Although no plant mortality was noted, the most pronounced reductions in leaf area (62.44%) and total plant biomass (68.16%) occurred in 2% microfiber cultures. Therefore, the ramifications of these findings may contribute to a deeper comprehension of the mechanisms and effects of MPs on soil properties and above-ground plant growth.

Introduction

Globally, microplastics (MPs) are increasingly recognized as major contaminants in soil ecosystems, owing to their pervasive distribution and associated environmental risks (de Almeida et al. 2023; Yang et al. 2021a). MPs encompass a variety of forms categorized as primary and secondary types. Primary MPs are directly introduced into the environment as microbeads, plastic pellets, and fibers, while secondary MPs originate from the breakdown of larger plastic items under UV radiation or through weathering processes (Avinash et al. 2023). These MPs vary widely in size, shape, color, and polymer composition, with polypropylene (PP) and polyethylene terephthalate (PET) commonly found in both soil and marine organisms (Yoon et al. 2022; Bi et al. 2023). In terrestrial environment, MPs accumulate through different pathways, including agricultural practices like vinyl mulching (Ng et al. 2018; Steinmetz et al. 2016), the use of compost as a soil amendment (Bradney et al. 2019), sewage irrigation (Nizzetto et al. 2016), application of coated fertilizers (Katsumi et al. 2021), tire abrasion (Kim et al. 2004, 2006), landfills, household appliances such as washing machines, tumble dryers (Rillig 2012) and personal care products (Cole et al. 2011). As a result, the annual deposition of plastics in soil ecosystems exceeds that in aquatic environments by a factor ranging from four to twenty-three times (Horton et al. 2017).

In soil environment, MPs exhibit vertical movement through interstitial gaps of soil to deeper layers (Li et al. 2021a) and over time, these become integrated into the soil matrix due to external influences such as bioturbation and human activities (Dissanayake et al. 2022). Soil organisms like earthworms and collembola species occasionally facilitate both vertical and horizontal transport of MPs through adhesion or excretion (de Souza Machado et al. 2019). Moreover, in arid climates soil fractures facilitate the movement of MPs below the surface of the soil (Yang et al. 2021a). In natural environmental settings, MPs undergo significant aging processes involving chemical, biological, and physical transformations (Zurier and Goddard 2021). These changes in MPs’ environmental behavior can impact soil physical and chemical properties, as well as plant growth indicators (Zeb et al. 2023). Soil physical parameters, such as bulk density (BD), maximum water holding capacity (MWHC), water-stable aggregation (WSA), porosity, and permeability are significantly influenced by MPs (Yu et al. 2023; Ingraffia et al. 2022; de Souza MacHado et al. 2018). For instance, Yu et al. (2023) observed a decrease in BD with the addition of polyester (PES) fibers and PP granules in silty loam soil, whereas BD remained unaffected in clay loam soil with PES fiber addition (Zhang et al. 2019). The photo-oxidation process of plastics releases chemicals into soil water, altering soil chemical characteristics (Maqbool et al. 2023), which occur through diverse and poorly understood pathways depending on polymer types and soil characteristics (Dissanayake et al. 2022). These substances, as noted by Bandow et al. (2017) alter soil cation exchange capacity and cation concentrations in the soil solution, consequently affecting soil pH, nutrient availability, soil organic matter (SOM), and soil urease activity (Maqbool et al. 2023; Hegan et al. 2015). For instance, the introduction of PET MPs into loam inceptisol soil reduced soil pH, with higher doses causing a significant pH decreased (Gharahi and Zamani-Ahmadmahmoodi 2022). Overall, the impact of MPs on soil physical and chemical properties is complex and varies depending on soil characteristics, MP properties, and environmental conditions.

Despite increasing attention to environmental MP pollution, research on the interactions between MPs and plants remains limited. Additionally, MPs have the potential to enter plant tissues and translocate to various plant parts (Li et al. 2024), leading to a range of adverse effects on plant health and development (Zeb et al. 2023; Jia et al. 2023; de Almeida et al. 2023; Dhevagi et al. 2024). According to Kumar et al. (2022) plants can be impacted by MPs contaminated soil via apoplastic (passive diffusion) and symplastic (osmotic uptake) pathways, followed by transport throughout the plant via vascular system. Research indicated that MPs could inhibit seed germination, root elongation, and nutrient absorption, as well as induce oxidative stress, cytotoxicity, and genotoxicity (Jiang et al. 2019; Mondal et al. 2022). They can alter plant development, mineral nutrition, photosynthesis, and metabolite production, and can be absorbed by roots and translocated to different plant parts (Dhevagi et al. 2024; Mészáros et al. 2023). The presence of MPs in soil negatively affects plant growth, leading to physical growth reduction, altered water, and nutrient uptake, drought induction, excessive reactive oxygen species (ROS) production, hormonal regulation disruption, and declines in chlorophyll content and photosynthesis (Wang et al. 2022a; Zeb et al. 2023; Jia et al. 2023). Furthermore, the type of polymer and the degradation state of the MPs can modulate their effects on plant-soil systems, with photodegradation exacerbating these impacts (Lozano et al. 2024).

