Assessment of microplastic pollution on soil health and crop responses: Insights from dose-dependent pot experiments

doi:10.21203/rs.3.rs-5127268/v1

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Assessment of microplastic pollution on soil health and crop responses: Insights from dose-dependent pot experiments

https://doi.org/10.21203/rs.3.rs-5127268/v1

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Microplastics (MPs) are increasingly recognized as environmental contaminants with complex impacts on soil health and crop productivity. This study investigates the effects of MP contamination in soil through field investigation and pot experiments. Field analysis revealed the presence of polyethylene terephthalate (PET) and high-density polyethylene in soil and plant roots, with visible pollution concentrated in the upper layers. In controlled pot experiments, Brassica juncea (mustard) and Lycopersicum solanaceae (tomato) plants were exposed to PET, polystyrene (PS) and nylon (NL) at different concentrations. The plants exhibited dose-dependent responses, with the treatments of 5% and 10% MPs showing the most detrimental effects (p < 0.05) on soil properties, particularly pH and available nitrogen levels. Both species displayed significantly different responses to treatments (p<0.05). Specifically, PS at a 5% concentration notably suppressed leaf area index while 10% NL resulted in reduced root length and chlorophyll content. PET posed the most significant hindrance to root growth. Interestingly, 10% NL with a high Potential Hazard Index score and a rating of hazard category IV, emerged as the most hazardous polymer at concentrations of 1% and 10%, while at 5%, PS had the most significant impact on plant properties. NL had an overall detrimental effect on mustard plants, whereas PS was more harmful to tomato plants. PET affected both species similarly. The results add to the growing information on the potential risks of MP pollution in the terrestrial environment which supports soil health and dependent organisms.

Agroecology

Microplastics

soil health

soil pollution

mustard

tomato

Microplastic contamination was assessed in soil and plants in natural as well as controlled environments.
PET and HDPE were detected in five out of eight soil samples and HDPE was detected in grass roots.
Nylon polymers cause more harms in higher concentrations, followed by PS and PET

PET MPs can infiltrate roots of mustard and tomato plants

Microplastics (MPs) are now a well-established feature in all environmental matrices. MPs are any solid plastic particle that is insoluble in water and has a dimension between 1 µm and 1000 µm (ISO 24187:2023). The United Nations Environment Programme considers MPs among the emerging threats to the ecosystem (UNEP 2021). Several studies have highlighted the vulnerability of soil ecosystems to MP contamination with agricultural fields particularly at risk due to practices like plastic mulching and the use of polymer-coated fertilizers (Lwanga et al., 2022; Zheng et al., 2022; Huang et al., 2020; Lian et al., 2021). The distribution of MPs within soil matrices is complex and influenced by various soil properties, with higher concentrations often found in upper soil layers (Boots et al., 2019; Lehmann et al., 2021; Meng et al., 2022; Johnson et al., 2020).

MPs in soil can disrupt nutrient absorption in plants and soil organisms, affect soil microbial communities and block soil pores, posing risks to soil health and ecosystem functions (Rillig et al., 2021a). Different polymers have been found to impact soil characteristics and plant traits, potentially leading to changes in plant biomass, root growth, and overall health. The variations in soil properties can further influence nutrition availability and uptake by plants affecting crucial processes like chlorophyll synthesis and photosynthesis (de Souza Machado et al., 2019; Boots et al., 2019).

MPs can accumulate in the soil over time, leading to persistent contamination and potential ecological risks. It is important to note that the impacts of MPs on soil structure can vary depending on factors such as the type, size, concentration and duration of exposure to MPs as well as the specific soil and environmental conditions. Despite extensive research on MP contamination in soil, gaps persist in our understanding of MP interactions with soil nutrient dynamics and their effects on soil health and plant vitality. This study aims to address this gap by investigating the effects of MPs on soil health and plant growth through field investigations and pot experiments. MPs derived from nylon (NL), polyethylene (PE) and polystyrene (PS) were added to soil at concentrations of 1%, 5%, and 10% w/v to assess their influence on soil characteristics and the growth of Brassica juncea (mustard) and Lycopersicum solanaceae (tomato). The study seeks to determine how varying MP concentrations and types affect specific soil and plant attributes.

