Traditional and biodegradable plastics improve pea (Pisum sativum) growth by promoting nutrient turnover in soil

Abstract

Microplastic accumulation caused by traditional plastic mulching can disturb plant nutrient-mining strategies. Biodegradable plastics may reduce these risks. However, the different effects of traditional and biodegradable microplastics on agroecosystems and optimal microplastic type for crop-soil systems remain largely unknown. A pot experiment was performed to identify the mechanisms underlying the effects of traditional [polypropylene (PP) and polyethylene (PE)] and biodegradable [polycaprolactone (PCL) and polyadipate/butylene terephthalate (PBAT)] microplastics at 0%, 0.1% and 1% (w/w) in a pea-soil ecosystem. Traditional microplastics caused greater carbon allocation to shoots, while PBAT did not significantly alter dissolved organic-carbon content. NH₄⁺-N increased with 1% (w/w) PP whereas NO^3–-N decreased owing to enhanced N-acetylglucosaminidase activity with 0.1% and 1% PP and PE, and 1% PBAT during pea growth. Biodegradable microplastics enhanced microbial biomass carbon, nitrogen and phosphorus, whereas traditional microplastics gave inconsistent results. Microplastics increased the complexity of bacterial and fungal networks and impacted ecosystem functions because they may serve as labile carbon resources for soil microorganisms, stimulating organic matter decomposition. However, once labile carbon in native soils is depleted, inadequate fresh labile carbon from root exudates fails to alleviate microbial carbon limitations, resulting in peas competing with microorganisms for scarce nitrogen resources to promote its growth.

Graphical abstract

Keywords

Biodegradable microplastics / microbial community / nutrient-acquisition strategies / standard microplastics

Highlight

	● Polypropylene (PP) and polyethylene (PE) and polycaprolactone (PCL) microplastics can bond to soil aggregates.
	● Polyadipate/butylene terephthalate (PBAT) microplastics can stimulate microbial growth, mining more available nutrients for pea root growth.
	● Microplastics increased N uptake during pea growth with increasing β -1,4-N-acetylglucosaminidase activity.
	● Increasing Shannon diversity was observed with 0.1% PP, PE and PCL, and with 0.1% and 1% PBAT at the seedling stage.
	● Microplastics at 0.1% and 1% did not affect material exchange between soil bacteria and fungi but did negatively effect on ecosystem functions.

1 Introduction

Plastic pollution, particularly that associated with microplastics (< 5 mm diameter particles), has emerged as a pervasive threat to ecosystem health and safety^[1]. Indeed, the extensive use of plastic mulch in agriculture has transformed agroecosystems into significant reservoirs of microplastic pollution. Once released from residual plastic films, microplastics readily interact with the surroundings in all ecosystems. As an exogenous input, microplastics have been shown to affect both abiotic and biotic soil processes, depending on particle characteristics and environmental conditions^[2]. Numerous studies have documented the general detrimental effects of microplastics on soil properties, microbial communities and soil biota, with some of their effects being lethal for soil health^[3,^4]. Further, microplastic debris can contribute to the concentration of other environmental pollutants, such as heavy metals and persistent organic pollutants, altering their distribution patterns and bio-accessibility in soils because of their persistence and large specific surface area^[5]. Additionally, microplastics can alter soil microbial community diversity and structure, thereby affecting ecosystem functionality and resilience^[6]. These effects are contingent upon the type and dose of the specific microplastics present^[7].

In agroecosystems, microplastics primarily originate from plastic films, both traditional and biodegradable types. Traditional microplastics, such as polypropylene (PP) and polyethylene (PE), pose emerging threats to soil health, plant growth and microbial diversity by altering soil structure, disrupting key nutrient cycles (e.g., carbon, nitrogen and phosphorus) and ultimately affecting ecosystem multifunctionality^[8–^10]. The different chemical structures of PE and PP result in distinct physicochemical and biological changes in the soil properties^[6]. In response to these challenges, environmentally friendly agricultural films composed of biodegradable microplastics, such as polycaprolactone (PCL) and polybutylene adipate terephthalate (PBAT), have been developed as alternatives to mitigate such ecological damage. Thus, there is an ongoing trend to replace non-degradable plastics with biodegradable ones^[11]. In particular, biodegradable microplastics (PCL and PBAT) have been engineered to readily decompose in natural environments, potentially reducing their long-term ecological footprint. PCL, a biodegradable polyester, shows distinct advantages over other biopolymers, such as its hydrophobic and slow-degrading nature^[12], while PBAT is an eco-friendly plastic with excellent heat resistance and ductility^[13]. Yet there are downsides to these biodegradable microplastics, as they can impair agroecosystem multifunctionality by reducing bacterial diversity, fungal diversity, nutrient availability and enzymatic activity, thereby hindering plant growth^[6]. PCL and PBAT may contain certain chemical additives that interfere with the biodegradation process and causes harm to the environment^[14]. Small plastic particles can infiltrate the soil, thus posing potential risks to soil biodiversity and health. However, their effects on the soil, in terms of toxicity and environmental impacts, remain largely unexplored. Also, the different ecological effects of traditional and biodegradable microplastics on agroecosystems, as well as the optimal biodegradable plastic to use during critical crop growth stages, have not been determined. Finally, most studies looking at the effects of microplastics focus mainly on a single growth stage; few have considered the different key stages of plant growth (seedling, flowering and maturity)^[10,^15]. Therefore, the ecological safety of biodegradable plastics must be assessed across all growth stages before promoting their widespread commercial application in crop production.