The impact of MPs on plant growth is influenced by several key factors, including size, shape, polymer composition, and concentration. As MPs fragment, their particle size diminishes, increasing their specific surface area and enhancing their potential to adsorb various contaminants and interact with plant roots (Jia et al. 2023). A study by Abbasi et al. (2020) demonstrated that MPs with larger surface areas absorbed more heavy metals around the rhizosphere, thereby significantly inhibiting the root growth of wheat. Similarly, Liao et al. (2019) showed that 100 nm polystyrene (PS) MPs reduced tomato (Lycopersicon esculentum L.) seed germination to a greater extent than 5 μm PS MPs. These findings are consistent with the studies by Bosker et al. (2019) and Wang et al. (2023) that larger particle sizes of MPs significantly impede plant growth compared to smaller ones. Physical differences also drive shape-related variations, with sharply-edged fragmented MPs being more detrimental to plants than those with smooth edges (Nematollahi et al. 2022). Experimental results showed that duckweed (Lemna minor) exposed to sharp-edged MPs exhibited significantly reduced root lengths compared to those exposed to smooth-edged MPs and control groups (Kalčíková et al. 2017). The polymer composition of MPs is another crucial factor, as various plastic types have distinct effects on plant growth (Zeb et al. 2023). For example, research by de Souza MacHado et al. (2019) demonstrated that the root biomass of spring onions (Allium fistulosum) was greater in treatments with polyethersulfone (PES) and PS compared to those with high-density polyethylene (HDPE), PP, and PET. Additionally, the concentration of MPs can differentially affect plants, with both low and high dosages producing significant impacts depending on the plant species and study conditions (Li et al. 2024; Wu et al. 2020). For instance, an experiment conducted by Yang et al. (2021b) demonstrated that high concentrations of biodegradable polylactic acid (PLA) substantially decreased maize shoot and root biomass, showing notable phytotoxicity, whereas low concentrations stimulated plant growth. Another study by Li et al. (2024) confirmed that cotton plants cultured in hydroponic conditions exhibited a dose-dependent phenotypic response to varying concentrations (100, 300, and 500 mg/L) of carboxyl-modified polystyrene (PS-COOH) MPs, with transcriptomic analysis revealing oxidative stress, altered sucrose catabolism, and the accretion of lignin and flavonoids.

The effects of MPs on various crops have been well-documented, including rice (Wu et al. 2020; Dong et al. 2021), spring onion (Rillig et al. 2019; de Souza Machado et al. 2019), carrots (Lozano et al. 2021), cotton (Li et al. 2024), maize (Wang et al. 2020a), wheat (Zhou et al. 2021), cucumber (Liu et al. 2024), lettuce (Lian et al. 2024), and perennial ryegrass (Boots et al. 2019). Since, different polymer types, concentrations of MPs, and incubation times, affect differently in each plant species. In this study, tomato (Solanum lycopersicum L.) was chosen as the test species due to its adaptability to various climates and its high lycopene and mineral content, which contribute to its economic and ecological importance (Veilumuthu and Christopher 2023; Mendonça et al. 2021). Previously, research on tomatoes has explored MPs’ effects on seed germination (Sahasa et al. 2023; Shorobi et al. 2023), phytotoxicity (Shi et al. 2022), nutrient uptake (Shorobi et al. 2023), physico-biochemical properties, root exudates in hydroponic conditions (Shi et al. 2023), and fruit development (Hernández-Arenas et al. 2021). However, earlier studies mainly focused on the effects of polyethylene (PE), PET, and PP polymers. Therefore, our study utilized commonly used microfiber towels composed of polyester (PES) and polyamide (PA) polymers, as well as straws derived from PP polymers. Microfibers, also referred to as MPs fibers, are the most frequently encountered MPs in the environment (Acharya et al. 2021). According to Galvão et al. (2020), an average 6 kg wash load of microfiber towels releases approximately 18 million synthetic microfibers, predominantly size shorter than 500 μm. Conversely, disposable plastic straws have drawn significant attention due to their short usage duration of about 30 min before disposal (Chitaka et al. 2020; Zhang et al. 2022). Hence, this study investigated the effects of these two prevalent types of plastic products (microfiber towels and straws) as MPs in soil ecosystems, both individually (PP/PES and PA) and in combination (PP + PES and PA), on soil physical and chemical properties, as well as the growth of Solanum lycopersicum L.

Materials and methods

Experimental design

A completely randomized design (CRD) was implemented for this study, encompassing the following key stages: (i) preparation of MPs and mixing with soil, (ii) seed collection and germination of seedlings, and (iii) transferring plants into experimental pots.

Soil sample collection

The soil used for the study was collected from urban agricultural land in Mirpur-1, Dhaka, Bangladesh. A random sampling approach was adopted for sample collection, and soil was collected and placed into a stainless-steel pot from the topsoil (0–10 cm). Overnight, the sampling stainless steel pot was soaked and cleaned in a 10% HNO₃ solution. Debris was removed from soil samples after these were air-dried. Before being stored for subsequent use and examination, the soil was crushed and sieved with a 2 mm mesh sieve (Alam et al. 2018; Rahman et al. 2012). The basic properties of the test soil were BD 0.96 ± 0.011 gm/cm³, MWHC 65.33 ± 4.73%, pH 5.44 ± 0.04, EC 440 ± 0.5 µS/cm, SOC 0.614 ± 0.015%, and SOM 1.076 ± 0.015%.

Collection of plastic materials and preparation of microplastics

Two types of plastics were used for this experiment. One is plastic straws, and the other is microfiber towels (Fig. A.1). Plastic straws made with PP (Fig. A.1 a) and fiber were a mixture of 70% PES and 30% PA (Fig. A.1 b) in this study. These two types of plastics were chosen because these are so small that these cannot be separated from ordinary waste for recycling. Their usage automatically increases day by day due to accessible applications. For using MPs from straws, plastic straws were randomly cut by scissors; then, the fragmented parts were blended using a high-speed blender and then sieved through a 2.36 mm sieve mesh (Verla et al. 2019). The microfibers were cut into smaller pieces by scissors, and the length of the fibers was 4 mm (Lozano et al. 2021).

Seed collection and germination of seedlings

Seeds of S. lycopersicum were collected from the Bangladesh Agricultural Development Corporation (BADC). Before planting, the viability of the seeds was tested using the flotation method. Then, seeds were first sown in the seedbed and allowed to grow until these were around 5–6 cm tall before transferring into the experimental pot.

Mixing of microplastics with soil

After preparing soil and MPs, these were mixed in three concentrations (0.4%, 1%, and 2%). To achieve a 0.4% concentration, 8 g of MPs were incorporated into 2000 g of soil; for a 1% concentration, 20 g of MPs were mixed with 2000 g of soil; and for a 2% concentration, 40 g of MPs were combined with 2000 g of soil. These specific concentrations were selected for each pot based on the potential for significant plant responses at these levels (Verla et al. 2019).