2.1. Field investigation

A preliminary field investigation was done to detect MPs in agricultural soils and to understand the impacts of MPs on soil-nutrient dynamics and plant health. Soil and plant samples were collected in November, from an agricultural field in Sonitpur district of Assam, northeast India (26.690º N, 92.831º E), where plastic mulching was being practiced to help improve crop yield. The soil samples were extracted from two depths – 0—7 cm and 7—15 cm, from four points selected along a 10 m × 10 m grid in the field. The depths were chosen considering plant root penetration depth as well as to detect the presence of MP in the deeper layers of soil. The soil was collected and stored in stainless steel boxes to prevent any further plastic contamination. The study area experiences hot, humid summers and dry winters with annual temperatures ranging between 11°C and 40°C while the soil texture is silty to sandy (Saikia and Baruah 2014; IMD CRIS).

Soil properties pH, electrical conductivity (EC), bulk density (BD), soil organic carbon (SOC) and available nitrogen (AN) were determined to understand the soil composition and characteristics. The pH and EC were measured using handheld digital meters after mixing a known quantity of soil sample with deionized H2O in a 1:5 ratio. BD was determined by the method given by Blake (1965). SOC was determined by the modified Walkley-Black wet oxidation method (FAO, 2019). AN was determined using an automatic nitrogen distillation system (Kelplus Elite EX-VA).

Detection of MP was first done visually using a stereozoom microscope (Model: Labomed CZM-6), and later confirmed via Fourier transform infrared spectroscopy (FTIR; Make: Perkin Elmer). Since the field was fallow at the time of sample collection, specimens of naturally growing grasses from the Poaceae family were collected. The plants were carefully uprooted and collected in order to determine whether MP could infiltrate the plant tissues. After a thorough washing, the specimens were oven-dried. The samples of the root, shoot and leaf blade were separated and ground into a fine powder in preparation for further analysis via spectroscopy and microscopy.

2.1.1 Microplastic extraction and indexing

Accurate extraction and characterization of MP in soil is a methodological challenge and subject to human errors. Following the methodologies in other studies, the samples were oven-dried before grinding into a fine powdery consistency and sieved for homogenization (Protyusha et al., 2024; Amrutha et al., 2021). Density separation with 5N NaCl solution (ρ = 1.2g/cm3) aided the separation of the lighter particles. The floating particles were removed and treated with Fenton’s reagent (30% H2O2 + FeSO4) to remove organic matter. The remaining particles were carefully filtered using vacuum filtration and collected on a glass fiber filter paper for further analysis via microscopy and spectroscopy. FTIR spectrum was analyzed using OpenSpecy developed by Cowger et al. (2021).

An MP abundance quotient was calculated as the ratio of the total occurrences of a specific MP type to the total number of samples analyzed. This quotient serves as a quantitative measure of the relative abundance of a given MP type in the study area, providing valuable insights into the prevalence of MP contamination in the environment. A polymer hazard assessment was also done using the formula given by Ranjani et al. (2021):

PHI = Σ P_n × S_n (1)

Where, PHI is the calculated polymer hazard index for a given MP type, P_n is the percent of a given MP type and S_n is the hazard score of a given polymer, as mentioned in Lithner et al. (2011). In this study, the MP abundance quotient was considered as the Pn variable for calculating the PHI, allowing hazard category evaluation for the selected study area. PHI scores indicate the importance of MP concentration in determining the score along with the polymer type. The reference for hazard category for each score has been adopted from Shekoohiyan & Akbarzadeh (2022), where PHI 0–1 indicates a hazard category of I, a score of 1–10 indicates a category of II, 10–100 is III, 100–1000 is IV and greater than 1000 indicates hazard category V.