The aim of this study was to examine the effects of both traditional and biodegradable microplastics on a crop-soil system across three key plant growth stages. Peas (Pisum sativum) were chosen as the experimental crop, being a nutritionally important and widely planted species. Numerous studies have reported that traditional microplastics (PE and PP) have significant negative effects on soil water and nutrient availability, microbial diversity, enzyme activity, photosynthesis and crop growth^[16–^18]. Considering the prevalence of microplastic pollution from traditional plastic film mulches in pea-soil systems, biodegradable plastic film mulches are currently under consideration as an alternative. However, whether the types and levels of degradable microplastics are superior to those of traditional microplastics during the critical plant growth stages in a pea-soil ecosystem remains an open question. Therefore, we hypothesized that biodegradable microplastics, as a C resource, included toxic and hazardous matter that is easily degradable during mineralize soil organic matter (SOM), releasing available nutrients for the growth of pea and offering a sustainable option for pea-soil systems. Therefore, these will have a better effect on a pea-soil system than traditional microplastics. This hypothesis was tested by a microcosm experiment in which traditional (PP and PE) and biodegradable (PCL and PBAT) microplastics were added at rates of 0.1% and 1% (w/w) in order to (1) assess soil nutrient availability, hydrolase activity and microbial diversity, (2) evaluate the effects of microplastics during the key pea growth stages and (3) examine the ecological effects of traditional and biodegradable microplastics on the pea-soil agroecosystem.

2 Materials and methods

2.1 Experimental design and setup

The experimental soil was collected from an edible bean-testing site in Haitong Times Agricultural Park, in Longshan Town, Cixi City, Zhejiang Province, China (30°03′ N, 121°28′ E). The region is characterized by a monsoon climate with an average annual temperature of 16.0 °C and average annual precipitation of 1273 mm. In November 2022, soil from the 0–20 cm tillage layer was sampled using a five-point sampling method. The test soil was cleared of impurities and plant residues, sieved through a 2-cm sieve and thoroughly mixed for the incubation experiments.

Four types of microplastics (powdered, pure polymer, 125 µm; Sigma-Aldrich, St. Louis, MO, USA) were selected for the experiment: PP, PE, PCL and PBAT. PP and PE represent traditional microplastics, and PCL and PBAT biodegradable microplastics. Microplastics were added to the soil at rates of 0%, 0.1% [1 g·kg^–1 dry soil (w/w)] and 1% [10 g·kg^–1 dry soil (w/w)] to simulate field conditions. The soil was mixed thoroughly with added nutrients and then divided into nine treatment groups with nine replicates each: (1) no microplastic addition (control), (2) 0.1% PP, (3) 1% PP, (4) 0.1% PE, (5) 1% PE, (6) 0.1% PCL, (7) 1% PCL, (8) 0.1% PBAT and (9) 1% PBAT (with all treatment concentrations expressed on a w/w basis). For treatments 2–9, PP, PE, PCL and PBAT were divided into multiple small particles and mixed separately with the soil to ensure a uniform distribution. Wet soil (~1.01 kg dry weight, about 60% water holding capacity) was added to PVC pots (14 cm × 11 cm). Pisum sativum cv. Zhong Qin No.1 seeds were germinated and three seedlings transplanted into each pot. Subsequently, water-soluble fertilizer (0.50 g·L^–1 urea, 0.30 g·L^–1 potassium phosphate and 0.08 g·L^–1 potassium chloride in deionized water) was added to each potted plant. The experimental pots were placed in an incubator with a day/night temperatures of 25/20 ± 1 °C and a relative humidity of 70%–80% until maturity. During the incubation, distilled water was added to maintain at 60% water holding capacity. Environmental factors were eliminated by arbitrarily rearranging the pots weekly. Two pea seeds of the same growth and health were sampled for each plot after 1 week.