Treatments

When planning the experiment, three aspects were considered. Single: straw/microfiber, mixture: (straw + microfiber), and MPs concentrations: (0.4%, 1%, and 2%). In each pot, 8 g, 20 g, and 40 g of MPs were added to 2000 g of soil for 0.4%, 1%, and 2% concentrations, respectively and 0% concentration contained no MPs which was termed as the control. For each treatment (0.4%, 1%, and 2%) and control (0% MPs) three replications (n = 3) were considered. In total, this study consisted of 30 pots of which 27 treatments (0.4%, 1%, and 2% MPs) and 3 controls (0% MPs) were used for planting seedlings (Table 1).

Table 1 An overview of the experimental setup

Incubation time

After mixing, the soil was incubated for 45 days. This initial incubation allowed contact between the soil microbiome, MPs particles, and potential plastic component leaching. The experimental soil was kept dark during the incubation period. To maintain high moisture levels, water saturation was monitored three times a week. A spray bottle was used to gently apply distilled water to soil surface (de Souza Machado et al. 2018).

Determination of physical and chemical properties of soil

After completing incubation time, soil properties (BD, MWHC, EC, pH, SOC, and SOM) were measured for nine treatments (each has three replications) and one control (each has three replications) by following standard methods. To determine soil BD, 50% of each soil sample was carefully placed in a graduated cylinder, and its weight was recorded without compaction. The cylinder was then tapped firmly on a bench to measure the volume of the compacted soil. Using the weight and volume data of each sample, the bulk density of the soil was calculated according to the methodology outlined by Hattery (1998). The MWHC of the soil was determined by the conventional method where at first, soil samples were dried in an oven (Model no-BODR-304) at 105 °C for 24 h. Subsequently, 25 g of the dried soil was placed on filter paper arranged in a funnel, and 25 mL of distilled water was added. The amount of water absorbed by the soil was measured to determine its MWHC (Hattery 1998). For soil EC measurement, 10 g soil sample was mixed with 50 ml distilled water in a beaker at a soil-to-water ratio of 1:5. The solution was agitated for 30 min and allowed to settle undisturbed for 1 h. The filtrate was then passed through filter paper (Whatman 125 mm), and EC was measured using an electric conductivity meter (Cond 3310) following the method described by Qi et al. (2020).

For soil pH determination, a 10 g soil sample was mixed with 25 mL distilled water in a beaker at a soil-to-water ratio of 1:2.5. The solution was agitated for 45 min until homogeneous, then allowed to settle undisturbed for another 45 min. The filtrate was passed through filter paper (Whatman 125 mm), and pH was measured using a multiparameter instrument (HANNA HI-9829) as per Jackson’s (1973) guidelines. SOC was estimated using the wet oxidation method described by Walkley and Black (1934) as adopted by Jackson (1973). A 2 g soil sample was mixed with 10 ml of 1 N potassium dichromate (K₂Cr₂O₇) and concentrated 10 ml sulfuric acid (H₂SO₄) in a 500 ml conical flask. After cooling, the mixture was diluted with distilled water, followed by additions of concentrated 5 mL phosphoric acid (H₃PO₄) and 0.2 g of sodium fluoride (NaF). The resulting solution was titrated with ferrous sulfate (FeSO₄) solution to a bottle green endpoint, with a blank experiment (3 replications) conducted using all chemicals except soil. SOM was determined by first calculating the proportion of SOC and then applying the traditional Van Bemmelen’s factor of 1.724, as described by Piper (1950).

Tomato seedlings shifting and measurement of growth metrics

When the tomato plants reached a height of 5–6 cm, these were transplanted into experimental pots and kept at mesocosm settings where temperatures were recorded at 25 ± 3 °C. Each pot (height = 25 cm, top diameter = 6 cm, and bottom diameter = 10 cm) contains 1 seedling in this experiment. Plant growth metrics (plant height, leaf numbers, leaf area, and girth diameter) were recorded for five weeks. Plant heights were estimated using a tape rule from the roots to the peak of plant. The number of leaves was physically counted and recorded. After drying, plant dry biomass was measured to a constant weight at 70 °C (using a Biolab Scientific Oven). The leaf’s length (L cm) and width (W cm) were measured at the apex and center, respectively by using a plastic ruler. The leaf area was then calculated using the proportion of the length and width (L×W) in cm² (Verla et al. 2019). For measuring plant girth diameter, a thread was used. At first, a thread was encircled around the perimeter; then, it was laid straight and measured.

Microplastics extraction from control soil

To evaluate whether MPs were present in the control soil (0% MPs), 5 g of dried soil was placed in a 50 mL tube, combined with 30 mL of distilled water, and shaken for 30 min at 150 RPM (revolutions per minute). Subsequently, tubes were centrifuged at 3000 RPM for ten minutes. The supernatant was filtered through a filter paper (Whatman No. 42). Then, sodium chloride (NaCl) 20 ml was added to the residue, agitated, and centrifuged again. If needed, the filtrate was passed through a fresh filter paper (Whatman No. 42), with the initial filter preserved in a petri dish for visual inspection. For a final extraction, a concentrated zinc chloride (ZnCl₂) solution (20 mL) was added to the centrifuge tubes containing the precipitate. After another centrifugation, the supernatants were filtered onto the same filters. Filters were air-dried for a full day before MPs detection and quantification (Corradini et al. 2019; Beriot et al. 2021).