2.2 Experimental design

For the laboratory-based pot experiments, the soil was collected from one of the least disturbed forested areas (26.703º N, 92.830º E) within the Tezpur University campus in Assam, India. Soil was collected from a depth of 0–7 cm using a trowel, and collected and stored in stainless steel boxes for further processing. The area had a dense canopy cover and thick understory and was devoid of any visible plastic pollution. The soil samples were dried and sieved to ensure that it is devoid of any visible MP pieces. The experiment was done in a controlled environment and appropriate measures were taken to ensure no cross-contamination from clothes or atmospheric deposition took place. Physico-chemical properties of the soil were noted before the experiment (pH = 5.97; EC = 0.16 mS/cm; BD = 1.08 g/cm³; SOC = 1.38%; AN = 14.93 mg/kg).

The MPs used in the experiment were NL, PET and PS. These MPs were selected based on the abundance of MPs detected from the field investigation as well as review of literature. To extract NL, PET and PS MPs, toothbrush bristles, PET bottles and thermocol packages were used, respectively. Previous studies have employed similar methodologies, such as extracting PET from common recyclable bottles (You et al., 2020; Mondal et al., 2022) and NL microfibers from lint filters of household washing machines (Boots et al., 2019). The bottles, thermocol pieces and bristles were cut or grated into smaller pieces which were cleaned with 70% ethyl alcohol to remove microbes and rinsed with distilled water, as described by You et al. (2020). The clean plastic pieces were frozen in liquid nitrogen and ground to a fine consistency using a mill. Subsequently, these finely ground MPs were sieved through a No. 200 (75 µm) stainless steel sieve to obtain uniform size. To achieve different concentrations, MPs in the proportion of 1%, 5%, and 10% of the total soil weight were added to 100 grams of soil. The soil and MP mixtures were sieved again to facilitate better assimilation of the MPs in the soil media. This process ensured the controlled introduction of MPs into the experimental soil. The mixture was then stored in small earthen pots with a top diameter of 5.5 cm, bottom diameter of 3 cm and height 8 cm, where the plants were later grown. Smaller pots were used to reduce the quantity of MP used in the experiment so that, after the end of the experiment, the MPs discarded into the environment could be minimized.

Two crop species, Brassica juncea (mustard) and Solanum lycopersicum (tomato) were chosen for the experiments. These plants were selected as they are grown on a larger scale in areas around the site of field investigation as well as due to their ease of cultivation and relatively quick germination. Seeds were bought from a local agriculture-based shop. 10 seeds of each species were planted in a pot against each treatment and the plants were harvested after six weeks. Plants were also cultivated in control pots simultaneously where no MP was added. The experiment was conducted in triplicates for all treatments. The pots were watered daily at the same time with 10 ml of distilled water.

To assess the impact of MPs on the plants, shoot growth was noted at weekly intervals and germination index (GI), leaf area index (LAI), root length (RL), shoot length (SL) and total chlorophyll (TChl) were observed after harvesting. GI was calculated according to the method given by Baruah et al., 2019. LAI serves as an indicator of photosynthesis and primary productivity and is calculated by dividing the total leaf area by the total ground area. RL and SL were measured using a graduated scale. TChl was calculated using the formula given by Arnon (1949). RL: SL ratio, an indicator of stressors to root growth, was also noted. To assess the uptake of MPs, roots, shoots and leaves of each plant were harvested, oven dried and ground into a fine powder for microscopy and FTIR analysis.

2.4 Statistical analyses

The statistical analyses were conducted utilizing SPSS (IBM SPSS Statistics version 25) and MS Excel 2016 (Microsoft Office Suite). Following an assessment of normality and homogeneity, the mean values of observations for each treatment were compared. Analysis of variance (ANOVA) tests were done to determine significant differences in response to the various treatments, for both polymer types and concentrations. Pearson correlation test was done to evaluate the relation between PHI scores and the observed soil properties. Paired t-tests were done to evaluate the significance of species-specific responses to treatments. Post-hoc tests were then applied to distinguish statistically significant differences within treatment groups. Significance was set at p < 0.05 for all comparisons.