2.2 Sampling and physicochemical properties for plants and soil

At the seedling (15 d after sowing), flowering (27 d after sowing) and maturity (41 d after sowing) stages of the pea growth (Fig.1), shoot and root samples were carefully collected, initially oven-dried (FD 115, Binder GmbH, Tuttlingen, Germany) at 105 °C for 2 h, then at 65 °C for 24 h to constant mass before weighing.

At the three growth stages, the two soil samples from each treatment was collected and stored in separate plastic ziplock bags at 4 °C and in centrifuge tubes at –80 °C until measuring soil physical-chemical properties and extracting genomic DNA, respectively. Soil microbial biomass C and N were assessed using the chloroform fumigation-potassium sulfate extraction method^[19]. Non-fumigated and fumigated extracts were analyzed for soil dissolved organic C (DOC) with a Multi N/C 2100 TOC/TN analyzer (Analytik Jena, Jena, Germany). Microbial biomass C (MBC) was calculated based on the difference in DOC between the non-fumigated and fumigated extracts using a k_ec factor of 0.45^[19]. In turn, microbial biomass N (MBN) was calculated to be similar to MBC using a k_en factor of 0.54^[19]. Non-fumigated extracts were analyzed spectrophotometrically to determine ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^–-N) concentrations with a PowerWave XS microplate spectrophotometer (BioTek Instruments, Winooski, VT, USA). Soil microbial biomass phosphorus (MBP) was determined using chloroform fumigation-sodium bicarbonate extraction followed by applying the molybdenum antimony colorimetric method^[19] with a UV-Vis spectrophotometer (UV-2450; Shimadzu, Kyoto, Japan). Briefly, two 5-g soil samples (non-fumigated and fumigated) were prepared, and a third 5-g fresh soil sample was treated with 0.5 mL of 250 μg·mL^–1 KH₂PO₄ to calculate the recovery efficiency of P. The three soil samples were then extracted with 80 mL 0.5 M NaHCO₃ (pH 8.5). MBP was calculated using k_p = 0.4 ^[20] and Olsen-P was determined using the soil NaHCO₃ extract (1:20 w/v)^[21].

The activities of the soil extracellular enzymes β-1,4-glucosidase (BG), β-cellobiohydrolase (CBH), β-1,4-N-acetylglucosaminidase (NAG), β-1,4-xylosidase (BX), polyphenol oxidase (PPO) and peroxidase (PER) were measured based on a modified version of a method used in previous studies^[22]. Briefly, BG, CBH, BX and NAG activities were measured fluorometrically (excitation at 365 nm and emission at 450 nm) using substrate links to a fluorescent tag (50 μL aliquot of 100 μmol·L^–1 4-methylmbelliferone in each sample well)^[23]. PPO and PER activities were measured colorimetrically (460 nm) using a 50 μL aliquot of 25 μmol·L^–1 L-dihydroxyphenylalanine in each sample well^[23]. Enzyme activities were quantified using an automated fluorometric plate reader (Victor3 1420–050 Multilabel Counter, PerkinElmer, Waltham, MA, USA).