Identification of microplastics in control soil

With caution, all components on the filter were gently brushed onto a glass plate, centralized to prevent particle overlap. A digital microscopic camera (Olympus-DP22; model U-TV0.5XC- 3, SN-5M01493) mounted on a stereomicroscope (LOTUS MC x51, AUSTRIA) was employed to capture images of the particles at 40x magnification. Subsequently, a second image was taken after heating the glass plate on a hot plate (Model: H-002 A) at 130 °C for ten seconds. Analysis of particle shape, color, brightness, and thermal response facilitated the differentiation of plastic particles from soil and organic materials (Zhang et al. 2018; Beriot et al. 2021).

Digestion and polymer group identification

For identifying the chemical composition of unknown plastic particles in soil, Fourier Transform Infrared Spectroscopy (FTIR) spectroscopy is a widely used method (Campanale et al. 2023). Addressing the challenge of finding plastic-free soil in nature, this study employed FTIR (IR-Prestige-21). This process involves digesting soil samples to remove all organic compounds and matter from the soil matrix (Hurley et al. 2018). Hydrogen peroxide (H₂O₂) is the most frequently used oxidant for this purpose (Liu et al. 2018). Hence, 30% H₂O₂ (v/v) was used for organic matter digestion, following the methodologies of Cunsolo et al. (2021) and Radford et al. (2021). After digestion, following the procedures stated by Dilshad et al. (2022) spectrum analysis was conducted where the wavenumber covered a range of 4000 to 500 cm⁻¹, and the spectral resolution was 8 cm⁻¹.

Quality control & quality assurance

Each set of samples (n = 10) included a blank (without soil) for quality control purposes. After completion of the analysis, the filter from this blank was stored in a petri dish and examined. This analysis considered potential laboratory contamination and the quality of reagents used. Glass funnels, petri dishes, and centrifuge tubes were employed for sample processing, along with a stainless-steel stirrer rod (Scheurer and Bigalke 2018; Mahon et al. 2017; Corradini et al. 2019).

Statistical analysis

With Origin Pro 2021 software, all the graphs in this study were developed. A two-way ANOVA test was employed in this study to investigate the effect of two distinct independent factors (MPs’ type and concentration) on dependent variable (soil properties / physiognomic trait) using IBM SPSS Statistics 26. This test was conducted to determine the significance threshold (p ≤ 0.05) between treatments and control. The significance of differences between treatment (0.4%, 1%, and 2% MPs) pairings with control (0% MPs) was then assessed using Tukey’s Honestly Significant Difference (HSD) test.

Results

Characteristics of microplastics found in control soil

In this study, different types of plastic fragments (Fig. 1a), films - colored (Fig. 1b), and transparent (Fig. 1c) were identified in control soil by using a stereomicroscope; these were classified according to their visual appearances (Fig. 1). The polymer type was confirmed by comparing the peak values with the standard peaks of FTIR. High-Density Polyethylene (HDPE) and Nitrile were prevalent (12 items/kg) in the control soil (Fig. 2). The spectrum showed the following bands of HDPE: C-H stretching at peak 2915 cm^− 1 and 2845 cm^− 1; CH₂ bend at 1462 cm^− 1 and 1472 cm^− 1. Nitrile: =C-H stretching at peak 2917 cm^− 1 and 2849 cm^− 1; CH₂ bend at 1440 cm^− 1 and 1197 cm^− 1; =C-H bend at 967 cm^− 1 in contrast to reference spectrum (Dilshad et al. 2022; Jung et al. 2018).

Fig. 1

Visual identification of microplastic particles under a stereomicroscope; grouped into three categories: (a) Fragments (hard), (b) Colored Films, (c) Transparent Films. The black bar below each picture indicates scale of 200 μm size

Fig. 2

IR spectrum of control soil shows significant absorption bands

Soil physical and chemical properties

Physical and chemical attributes of soil exposed to MPs over 45 days are illustrated in Fig. 3(a, b, c, d, e, f). Physical parameters examined included MWHC, BD, and EC, while chemical properties were pH, SOC, and SOM. The integration of MPs in soil treatments produced significant variations in soil properties. Specifically, the cultures with 1% microfiber resulted the most substantial increased in soil pH (17.84%). Conversely, 2% mixed cultures of MPs led to the highest reductions in EC and BD, with decreased 67.3% and 7.29%, respectively. Additionally, variations in MPs concentrations (0.4%, 1%, and 2%) and types (straw, microfiber, and mixed) elicited notable changes in MWHC, SOM, and SOC, demonstrating the differential impacts of these treatments (+ MPs) on various soil properties.

Fig. 3

Soil bulk density (a), maximum water holding capacity (b), electric conductivity (c), pH (d), organic carbon (e), and organic matter (f) after 45 days of microplastic exposed soil. Vertical bar indicates the standard deviation (± SD). * p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001, and n.s. denotes non-significant in Tukey’s test comparison with the control samples

Visual appearances of tomato plants after five weeks

After five weeks, tomato plants in the control (0% MPs) and treatment (0.4%, 1%, and 2% MPs) cultures in soils are shown in Fig. A.2. In comparison to other treatment cultures, the proliferation in controlled soil (Fig. A.2a) was higher. The growth of the treatment plants was higher in cultures containing 0.4% straw (Fig. A.2b), microfiber (Fig. A.2c), and mixed MPs (Fig. A.2d), while the lowest growth was found in cultures containing 2% straw (Fig. A.2e), microfiber (Fig. A.2f), and straw mixed with microfiber (Fig. A.2 g).

Physiognomic characteristics & total plant biomass (TPB) of test tomato plant

Plant heights, leaf numbers, leaf areas, and girth diameters were recorded over five consecutive weeks to assess the physiognomic traits of test plants grown in control soil (0% MPs) cultures versus those cultivated with various concentrations of MPs (0.4%, 1%, and 2%). The comparative analysis of plant height (Fig. 4a), leaf number (Fig. 4b), leaf area (Fig. 4c), and girth diameter (Fig. 4d) indicated that plants in control soil (0% MPs) exhibited more rapid growth in these traits. In contrast, slower growth was observed in plants subjected to 2% treatments of straw, microfiber, and mixed MPs. The most significant reduction (56.37%) in plant height was observed in cultures containing 2% straw. The number of leaves showed a notable reduction of 54.37% in cultures with 2% straw and 2% mixed MPs. Additionally, the leaf area experienced a substantial decreased (62.44%) in cultures with 2% microfiber. The girth diameter also exhibited the highest reduction (56.43%) in cultures with 2% straw. TPB was measured in the fifth week (Fig. 4e) and TPB fell including the reduction in plant growth as the number of MPs in soil increased. The maximum TPB (3.29 g) was recorded in plants cultivated in control soil (0% MPs) whereas, the highest reduction (68.16%) in TPB was observed in plants grown in cultures with 2% microfiber.