3.1. Field observations for soil and plants

From field investigations, MPs were detected in five out of the eight soil samples using FTIR. In two sites, MPs of PET and High-Density Polyethylene (HDPE) were found to have leached into the deeper layers of soil. There were no significant differences in soil properties between sites where MPs were detected and sites where MPs were not detected. Factors such as pH, EC, BD, SOC and AN showed variability across all sites, suggesting that MP presence may not strongly correlate with the soil parameters. In the grass samples, HDPE MPs were detected in the root samples. No MPs were detected in the grass shoot or leaf blade samples. This suggests the potential uptake or accumulation of MP by other plants as well, including crops.

ANOVA and posthoc Duncan’s multiple range test indicate that there is no statistically significant difference in pH and EC in the samples, however, BD, SOC and AN are significantly different (p < 0.05) in the 0–7 and 7–15 cm soil depths. Pearson correlation test was done for soil parameters at both depths. No significant changes were found and no significant influence of MPs on the soil properties was observed.

According to the MP Abundance Quotient, PET was found to be the most prevalent MP in the study area with a score of 0.63, followed by HDPE with a score of 0.5. HDPE has a higher polymer hazard score of 11. Both PET and HDPE were calculated to be hazard category II based on their abundance and potential ecological impacts.

3.2 Soil characteristics in pot experiment

Following a six-week incubation period, systematic analyses were conducted to assess the effects of MP exposure on soil quality and plant health. Average values of the triplicates are taken. The results, as shown in Fig. 1(a-e), demonstrate the fluctuations in soil characteristics in response to the different doses of MPs. With the increase in concentration of MPs, a decreasing trend could be noticed for each parameter. It helps provide insights into the potential impacts of MP pollution on various soil properties. Statistical analysis further elucidates the significance of these observations and helps discern the specific impacts of MP exposure on soil characteristics.

(a) (b)

(e) (f)

Figure 1: (a-e) Effects of different concentrations and different MP types on soil properties, where the blue line represents mean observations in the control pot, the x-axis represents the various treatments and y-axis denotes the various parameters. (f) PHI scores for the different concentrations of MPs where II, III and IV represent the hazard category.

ANOVA and Duncan’s post hoc Multiple Range Test were conducted to assess mean differences among MP concentrations. At a 1% MP concentration, pH levels exhibited significant disparities, ranking as PET^a > Control^b ≥ NL^bc ≥ PS^c, where a, b, c represent significance in differences at p < 0.05. At 5% MP concentration, the pH was most affected by Control^a > PET^b ≥ NL^bc ≥ PS^c, and for 10% concentration, variation in pH values were significant at Control^a > PET^b > NL^c = PS^c. Similarly, significant variations were observed across treamtnets in EC.with significant variation observed at 10% concentration where Control^a > NL^ab > PET^b = PS^b. Regarding BD, there was no significant variation at 1% concentration of MPs (max = 1.07, min = 1.06), whereas for 5% and 10% concentration of MP, variation was found to be significant with Control^a > NL^ab ≥ PS^bc ≥ PET^c.

In this study, variation of SOC at 1% concentration of MPs was found to be at Control^a > NL^a > PET^b > PS^b, at 5%, the variation was Control^a > NL^a > PET^b > PS^c, and for 10%, the variation was significantly different for all MPs as Control^a > NL^b > PET^c > PS^d. Regarding impact of MPs on AN, the observations indicate that at lesser concentration, i.e. 1%, all three MPs – NL, PET and PS have similar AN as the control. However, with increasing concentration of MPs, the AN reduced. Comparison analysis within groups (NL^a = PET^a > CONTROL^a> PS^a) indicate no significant differences at 1% concentration of MPs, whereas at 5% concentration, the variation is Control^a > NLA^b = PET^ab ≥ PS^b and for 10%, it is Control^a > PET^b > NL^ab = PS^ab.