2.3 DNA extraction, amplification, high-throughput sequencing and bioinformatic analysis

Total genomic DNA was extracted from 0.5 g of frozen soil (–80 °C) using a DNeasy PowerSoil Kit (Qiagen, Hilden, Germany) following the manufacturer’s protocol. DNA concentration and quality were measured using a NanoDrop spectrophotometer (ND-100, Thermo Fisher Scientific, Waltham, MA, USA). The universal forward primer 341 F (CCTAYGGGRBGCASCAG) and the reverse primer 806 R (GGACTACNNGGGTATCTAAT) were used to amplify the bacterial hypervariable V3–V4 regions of the 16S rRNA gene^[24]. For fungi, the ITS region was targeted by the primers ITS1F and ITS2^[6]. Polymerase chain reaction conditions included an initial denaturation for 3 min at 95 °C, followed by 30 cycles of denaturation at 95 °C for 5 s, annealing at 58 °C for 30 s and extension at 72 °C for 30 s, and a final extension at 72 °C for 10 min. The polymerase chain reaction products were purified using a gel extraction kit (Qiagen) and sequenced on an Illumina MiSeq platform (Novogene Co., Ltd., Beijing, China). Sequencing libraries were generated using NEBNext® Ultra™ II DNA Library Prep Kit for Illumina® (New England Biolabs, Ipswich, MA, USA) following manufacturer instructions. Index codes were then added. The library quality was assessed using a Qubit® 2.0 Fluorometer (Thermo Fisher Scientific, Waltham, MA, USA). Finally, the library was sequenced on an Illumina platform and 250 bp paired-end reads were generated. The raw sequences were processed using Quantitative Insights Into Microbial Ecology (QIIME, version 1.7.0)^[25]. In the first step, sample splitting was performed to determine the sample from which each sequence originated. Sequence denoising was performed to obtain amplicon sequence variants (ASVs) to reduce sequencing errors and redundancy. After obtaining the feature list (ASV list) and representative sequences, species annotation was performed for bacteria, fungi and protists. ASVs with total sequences length of < 20 were removed and the resulting bacterial, fungal and protist samples were individually flattened.

2.4 Statistical analysis

All figures were generated and statistical analysis was performed using R software (version 4.0.0). Levene’s test for homogeneity of variance was performed and treatment means were compared using the R package “agricolae” with a least significant difference of 5% (LSD_0.05). FastSpar (version 1.0.0), which incorporates the SparCC algorithm, was used to infer microbial correlation networks. All treatment data were divided based on bacterial and fungal communities to create co-occurrence networks of bacterial and fungal communities. The abundance of ASVs was > 0.1% in each treatment. Subsequently, the correlations and p-values among the ASVs were inferred using 50 iterations and 1000 permutations. Significant (p < 0.05) correlations at > 0.7 and < 0.7 were used. The correlations were visualized and their topological features calculated using Gephi (version 0.9.2). Key network metrics, including the number of nodes and edges, were calculated by combining each treatment at the three key growth stages. Structural equation modeling (SEM) was performed using the R package “lavaan” to test the significance of the hypothesized causal relationships among the type and content of microplastics, microbial biomass, soil enzyme activity, available nutrients, microbial diversity and plant growth. Before SEM analysis, DOC, DON and Olsen-P were downgraded to available nutrients, MBC, MBN and MBP to microbial biomass, BG, CBH, XYL, NAG, PPO and PER to soil enzyme activity, and the type and dose of microplastics and biomass of the pea plants were standardized. The best-fit model was determined using the chi-square test (χ²), p-values, goodness-of-fit index (GFI), root mean square error of approximation (RMSEA) and standard root mean square residual (SRMR) methods. Based on the removal and addition relationships between the observed variables, an optimal model was finally built and the standardized estimate value (R²), standardized total effects and effect of the addition of microplastics on the pea biomass were determined.

3 Results

3.1 Plant biomass

Pea shoot biomass decreased by 43.4% (p < 0.05) with 1% PP and by 10.8% with 0.1% PBAT. Pea root biomass increased by 35.3% (p < 0.05) with 0.1% PBAT at the seedling stage, compared to that of the control plants. The shoot:root ratio (S:R) increased by 36.8% (p < 0.05) with 0.1% PP (Tab.1). Consistently, at flowering, shoot biomass increased by 126.1% with 0.1% PP, 88.0% and 68.7% with 0.1% and 1% PE, respectively, and 103.0% with 1% PBAT, compared to that of the control plants (p < 0.05); meanwhile, root biomass increased by 34.3% with 1% PP and 0.1% PBAT compared to that of the control (p < 0.05) at flowering. S:R increased by 110%, 60.7%, 57.3% and 50.1% with the 0.1% PP, 1% PE, 0.1% PCL and 1% PCL (p < 0.05), respectively, but decreased by 28.4% with 0.1% PBAT (p < 0.05) (Tab.1). At maturity, shoot biomass increased by 72.5% to 193.0% (p < 0.05) with the addition of microplastics (except for 1% PCL), while root biomass increased by 66.4%, 110.0%, 103.4% and 104.9% (p < 0.05) with the 1% PP, 1% PE, 0.1% PBAT and 1% PBAT, respectively, compared to that of control plants. Consequently, S:R increased by 97.3% and 73.7% (p < 0.05) with the 0.1% PP and 0.1% PE, respectively (Tab.1).