Fig. 4

Plant height (a), leave numbers (b), leaf area (c), girth diameter (d), and biomass (e) for different treatments (+ MPs) and control (-MPs) after five weeks. Vertical bars indicate the standard deviation (± SD). *p ≤ 0.05, **p ≤ 0.01, ***p ≤ 0.001 and n.s. denotes non-significant in Tukey’s Honest Significant Difference (HSD) test comparison with the treatments (0.4%, 1%, 2%) and control (0%)

Effects of MPs on soil and the growth of test plant

The outcomes of two-way ANOVA determine the effects of different types of MPs (straw, microfiber, and mixed) and their concentrations (0.4%, 1%, and 2%) on physical / chemical properties of soil and physiognomic traits shown in Table 2. Types of MPs had a substantial impact on MWHC, EC, SOC, and SOM, whereas the concentrations of MPs primarily affected MWHC, EC, pH, SOC, SOM, leave number, area, and plant biomass. According to Table 2, the most affected parameters in soil were EC, pH, SOC, and SOM when both type and concentration were considered.

Table 2 Effects of MPs types and concentration on physical / chemical properties of soil and physiognomic traits

Discussion

Effects of microplastics on soil physical properties

Soil bulk density (BD) is a crucial parameter influencing soil porosity and root development within the soil-plant-atmosphere system (Bowman and Hutka 2002; McKenzie et al. 2004). In this investigation, compared to control soil (0.96 ± 0.011 g/cm³), BD decreased with the addition of straw and microfibers (Fig. 3a). However, two-way ANOVA showed that BD did not vary significantly when compared with control (0% MPs) (Fig. 3a; Table 2). Incorporation of MPs into the soil slightly reduced BD, as MPs generally have a lower density than soil particles (Mbachu et al. 2021). Previous studies (de Souza Machado et al. 2018; Souza Machado et al. 2019) have indicated that MPs exposure decreased soil BD. Unlike spherical MPs like beads that closely resemble natural soil particles, Wang et al. (2022b) found that linear MPs such as fibers and fragments had more pronounced effects on BD reduction. The irregular shapes of straw and microfiber fragments in this study likely contributed to the slight declination in BD, with significant reductions observed when these MPs were mixed. Among the tested cultures, BD reduction percentages showed that cultures with 2% mixed MPs exhibited the highest reduction (7.29%), followed by cultures with 0.4% straw, 2% microfiber, 0.4% mixed MPs, and 1% mixed MPs (5.21%). Smaller reductions were observed in cultures with 2% straw (4.17%), 1% microfiber (3.13%), and 1% straw (2.08%), while no reduction was noted in cultures with 0.4% microfiber (0%). These findings underscore the significant influence of MPs size, concentration, and shape on BD reduction. Conversely, Yu et al. (2023) demonstrated that PES fibers (0.5% w/w) effectively reduced BD in silty loam soil compared to PP granules (2% w/w), highlighting the importance of structural characteristics in MPs-mediated changes in soil properties.

A particular soil’s maximum water-holding capacity (MWHC) refers to how much water can be stored for crop usage. In this investigation, the MWHC of the control soil (0% MPs) was 65.33% (Fig. 3b). Following incubation with MPs, the percentage increased in MWHC compared to the control was observed in soil cultured with microfibers which were as follows 8.17%, 15.8%, and 16.4% for 0.4%, 1%, and 2% microfiber concentrations, respectively. In soils containing mixed MPs, MWHC enhancements were recorded at 4.6%, 11.7%, and 15.8% for 0.4%, 1%, and 2% MPs concentrations, respectively. Conversely, MWHC decreased in soils treated with straw, with reductions of 13.3%, 4.8%, and 1.53% for 0.4%, 1%, and 2% straw concentrations, respectively. Also, according to the results of Tukey’s HSD tests, soil MWHC was statistically significant (p ≤ 0.05) in the cultures with straw 2%, microfiber 2%, mixed 1%, and mixed 2% treatments compared with control (0% MPs) and cultures with microfiber 0.4% and mixed 0.4% were statistically significant (p ≤ 0.05) with their respective cultures with 1% and 2% concentrations (Fig. 3b; Table 2). Comparable findings found by Wang et al. (2022c) documented that PP significantly reduced soil MWHC compared to polyvinyl chloride (PVC) and PE, as evidenced in studies on soil moisture dynamics. This reduction in MWHC in straw-amended soils is likely attributed to the superior MWHC of fibers compared to straw. Research by Jazaei et al. (2022) concluded that the incorporation of fiber MPs enhanced the water-retention capacity of fine sandy soil samples, facilitated by the lightweight and flexible nature of fibers that created additional microscale pore spaces. Similarly, studies by de Souza Machado et al. (2018, 2019) indicated that polyethersulfone (PES) fibers enhanced water retention capacity relative to other MPs such as PES fragments, polyacrylic (PC) fibers, and PA beads. It is therefore difficult to make broad generalizations regarding the capacity of soil to retain water as different compositions yield various kinds of results (Mbachu et al. 2021).