The PHI score for each treatment used in the pot experiment has been shown in Fig. 1(f). Pearson correlation tests were conducted between PHI scores and individual soil parameters pH, EC, BD, SOC, and AN. Figure 2 displays the correlation coefficient (r) for each respective parameter. The results indicate overall negative and weak correlation.

3.3 Plant characteristics after pot experiment

Weekly shoot growth for both species was recorded for all the treatments over a six-week period. Variations in germination was observed as well. Figure 3 shows the influence of the treatments on the various plant parameters in increasing order in a given group. T-test was done to determine if different concentrations of MPs impacted the two species differently and it was found that difference in response to treatments was significantly different in case of GI and total chlorophyll. Duncan’s multiple range test was done to determine significance at p < 0.05.

(A1) (B1)

(A2) (B2)

(A3) (B3)

(A4) (B4)

(A5) (B5)

(A6) (B6)

(A7) (B7)

Figure 3: Effects of MP particles on plant properties. Impact of various MP concentrations on the growth of mustard (A1 to A7) and tomato (B1 to B7) over six weeks; Impact of the different types and different concentrations of MPs on GI, RL, SL, TChl and LAI have been presented for enhanced visual comparison. Mean values of triplicates were taken for the visualization after mean comparison tests. The charts show response to various treatments against control highlighted in yellow for mustard, also mentioned as (M) in the chart title, and orange for tomato, mentioned as (T) in the chart title. The difference in response to the treatments in both species is evident from the graphs.

Descriptive statistics suggested approximate normality within each group. Multivariate analysis revealed significant effects for both the concentration factor (F(2, 18) = 3.340, p = 0.05) and the polymer factor (F(2, 18) = 9.037, p = 0.02). These findings indicate that both concentration levels and polymer types exert statistically significant impacts on the dependent variables. A significant interaction effect was observed between the concentration and polymer factors (F(8, 72) = 2.880, p = .018), suggesting that the influence of concentration on the dependent variables is contingent upon the specific polymer type, and vice versa.

In mustard plants, significant differences were observed in physiological responses to the different doses. GI exhibited a reduction of up to 50%, indicating adverse effects on seed germination. RL displayed upto 66% difference in response to the treatments, suggesting a substantial decrease in root elongation attributed to MP exposure, potentially impairing nutrient uptake and overall plant health. SL also demonstrated a significant reduction, with a maximum difference of 36.78%, indicative of compromised above-ground biomass and productivity under MP influence. Furthermore, the root length to shoot length ratio demonstrated a significant imbalance with a difference of 69.24%, suggesting alterations in root-shoot allocation strategies under MP influence. TChl exhibited a considerable decrease of up to 57.89%, highlighting disruptions in photosynthetic activity and plant metabolism due to MP exposure. LAI showed the most pronounced difference at 82.15%, indicating substantial reductions in foliage density and light interception capacity, which can detrimentally impact overall plant productivity. Plants treated to 10% NL exhibited lower values across all parameters while plants exposed to 10% PS had the best growth as shown in Fig. 3 (A1).

In tomato plants, pots treated with 10% PET exhibited lower values for all parameters. The GI exhibited a reduction of 40.00%, and RL displayed a pronounced decrease of 61.54%, impacting nutrient uptake and plant anchorage in the soil. SL showed a notable decrease of 57.78%, affecting above-ground biomass and plant productivity. TChl exhibited a significant decrease of 78.01%, and LAI displayed a substantial reduction of 72.63%. Additionally, the RL:SL showed an imbalance of 64.30%, potentially altering nutrient acquisition, water uptake, and overall plant development dynamics. The growth rate was less than control across all treatments. Plants exposed to 10% NL had the slowest growth rates for both species.