3.2 Soil available nutrients and microbial biomass

Soil DOC content was lower (p < 0.05) across all microplastics treatments (except for PBAT) than in the control at the seedling stage. At flowering and maturity, DOC content was not affected (p > 0.05) by microplastics addition relative to the control (Tab.2). Similarly, soil NH₄⁺-N content was not affected (p > 0.05) by microplastics addition at the seedling and flowering stages, although at maturity it was higher (p < 0.05) with 1% PP than in the control. Soil NO₃^--N was higher (p < 0.05) with PCL (0.1% and 1% w/w) than in the control at the seedling stage, but was not affected at flowering (p > 0.05), and was lower (p < 0.05) at maturity with PP (0.1% and 1%), PE (0.1% and 1%) and PBAT (1% w/w) than in the control. Soil Olsen-P concentration was lower (p < 0.05) with 0.1% PP than in the control at the seedling stage, but remained unaffected at flowering and maturity (p > 0.05). Soil MBC content was higher (p < 0.05) with 1% PCL and 1% PBAT than in the control at the seedling stage. At flowering, MBC was higher (p < 0.05) with 0.1% PP, 1% PCL and 1% PBAT than in the control. At maturity, MBC was lower (p < 0.05) with 1% PE and higher (p < 0.05) with 0.1% and 1% PCL than in the control. Soil MBN content was higher (p < 0.05) with 0.1% PP, 0.1% PE, 0.1% PCL, 0.1% PBAT and 1% PBAT than in the control at the seedling stage. At flowering, MBN was higher (p < 0.05) with 0.1% PP, 1% PE, 0.1% PCL and 1% PCL than in the control. At maturity, MBN was higher (p < 0.05) across all microplastics treatments (except for 0.1% PP and PCL) than in the control. Soil MBP content was higher (p < 0.05) with all microplastics treatments (except for 0.1% PE and 1% PCL) than in the control at the seedling stage. At flowering, MBP was higher (p < 0.05) with 0.1% PP, 0.1% PE and 1% PE than in the control. At maturity, MBP was lower (p < 0.05) across all microplastics treatments (except for 1% PE) than in the control.

3.3 Soil potential enzyme activities

Soil BG activity was higher (p < 0.05) with 0.1% PE, 0.1% PCL and 0.1% PBAT, but lower (p < 0.05) with 1% PCL, than in the control at the seedling stage. At flowering, BG activity was lower (p < 0.05) with 0.1% PE and 1% PCL than in the control. At maturity, BG activity was not affected (p > 0.05) by the addition of microplastics compared to that in the control (Tab.3). Soil CBH activity was higher (p < 0.05) with 0.1% PE, PCL and PBAT, and lower (p < 0.05) with 1% PCL, than in the control at the seedling stage. At flowering, CBH activity was not significantly affected (p > 0.05) by microplastic addition. At maturity, CBH activity was higher (p < 0.05) with 1% PE than in the control. Soil NAG activity was higher (p < 0.05) with 0.1% PP, 0.1% PE, 1% PE, 0.1% PCL and 0.1% PBAT than in the control at the seedling stage. At flowering, NAG activity was higher (p < 0.05) with 0.1% PP, PE, PCL and PBAT, as well as with 1% PBAT than in the control. At maturity, NAG activity was higher (p < 0.05) with 0.1% PP, 0.1% PE, 1% PE, 1% PCL, 0.1% PBAT and 1% PBAT than in the control. Soil BX activity was higher (p < 0.05) across all 0.1% microplastics treatments (except for PP) than in the control at the seedling stage. At flowering, BX activity was lower (p < 0.05) with 0.1% PE and 1% PLA than in the control. At maturity, BX activity was higher (p < 0.05) with 0.1% PE, PCL and PBAT, as well as 1% PBAT, than in the control. Soil PPO activity was lower (p < 0.05) with 1% PE but higher (p < 0.05) with 0.1% PCL, 1% PCL and 1% PBAT than in the control at the seedling stage. At flowering, PPO activity was higher (p < 0.05) with all 0.1% microplastics treatments, and with 1% PP and PBAT than in the control. At maturity, PPO activity was lower (p < 0.05) with the addition of any microplastic (except for 0.1% PP and 1% PBAT) than in the control. Finally, soil PER activity was lower (p < 0.05) with 0.1% PP, 1% PCL and 1% PBAT, but higher (p < 0.05) with 0.1% PE, 0.1% PCL and 1% PE than in the control at the seedling stage. At flowering, PER activity was higher (p < 0.05) across all microplastics treatments, except for 1% PP, than in the control. At maturity, PER activity was lower (p < 0.05) with 0.1% PP, PCL and PBAT as well as 1% PCL than in the control.