In comparison to other treatments, according to this study, EC was the highest (440 ± 0.5 µS/cm) in control soil (0% MPs), and second highest (327.33 ± 0.5 µS/cm) in cultures with 1% of straws (Fig. 3c). EC in soil typically ranges from 200 to 1200 µS/cm and under this level shows an insufficient supply of nutrients whereas a conductivity level beyond this level indicates a high quantity of salt (Afrin et al. 2020). In this investigation, the EC was found to be < 200 µS/cm in cultures with 2% of straws (158.77 ± 0.25 µS/cm) and 2% of mixed (143.2 ± 0.15 µS/cm) MPs, but 230 µS/cm in culture with 2% of microfibers. The most pronounced decreased in EC compared to the control was observed in the culture with a 2% concentration of mixed MPs, demonstrating a reduction of 67.3%. This was followed by a 63.9% reduction in cultures with 2% straw and other significant reductions included 51.1% in 0.4% straw cultures, 47.8% in 2% microfiber, 47% in 0.4% mixed MPs, 46.2% in 1% mixed MPs, 31.8% in 1% microfiber, 29% in 0.4% microfiber, and 25.74% in 1% straw cultures. Besides, in Tukey’s HSD tests, all the treatments of EC with control (0% MPs) and between treatment cultures (0.4%, 1%, and 2%) demonstrated significance at p ≤ 0.001 (Fig. 3c; Table 2). These findings suggest that higher concentrations of mixed MPs in soil may adversely affect EC, accounting for the lower conductivity observed in such treatments. Qi et al. (2020) reported comparable results, indicating that soil EC decreased after two months of incubation with low-density polyethylene (LDPE) and biodegradable plastic mulch films. Contrarily, another study by Yuan et al. (2023) found that PP increased EC in most treated soils after 120 days of incubation compared to PE.

Effects of microplastics on soil chemical properties

In the control soil (0% MPs), the pH was measured at 5.44 ± 0.04. Cultures containing 1% microfiber exhibited the highest pH 6.41 ± 0.06, whereas the lowest pH, 5.76 ± 0.04, was recorded in cultures with 0.4% microfiber (Fig. 3d). This study revealed that the presence of MPs caused minor variations in soil pH, with the most significant elevation observed in cultures containing 1% microfiber. Tukey’s HSD test also showed significance (p ≤ 0.01 and p ≤ 0.001) in all treatments when compared to the control (0% MPs) and between treatments (0.4%, 1%, and 2% MPs), except for cultures with 2% and 0.4% straws (Fig. 3d; Table 2). Previous research by Zhao et al. (2021) and Qi et al. (2020) supports the finding that MPs can influence soil pH, with amendments of bioplastics and LDPE resulting in higher pH values compared to control soils (without MPs). This rise in soil pH is likely due to the leaching properties of MPs, which enhance soil aeration, porosity, and related characteristics. In contrast, Boots et al. (2019) found that the presence of HDPE MPs led to a decrease in soil pH relative to control conditions. Further studies by Li et al. (2021b, 2022) demonstrated that soil pH response to MPs depends on soil type; addition of PE to acidic soils tends to lower pH, whereas it raises pH in alkaline soils. In this investigation, despite the soil being acidic, pH increased in all treatments. The increase in pH followed this order: 1% microfiber (17.84%) > 2% mixed MPs (15.80%) > 2% straw (15.06%) > 0.4% straw (13.98%) > 2% microfiber (12.87%) > 1% mixed MPs (12.68%) > 1% straw (9.38%) > 0.4% mixed MPs (6.06%) > 0.4% microfiber (5.88%).

The type of MPs deposited at the surface influences SOC structures (Kumar et al. 2022). Plastic polymers can act as a carbon source in the environment because these contain a high percentage of inert compounds that are more than 90% carbon (Rillig 2018; Rochman 2018). According to Chen et al. (2020) MPs contaminated soils had fewer macro-aggregates with lower stability than soil free of MPs. As MPs and the soil microbiome interact, the physicochemical properties of the soil are modified accordingly (Wang et al. 2020b). In this study, cultures with 0.4% straw and 0.4% mixed MPs have slightly higher SOC, as opposed to microfiber-containing cultures (0.4%, 1%, and 2%) and control (0% MPs), which contain 0.62 ± 0.02% of SOC. However, in other treatments (1% and 2%) SOC reduced gradually (Fig. 3e). Only the culture with mixed 1% and control (0% MPs) demonstrated non-significance in Tukey’s HSD test; the other treatments (0.4%, 1%, and 2%) demonstrated significance (p ≤ 0.01 and p ≤ 0.001) both comparing with the control (0% MPs) and among treatments (Fig. 3e; Table 2). Overall, SOC increased 9.68% in cultures with 0.4% straw and mixed MPs, and 1.61% in cultures with 2% mixed MPs. In contrast, other treatments exhibited a reduction in SOC relative to the control. Specifically, SOC decreased by 30.65% with the addition of 1% microfiber, 25.81% with 2% straw, 20.97% with 1% straw, 11.29% with 0.4% microfiber, 6.45% with 2% microfiber, and 4.84% with 1% mixed MPs. Opposite results obtained by Liu et al. (2017) showed that a higher MPs dose increased SOC after 30 days of incubation in a climate-controlled room.

SOM was 1.076 ± 0.02% in the control soil (0% MPs). The amount of SOM was the highest in the cultures with 0.4% of straws (1.16 ± 0.012%) and mixed (1.16 ± 0. 014%) treatments whereas the lowest amount of SOM was 0.786 ± 0.003% found in the culture with 2% of straws (Fig. 3f). Though a handful number of studies have been carried out on the effects of MPs in SOM, the outcome was different from one another, and some studies even showed non-significance results (Liu et al. 2017; Chen et al. 2020; Ren et al. 2020; Dong et al. 2021). Also, in this study, only treatment culture with mixed 1% and 2% MPs showed non-significance, however, other treatments (0.4%, 1%, and 2%) showed significance compared with the control (0% MPs) (Fig. 3f; Table 2). Soil in this study was acidic which resulted in a minor elevation in SOM levels. 7.41% in cultures with mixed MPs and 8.33% in cultures with 0.4% straw when mixed MPs were applied. Piccardo et al. (2020) reported a similar perception that MPs increased SOM in an acidic environment. However, other treated samples exhibited a downward trend in SOM, with reductions observed as follows: 1% microfiber (30.56%), 2% straw (26.85%), 1% straw (21.30%), 0.4% microfiber (12.96%), 2% microfiber (7.41%), 1% mixed (4.63%), and 2% mixed (1.85%).