Multivariate ANOVA and posthoc tests found no significant effect of MP types on mean GI in both mustard and tomato plants. However, paired t-test analysis revealed a disparity in GI values between both plants (t(9) = -12.348, p < 0.001), indicating distinct physiological responses under MP exposure. Significant differences in LAI were found among treatment groups in mustard plants, showing varied impacts of different polymer types. However, no significant difference was observed in tomato LAI treatments. For RL, a significantly higher impact was observed in mustard with PET compared to NL and PS (p = 0.001), while no significant difference was found in tomato RL. Significant variations were noted in SL responses to MP types in both mustard and tomato plants. Dose-dependent responses for all parameters are shown in Table 1. Significant differences were observed in TChl in mustard but not in tomato. Paired t-tests indicated significant variations in TChl for both species (p < 0.05), aligning with previous studies suggesting the detrimental impact of MPs on soil health and plant growth (Mondal et al., 2022).

Table 1

Dose-dependent impact of MPs on various plant properties in ascending order of impact for mustard and tomato plants. No significant difference was found in the effect of polymer types or concentrations on GI.
	MUSTARD	TOMATO
Concentration	Leaf Area Index
1%	NL < PET < Control < PS	Control < PS < NL < PET
5%	NL < PET < PS < Control	PS < PET < NL < Control
10%	NL < PET < Control < PS	PET < PS < NL < Control
	Root Length
1%	PET < NL < PS < Control	PS ≤ NL ≤ Control = PET
5%	PET < PS < NL < Control	PS < PET ≤ NL ≤ Control
10%	PET < PS < NL < Control	NL = PS = PET < Control
	Shoot Length
1%	Control < NL < PET < PS	PS < PET < Control < NL
5%	Control = PET = NL < PS	PS < PET < Control = NL
10%	NL ≤ Control ≤ PET < PS	PS < PET < NL < Control
	Total Chlorophyll
1%	NL < PS < PET < Control	PS < PET < Control < NL
5%	NL < PS < PET < Control	PET < NL < PS < Control
10%	NL < PET < PS < Control	PS < PET < NL < Control

FTIR analyses revealed the uptake of PET MP in roots of both species at 10% concentration whereas no MPs were detected in the shoots and leaves or for the other polymers at any concentration level.

In the field investigations, PET and HDPE were detected in soil samples whereas HDPE, commonly used in plastic mulching applications (Kasirajan & Ngouajio, 2012), was also detected in root samples, suggesting potential pathways for MP accumulation in agricultural soils as well as uptake in crops. On the other hand, PET is a product that is found in several commonly used items such as carry bags, bottles, wrappers, etc., and may be deposited in the soil either via littering or by atmospheric deposition. This finding underscores the potential for MPs to accumulate and persist in soil environments. Considering the widespread use and persistence of plastic materials in modern farming practices, further studies concerning spatial exploration and the long-term effects of MP contamination on soil health and agricultural productivity are required. The results from this field study may help in comparing and validating the laboratory-based pot experiments, providing valuable insights for developing effective mitigation strategies to address plastic pollution in agricultural soils.

In the laboratory based experiments, significant disparities in soil parameters indicate detrimental effects with increasing MP concentration. Soils treated with PS exhibited reduced pH, EC, BD, SOC and AN levels, compared to the control. Previous studies have indicated that higher MP concentrations may lead to soil acidification (Boots et al., 2019; Wang et al., 2021). However, further research is needed to confirm this hypothesis (Levchik et al., 1999; Myren et al., 2020). In contrast, Zhao et al. (2021) observed an initial decline in soil pH followed by an increase, suggesting complex interactions between MPs, soil biota, and soil pH. No significant differences were found for EC in response to treatments, consistent with findings by Chia et al. (2022). However, within groups, significant variations in EC were observed at 10% concentration, with all treatments showing lower EC than the control, particularly in soils treated with PS. Regarding BD, significant variations were observed at 5% and 10% MP concentrations, with a decreasing trend observed with increasing MP concentrations. This aligns with previous findings indicating that MP contamination may reduce soil bulk density by preventing soil aggregation (de Souza Machado et al., 2018; Chia et al., 2021). The presence of MPs may disrupt soil microbial biota, affecting SOC content. SOC variations were significant across all MP concentrations, with PS-treated soils exhibiting the greatest difference compared to the control. MPs may also impact soil AN dynamics, with all MPs showing similar AN levels at 1% concentration but reducing AN at higher concentrations. Soil microbes play a crucial role in nitrogen regulation, but MPs disrupt these processes, impacting nitrogen mineralization and bioavailability (Shi et al., 2022), with specific effects observed for polyethylene MPs (Li et al., 2020).