3.4 Microbial community diversity and co-occurrence network

None of the microplastics had any significant effect on bacterial richness, Chao1 and ACE at the seedling stage. However, an increasing trend in Shannon diversity (p < 0.05) was observed with 0.1% PP, PE and PCL, and with 0.1% and 1% PBAT (Tab.4). In contrast, microplastics did not significantly affect bacterial richness, Chao1, Shannon and ACE at flowering and maturity (Tab.4). In contrast, none of the microplastic treatments significantly affected fungal richness, Shannon, Chao1 and ACE across any of the three growth stages studied (Tab.5). Nonetheless, all microplastic treatments enhanced the complexity of the bacterial and fungal networks by altering bacterial and fungal community structure, as well as increasing key network metrics (nodes and edges), although they did not change the bacterial communities dominated by Proteobacteria, Acidobacteriota and Crenarchaeota, nor fungal communities dominated by Ascomycota, Mortierellomycota and Chytridiomycota (Fig.2).

3.5 Effect of microplastics on pea biomass

At the seedling stage, the structural equation model SEM (χ² = 0.358, df = 2, p = 0.836, GFI = 0.997, RMSEA = 0.000 and SRMR = 0.013) indicated that all factors (biotic and abiotic) accounted for 21.3% of the variance in pea biomass. Further, microplastic type (0.308), microbial biomass (0.216), microbial diversity (0.059) and soil enzyme activity (0.687) positively affected pea biomass, whereas microplastic concentration (–0.478) and available nutrients (–0.123) had negative effects on pea biomass (Fig.3). At flowering, SEM (χ² = 0.211, df = 2, p = 0.900, GFI = 0.998, RMSEA = 0.000 and SRMR = 0.015) revealed that all factors explained 14.2% of the variance in pea biomass. Microbial diversity (0.09), microplastics concentration (0.134), soil enzyme activity (0.192), microbial biomass (0.158) and available nutrients (0.307) all positively influenced the pea biomass (Fig.3). At maturity, SEM (χ² = 0.521, df = 2, p = 0.771, GFI = 0.996, RMSEA = 0.000 and SRMR = 0.029) revealed that all factors explained 62.8% of the variance in pea biomass. Further, microplastic concentration (0.124), microplastic type (0.196) and microbial biomass (0.734) were positively related to pea biomass, whereas microbial diversity (–0.075), soil enzyme activity (–0.041) and available nutrients (–0.262) had a negative relationship to pea biomass (Fig.3).

4 Discussion

The impacts of microplastics on crop-soil systems in agriculture remain a contentious issue, largely because of the diverse types and doses of the specific microplastics involved^[26,^27]. The phytotoxicity of microplastics depends on characteristics such as concentration, size, shape, composition and charge, as well as on plant species and growth stage^[28]. This study found inconsistent and contradictory results regarding the effects of four microplastics on pea roots and shoots at three stages of plant growth. Microplastics can adhere to plant surfaces, enter the plant through the root tips, affect root development, inhibit seed germination and induce oxidative stress responses, alter photosynthetic efficiency, and cause cytotoxicity and genotoxicity, thereby affecting plant metabolism and nutrient uptake^[17,¹⁸,^29]. Notably, the increase in root biomass observed at maturity in PBAT-amended soils indicates that PBAT, which shows high heat resistance and ductility, is relatively eco-friendly and supports pea root growth. PBAT microplastics can stimulate microbial growth and mine more available nutrients for pea root growth^[6]. In contrast, our results showed variable impacts of standard microplastics (PP and PE) on plant growth. At the seedling stage, shoot biomass increased only in 1% PP-treated plants and, at flowering, shoot biomass increased with 0.1% PP, whereas root biomass increased only with 1% PP. Shoot biomass increased with both 0.1% and 1% standard plastics, while the root biomass increased only with 1% PP and PE at the maturity stage. These results indicate that different types and levels of standard microplastics have different effects on plant growth, potentially owing to their phytotoxicity, which depends on their characteristics and dose^[10]. In addition, microplastics can reduce root penetration, improve soil aeration, and stimulate root growth and extension by altering root architecture and morphology^[30,^31]. S:R increased with 0.1% PP across all three growth stages; at flowering, the it increased with 1% PE and 0.1% PCL, and at maturity, with 0.1% PE. These findings indicate that more carbon was allocated to the shoots in the standard microplastics (PP and PE) treatments than in the PBAT treatment during pea growth.