Effects of microplastics on the physiognomic characteristic of tomato plant

At the end of the fifth week, the highest plant height was observed in the culture containing 0.4% microfibers, with an average height of 37.07 ± 1.30 cm, slightly exceeding the control group (0% MPs), which had an average height of 36.83 ± 2.02 cm. Moderate plant heights were recorded in cultures with 0.4% straw (27.83 ± 2.12 cm), 1% straw (23.63 ± 1.25 cm), 1% microfibers (29.7 ± 2.12 cm), 0.4% mixed MPs (32.43 ± 2.91 cm), and 1% mixed MPs (25.47 ± 1.16 cm). The cultures with the most stunted plant heights were those containing 2% straw (16.07 ± 2.42 cm), 2% microfibers (17.43 ± 1.27 cm), and 2% mixed MPs (20.37 ± 1.33 cm) (Fig. 4a). Only the culture with 0.4% microfiber experienced a slight increased plant height (0.65%) compared to the control. Other cultures exhibited a reduction in plant height, following this pattern: 2% straw (56.37%) > 2% microfiber (52.67%) > 2% mixed MPs (44.71%) > 1% straw (35.83%) > 1% mixed MPs (30.82%) > 0.4% straw (24.43%) > 1% microfiber (19.36%) > 0.4% mixed MPs (11.96%). Additionally, Tukey’s HSD test revealed that all treatments were significant (p ≤ 0.05), except for the cultures containing 0.4% mixed MPs, 0.4% microfibers, and 0.4% straw, compared to the control plants (0% MPs) (Fig. 4a; Table 2). The most significant decline in plant height was observed in cultures containing 2% straw (PP). Verla et al. (2019) similarly reported that PP plastic negatively impacts the growth of young lime trees, significantly reducing plant height during vegetative growth. According to Jia et al. (2023) the stress induced by PP MPs blocks the pores in the seed coat or roots, altering water and nutrient uptake, and causing drought-like conditions, which ultimately affect plant height.

According to Iqbal et al. (2023) MPs adversely affect growth and tissue development in plants. In this study, the control group (0% MPs) exhibited the highest average leave count (19 ± 2.64). In contrast, leave numbers in the treatment groups varied based on the type of MPs used (straw, microfiber, and mixed) and their concentrations (0.4%, 1%, and 2%). Specifically, treatments with microfiber showed leave counts of 16 ± 3 at 0.4%, 9 ± 1 at 1%, and 11.33 ± 2.08 at 2% concentrations. Similarly, mixed MPs had leave numbers of 15 ± 1 at 0.4% and 10 ± 2 at 1% concentrations. Notably, straw treatments had the lowest leave numbers, with 11 ± 3 at 0.4% and 9 ± 1.73 at 1%, while both 2% straw and 2% mixed MPs resulted in the least leaves (8.67 ± 0.57) (Fig. 4b). The observed reductions in leaf number were significant, with a decline of 54.37% for 2% mixed MPs and 2% straw, 52.63% for 1% straw, 47.37% for 1% mixed MPs, 42.11% for 0.4% straw, 40.37% for 2% mixed MPs, 36.84% for 1% mixed MPs, 21.05% for 0.4% mixed MPs, and 15.79% for 0.4% microfiber. Furthermore, cultures with 0.4% microfibers and mixed MPs were not statistically significant compared to the control in Tukey’s HSD test (p > 0.05), whereas other treatments showed significance (p ≤ 0.05) depending on concentration and MPs type (Fig. 4b; Table 2). Given that plant height was significantly affected by the 2% MPs treatments, the reduction in leaf number was also prominent.

According to Ci et al. (2015) the size of the leaf area serves as an indicator of plant health. In this study, control plants (0% MPs) exhibited the largest leaf area, averaging 118.32 ± 21.41 cm². In contrast, plants treated with 2% microfiber MPs showed the smallest leaf area at 44.46 ± 12.19 cm². Moderate leaf areas were observed in cultures treated with 0.4% microfiber (75.21 ± 10.27 cm²), 0.4% straw (71.59 ± 8.21 cm²), and 1% microfiber (71.43 ± 13.05 cm²) (Fig. 4c). Further, all plant leaf areas showed significance (p ≤ 0.05) according to Tukey’s HSD results, except for the cultures containing 0.4% straw and microfiber (Fig. 4c; Table 2). Compared to the control, the leaf area decreased by 62.44% for cultures with 2% microfiber, 51.16% for cultures with 2% mixed MPs, 44% for cultures with 2% straw, 40.53% for cultures with 1% mixed MPs, 39.63% for cultures with 1% microfiber, 39.51% for cultures with 0.4% straw, 36.46% for cultures with 0.4% microfiber, 28% for cultures with 1% straw, and 26.17% for cultures with 0.4% mixed MPs. Similar results were reported by Lian et al. (2021) where the application of polystyrene nanoplastics (PSNPs) to lettuce (Lactuca sativa) over 30 days led to a significant reduction in leaf area, inducing oxidative stress and impairing the antioxidant system of the leaves.