The results indicate that PS impacted all the factors negatively and was found to be the most detrimental to soil health. While the PHI score for PET may be less in this study, it is one of the most common MPs to be detected across environmental matrices as well as it has numerous applications (Kim et al., 2021; Abbasi et al., 2021). Due to poor recycling and waste management, PET MPs inadvertently end up in the environment thereby increasing their concentration thereby elevating their hazard category. The results of the pot experiments reveal diverse effects of the polymers on plant properties. Given that each MP originates from distinct parent polymers, their environmental impact also exhibits variability. For instance, PET is a polymer made by the esterification of terephthalic acid (Oku et al., 1996). Although PET has a low hazard score of 4 but its risk potential is uncertain as it may contain up to 60% of non-classified compounds (Lithner et al., 2011; Ranjani et al., 2021). PS is made from styrene monomers derived from petroleum. PS is of foam consistency and is slow to degrade and may acts as vectors for other elements (Hwang et al., 2020; de Souza et al., 2022). PS has a hazard score of 30 and may cause acute toxicity, skin and eye irritation (Lithner et al., 2011). All these factors lead to uncertainties in affirming the impacts of PS in the environment. NL is a type of polyamide usually consisting of aminocaproic acid monomers that contain six carbon atoms and an amino group. NL grade 6.10, commonly used in toothbrush bristles, was used in this experiment. It has a high hazard score of 47 which may be understated as > 10% weight of the polymer consists of unclassified substances (Balyeat et al., 1988; Lithner et al., 2011; Zheng et al., 2022). MPs derived from NL at 10% concentration has the highest PHI score followed by PS as shown in Fig. 1(f). These varying PHI scores have been found to have specific impacts on the plant and soil properties. In this study, PET MPs have been found to impact all the parameters significantly and it can be speculated that with even more concentration, the effects could be detrimental to soil health and the terrestrial environment as a whole.

Univariate test revealed that the PHI scores differ significantly (p < 0.05) within the group. Pearson correlation indicates strong negative correlation between PHI score and pH and AN at r=-0.604 and r=-0.665, respectively. For EC and BD, a weak correlation was noted at r=-0.198 and r=-0.191, respectively. However, a weak relationship between PHI scores and SOC was observed at r = 0.096. Similar effect can be observed in the results of the pot experiment. Thus, it can be deduced that higher PHI scores are associated with lower soil pH, lower electrical conductivity, lower bulk density, and lower available nitrogen levels. The reduction in pH and AN may impact the soil ecosystem as a whole and hamper plant health. Rillig (2018 & 2021b had implied that MPs may mimic as SOC which can have implications in soil carbon studies as well as the global carbon cycle. However, environmental conditions, including soil type, climate, and crop management practices, can significantly influence soil properties and responses to treatments. In this study, the pot experiments were conducted under conditions mirroring those of actual field studies, ensuring that the findings are representative of natural environmental dynamics. Therefore, the observed outcomes may be attributed to the broader context of the natural environment.

The data exhibits a discernible dose-response relationship between polymer concentration and plant parameters, wherein higher concentrations of polymers correlate with diminished values in parameters such as shoot length, root length, total chlorophyll content and leaf area index relative to the control group. Notably, distinct polymers cause varying impacts on plant physiology. For instance, PET MPs induce a pronounced reduction of 50% in shoot length compared to the control, whereas PS MPs elicit a comparatively modest decline of 10% in total chlorophyll content. This underscores the nuanced influence of polymer composition on specific plant responses. The observations are derived from a six-weeks exposure period, prompting inquiry into the persistence of these effects over prolonged durations and the potential evolution of plant responses over time.