Soil DOC content was not affected at flowering and maturity, indicating that at the seedling stage, PP, PE and PCL microplastics might have bonded with soil aggregates, protecting SOM from microbial decomposition^[32]. This finding is consistent with a previous study which reported that PE did not affect the processes responsible for the degradation of organic matter^[33]. Olsen-P decreased only in the 0.1% PP treatment at the seedling stage, indicating that PP microplastics present in the soil can reduce the number of adsorption sites^[34]. Therefore, PP microplastics could cause peas to use more NO₃^–-N than NH₄⁺-N during growth. Notably, soil NO₃^–-N increased at the seedling stage with the addition of 0.1% and 1% PCL but decreased upon addition of 0.1% PP, 1% PP, 1% PE and 1% PBAT compared to that in the control at maturity. This indicates that PCL caused cells to form aggregates entangled in exopolymers at the seedling stage due to these being immobilized by PCL in the soil as a biofilm^[9,^35]. Therefore, PP, PE and PBAT enhanced NO₃^–-N uptake by pea roots^[10,^35]. However, this study did not analyze the microbiome, so it was not possible to determine the effect of microplastics on rhizospheric microorganism nutrient mineralization and root nutrient acquisition strategies during pea growth.

Microplastics can alter the resource acquisition strategies of both crop roots and microbes through their impact on the amount and composition of root exudates and subsequent effects on nutrient availability^[27]. Our results demonstrated that biodegradable microplastics (0.1% PCL, 1% PCL and 1% PBAT) increased MBC, MBN and MBP at three growth stages (MBP decreased only at maturity), indicating that PCL and PBAT serve as biodegradable C sources that support SOM decomposition and nutrient cycling during pea growth owing to the altered metabolic status of the microbial community^[36,^37]. In this process, the highly biodegradable PCL was degraded by lipase and esterase activities, as Proteobacteria were the dominant phyla on the surface of the PCL polymer^[38], resulting in more C being released for microbial activity, thus enhancing soil microbial network complexity^[39,^40], as PCL byproducts (linear and cyclic polymers and oligomers) can cause severe membrane depolarization by impairing membrane proton pumps and altering normal membrane ion-permeability^[41]. PCL and PBAT microplastics increased the Shannon index of bacteria only at the seedling stage, illustrating that biodegradable microplastics promote the growth of bacterial taxa involved in their hydrolysis. This finding is inconsistent with a study that showed that biodegradable microplastics have an adverse effect on other taxa^[6], which is related to soil and crop plant types. Additionally, biodegradable microplastics did not have a strong toxic effect on bacterial interactions, but rather appeared to enhance bacterial network complexity^[42]. Biodegradable microplastics create a resource-enriched environment in the soil that alleviates microbial resource competition and allows for mutualistic symbiosis^[43]. Therefore, high auxiliary C metabolism in biodegradable microplastic-amended soils promoted microplastic-derived C into microbial biomass C, N and P. In contrast, traditional microplastics (PP and PE) introduced lysate and altered soil chemodiversity, thus facilitating metabolism with high C investment. However, pea root exudates can disrupt microbial nutrient acquisition when microplastics are added, leading to inconsistencies in MBC, MBN and MBP because traditional microplastics (PP and PE) are more resistant to degradation and form microplastic-soil aggregates that provide an additional habitat for soil microorganisms^[16,^44]. At the lower dose tested (0.1%), PE, PLA and PBAT showed higher activities of C-acquisition enzymes (i.e., BG, CBH and BX) compared to those with the higher dose (1%) at the seedling stage. A decline in BG and BX activity was observed with increasing PCL levels at seedling and flowering stages. Further, higher CBH activity was observed at the seedling stage with 0.1% PE whereas, at maturity, CBH was higher with 1% PE. In turn, at the maturity, BX activity was observed in 0.1% PE-, PCL- and PBAT-amended, and 1% PBAT-amended soil, but not in PP-amended soil. These findings indicate that increased labile C from root exudates did not alleviate microbial C limitations with microplastic addition. Also, NAG activity was higher with 0.1% PP, PCL and PBAT from the seedling to maturity stages, and with 1% PE and PBAT treatment at the seedling and maturity stages, respectively. This indicates that microplastics increased N uptake during pea growth, resulting in increased NAG activity.