After the end of the fifth week, the control plants (0% MPs) exhibited the widest girth diameter, measuring 1.4 ± 0.2 cm. In contrast, cultures treated with 2% mixed MPs (0.63 ± 0.08 cm) and 2% straw (0.61 ± 0.01 cm) demonstrated the smallest girth diameters (Fig. 4d). Compared to the control, all treatment cultures showed a significant reduction in girth diameter, with decline percentages as follows: 56.43% for 2% straw, 55% for 2% mixed MPs, 51.43% for 0.4% mixed MPs, 50% for 1% microfibers, 49.29% for 1% mixed MPs, 48.57% for 0.4% straw, 47.86% for 2% microfibers, 46.43% for 1% straw, and 44.29% for 0.4% microfibers. All treatment cultures were found to be significant (p < 0.05) in the Tukey’s HSD test, with the notable exception of cultures containing straw 0% and straw 0.4%, microfiber 0% and 0.4%, microfiber 1% and 2%, and mixed 1% and 2% culture (Fig. 4d; Table 2). Similar results were reported by Zhou et al. (2023) that the presence of PP and PES MPs significantly decreased the culm diameter of Zea mays, Glycine max, and Arachis hypogaea plants under field conditions, with the extent of the reduction varying depending on the MPs type and plant species.

The control plants (0% MPs) exhibited the highest total plant biomass (TPB), measuring 3.29 ± 0.34 g. Compared to cultures with 1% and 2% MPs, all cultures with 0.4% MPs showed higher TPB. The lowest TPB was recorded in cultures with 2% MPs. In this study, TPB of all treatments with control (0% MPs), and among treatment cultures (0.4%, 1%, and 2%) exhibited significance (p ≤ 0.05) in Tukey’s HSD test (Fig. 4e; Table 2). Over time, the trend in TPB influenced by MPs was observed as follows control (3.29 ± 0.34 g) > microfiber 0.4% (2.74 ± 0.35 g) > mixed 0.4% (2.68 ± 0.13 g) > microfiber 1% (2.43 ± 0.37 g) > mixed 1% (2.26 ± 0.20 g) > straw 0.4% (2.06 ± 0.25 g) > straw 1% (1.71 ± 0.26 g) > mixed 2% (1.44 ± 0.21 g) > straw 2% (1.22 ± 0.19 g) > microfiber 2% (1.05 ± 0.003 g) (Fig. 4e). This trend demonstrates that increasing MPs concentrations in the soil significantly reduced TPB, with a decrease of 62.59% in cultures containing 2% straw, 68.16% for 2% microfiber, and 56.22% for 2% mixed MPs. Consequently, plants grown in 2% MPs-containing soils developed more slowly than those in other treatments, resulting in lower total dry biomass. This reduction in growth is attributed to MPs in the soil obstructing the availability of nutrients and water to the plant roots, thereby impeding proper growth and affecting TPB (Okeke et al. 2023). Similar observations were reported by van Kleunen et al. (2020) where ethylene PP-derived MPs at a 5% concentration substantially reduced plant biomass. Additionally, He et al. (2023) confirmed that biomass significantly decreased across all plant species, regardless of the diversity of MPs (EPS, PET, PP, HDPE, PLA, PA6).

Overall, tomato plants exhibited no mortality in this study, though their physiognomic traits were impacted by MPs exposure compared to the control (0% MPs). Cultures with 0.4% microfiber and 0.4% mixed MPs showed enhanced growth relative to the control group. This favorable growth condition can be attributed to two factors. First, the low concentration of plastic, with microfiber present in both types which can retain more water than straw, thus providing better nourishment to the plants. Second, the presence of earthworms in the soil, which can ingest plastics and mitigate their deleterious effects (Wan et al. 2019). Also, PP (straw) exhibited more negative impacts than the mixture of PES and PA (microfiber). Similar findings were reported in an experiment involving macro- and micro-sized plastics on the growth of juvenile lime trees, where soil containing PP had a more pronounced effect (Verla et al. 2019). Moreover, this study revealed that an increasing MPs concentration led to a decline in total plant biomass. However, Shorobi et al. (2023) reported contrary findings, noting that the dry weights of tomato and cherry tomato plants increased after seven days of MPs PP exposure compared to the control. Additionally, our study indicated a dose-response relationship in tomato plants, where increasing MPs concentrations decreased plant growth. All physiognomic traits were lowest in cultures with 2% MPs, although some plant cultures with 0.4% MPs outperformed the control (0% MPs). These results align with Weerasinghe and Madawala (2022) indicating that increasing amounts of PET and LDPE MPs reduced the relative growth rate of S. lycopersicum. Furthermore, Sahasa et al. (2023) reported that MPs, particularly PE, could slightly affect the seed germination rate of S. lycopersicum after 24 h in an aqueous solution. Therefore, the presence of straw (PP), microfiber (PES + PA), and mixed MPs (PP + PES + PA) influenced the growth of tomato plants depending on the concentrations (0.4%, 1%, and 2%).

Conclusion

The results of this investigation demonstrated that the inclusion of MPs in the soil significantly reduced soil BD and EC, while simultaneously increased pH levels across all treatment cultures. In contrast, the MWHC, SOC, and SOM exhibited variable responses contingent on the types and concentrations of MPs introduced. Additionally, the physiognomic traits of S. lycopersicum revealed optimal growth in cultures containing 0.4% MPs (PP, PES + PA, and PP + PES + PA) compared to the control group (0% MPs), with the most adverse effects observed in cultures with 2% MPs. Furthermore, TPB was inversely proportional based on the types and concentrations of MPs. Therefore, it is plausible that prolonged exposure to MPs in soil could entirely impede the growth of tomato plants and further deteriorate soil properties. This study’s focus on two specific types of MPs (straw and microfiber) limited insights into how different types (PA, PP + PS + PA) and concentrations (0.4%, 1%, and 2%) of MPs affect soil parameters and plant growth metrics. This study also highlights the urgency for sustainable agricultural practices and effective MPs mitigation strategies to maintain soil quality and promote healthy growth of plants, while advocating for environmentally friendly plastic alternatives within a socio-economic framework that includes enhanced monitoring, research, technical innovation, proper training, and socio-economic incentives. Consequently, further research on a broader range of MPs types and concentrations is essential to fully understand their threshold level and impact on soil-plant systems, considering their emerging role as potential contaminants in terrestrial environments.