Several studies have indicated that MPs can significantly impact the growth of plants (Campanale et al., 2022; Lian et al., 2022). The control group demonstrated healthier root and shoot lengths, total chlorophyll content and leaf area index compared to the MPs-treated groups. Variations in germination was observed as well where tomato exhibited higher germination indices compared to mustard across treatments. A probable reason for lower GI in mustards could be poor quality of seeds but significant variation between control and other treatments was observed for all other parameters. MP treatments, particularly at higher concentrations, tended to have reduced root and shoot lengths as well as total chlorophyll content and leaf area index, in both plant species. Among MPs types, NL showed the most pronounced negative effects on plant growth parameters, followed by PET and PS. These findings underscore the considerable impact of MP pollution on various aspects of plant physiology and growth dynamics, emphasizing the urgency of implementing mitigation strategies to preserve ecosystem health and agricultural productivity. These findings underscore the considerable impact of MP pollution on various aspects of plant physiology and growth dynamics, highlighting the urgent need to address MP contamination to safeguard plant health and ecosystem functioning.

5.1 Challenges and future scope

Further analysis and additional data may provide deeper insights into potential relationships between MPs contamination and soil characteristics. Extending the experiment duration beyond plant mortality could reveal additional insights into long-term effects and ecosystem dynamics. Additionally, investigating enzyme activity and other biochemical responses would offer valuable insights. MP accumulation in plants may be exploited to discover phytoremediation techniques to tackle the issue of MP pollution. Moving forward, exploring practical applications and mitigation strategies in real-world settings will be crucial, necessitating innovative approaches to address MPs impacts on agricultural systems effectively. Addressing these limitations will advance the ability to manage and mitigate the impacts of MPs pollution on soil and plant health, and overall environmental sustainability.

The impact of three MP concentrations on soil health and plant growth parameters were observed. The findings reveal varying degrees of hazard associated with different MP concentrations. AN and BD were the most impacted soil parameters whereas increasing concentration of any MP impacted all properties. In plants, NL was found to be the most hazardous at concentrations of 1% and 10%, whereas as 5%, PS the impacted the most plant properties. NL hindered mustard growth while PS impacted the growth of tomatoes the most, while PET had similar impacts on both plants. Like soil, plants exhibited signs of stress and potential mortality with increasing concentrations, suggesting a dose-dependent response to MP exposure. Notably, regardless of concentration, NL MP emerges as the most concerning, consistently influencing plant health across all levels. The relationship between MP presence and altered plant behavior underscores the need for further investigation into the implications of MP contamination on agricultural ecosystems and plant performance.

Credit authorship contribution statement

Ankita Saha: Conceptualization, Resources, Investigation, Methodology, Data curation, Software, Validation, Visualization, Writing – original draft, Writing – review & editing. Parishmrita Baruah: Conceptualization, Formal analysis, Methodology, Software, Writing – original draft. Sumi Handique: Conceptualization, Methodology, Resources, Writing – review & editing, Supervision

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Supplementary data available on request

Funding

This work was supported by the DST Core Research Grant (CRG/2022/000538); and the UGC SJSGC Fellowship (202223-UGCES-22-GE-ASS-SJSGC-16445).

Acknowledgement

The authors would like to thank the staff at Sophisticated Analytical Instrumentation Centre at Tezpur University for providing access to FTIR for this study.

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The authors declare no competing interests.

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Assessment of microplastic pollution on soil health and crop responses: Insights from dose-dependent pot experiments

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Highlights

1. Introduction

2. Materials and methods

2.1. Field investigation

2.1.1 Microplastic extraction and indexing

2.2 Experimental design

2.4 Statistical analyses

3. Results

3.1. Field observations for soil and plants

3.2 Soil characteristics in pot experiment

3.3 Plant characteristics after pot experiment

5. Discussion

5.1 Challenges and future scope

6. Conclusion

Declarations

References

Additional Declarations

Supplementary Files

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