The decomposition of biodegradable microplastics (PCL and PBAT) can produce water and labile C, thus providing energy for soil microorganisms^[6,^12] and promoting SOM decomposition through increased PPO activity in 0.1% PCL-, 1% PCL- and 1% PBAT-amended soil at the seedling stage and 0.1% PCL-, 0.1% PBAT- and 1% PBAT-amended soil at flowering, but decreased PPO activity in 0.1% PCL-, 1% PCL- and 0.1% PBAT-amended soil at maturity. In this process, PCL can degrade into nanoplastics and oligomers, which can be potentially toxic to surrounding organisms^[44]. However, the cross-linked structure of PBAT restricts the access of water and microorganisms to the polymer chain, making it more resistant to degradation than PCL^[39]. Once labile C is depleted, pea roots release root exudates (labile C), obviating the need for microbial decomposition of SOM until the maturity stage. However, PP-bonded soil aggregates are resistant to breakdown because of their higher malondialdehyde and hydrogen peroxide content, resulting in lower PER activity in 0.1% and 1% PP-amended soil at the seedling stage^[45]. Conversely, higher PER activity may occur with 0.1% and 1% PE due to oxygen deficiency in PE-bonded soil aggregates by altering the antioxidant system, potentially causing the accelerated growth of pea plants^[46]. However, PER activity also varied at flowering and maturity due to differing pea growth strategies. Microplastics can also alter the characteristics of plant–soil microbial communities, indirectly affecting crop growth^[17,^31]. Notably, an increasing trend in the Shannon diversity of bacteria was observed at the seedling stage in 0.1% PP-, PE-, PCL- and PBAT-amended soil, as well as in 1% PBAT-amended soil. However, the microplastic type and dose did not significantly affect the bacterial richness, Shannon, Chao1 and ACE indices across all growth stages, indicating that bacteria dominated microbial communication with increasing bacterial and fungal network complexity owing to microplastic addition at the seedling stage^[47]. The reduction in bacterial dominance during flowering and maturity may have resulted from microplastics serving as a labile C source. However, when this source was depleted by microorganisms, the fresh labile C from root exudates was insufficient to sustain bacterial growth. Therefore, microplastics at the 0.1% and 1% doses did not affect material exchange among soil bacteria and fungi, but did impact ecosystem functions during microbial adaptation to the microplastic-contaminated environment^[3,^4]. Additionally, we conclude that, because of the short incubation time, the full toxicity of microplastics was not realized. Thus, these plastics represent a lower threat to soil microbial taxa and had a greater effect on nutrient cycling for pea growth. SEM analysis also demonstrated that the microplastic dose had a significantly negative direct effect on pea biomass whereas the microplastic types had a significantly positive indirect effect on pea biomass through microbial biomass and enzyme activities at the seedling stage. However, neither microplastic type nor dose significantly affected pea biomass, directly or indirectly at flowering and maturity, indicating that the accumulation of microplastics altered soil properties by serving as a C source for microbial growth, thereby influencing the size of related C, N and P pools which then affects pea growth.

Nevertheless, based on this short-term pot experiment, the differential ecological effects of traditional and biodegradable microplastics, and the safety of biodegradable plastics as replacements for traditional plastics in pea-soil ecosystems remain uncertain. Consequently, there are some uncertainties that need to be taken into account. The differential effects of traditional versus biodegradable microplastics in this study were mixed and thus there are no clear winner. However, given this was a short-term pot experiment, long-term field experiments with pea (or other legumes) that are fully dependent on biological-fixed N will be needed to fully evaluate whether these microplastics pose a threat to pea-soil ecosystems.

5 Conclusions

Our results showed that the type and dose of microplastics influenced the characteristics of the pea-soil system. Notably, PBAT appeared to be relatively eco-friendly and supported pea root growth, as it is a biodegradable C source that supports SOM decomposition and nutrient cycling during pea growth. Alternately, traditional microplastics (PP and PE) had varying effects on crop growth depending on their characteristics (e.g., dose, size, shape, composition and charge). However, once the labile C was depleted, fresh labile C from root exudates failed to alleviate microbial carbon limitations, forcing pea plants and microorganisms to compete for limited soil N and P resources. Consequently, at the seedling stage, easily degradable matter in microplastic-bonded soil aggregates may have broken down, leading to bacterial dominance in microbial communication. Bacterial dominance decreased as pea growth progressed because of the depletion of labile C from microplastics, thereby shifting plant resource acquisition strategies toward competition for limited N and P. However, whether traditional or biodegradable microplastics are preferable for pea growth remains to be clarified. Specifically, long-term field experiments are required to assess the potential ecotoxicological effects of microplastics on the pea-soil system.

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The Author(s) 2025. Published by Higher Education Press. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0)